Mechanical Sand Control Methods

This article describes mechanical sand control methods and covers the following topics:

·Gravel packing techniques i.e. Internal Gravel Packing (IGP) and External Gravel Packing (EGP), and the

various gravel packing systems presently available.

·Post-gravel pack operations and remedial treatments with special emphasis on through tubing techniques.

·Other mechanical sand control methods i.e. non-gravel packed screens and external casing packers.

·Special sand control applications including horizontal wells, fracpacking, and the Auger system.

Two main objectives were pursued when preparing this chapter:

. To provide all the required information for designing a mechanical sand control completion,

·To promote a common and consistent approach to gravel packing within the Group.

The latter objective was more difficult to achieve as gravel packing is still being developed through extensive

research and field trial programmes.

1 General gravel pack design considerations

1.1 What is a successful gravel pack?

Effective sand control was long considered to be the main measure of success for a gravel pack and current

procedures allow this objective to be achieved with confidence for most applications. Some reduction in well

productivity as a result of gravel packing has long been recognised, and accepted by many companies as being

a price to be paid for sand control. However recent tests have highlighted that this damage, when expressed as

Productivity Index reduction, may be of the order of 50-80%. Well damage of this magnitude is often

unacceptable in terms of deferred production and energy losses in the production system.

The requirements for a successful gravel pack are to reduce sand production to a tolerable level whilst

minimising productivity impairment. The emphasis today is to optimise gravel packing methods with the prime

objective of minimising productivity impairment. Despite all the efforts of the past the industry has been

unsuccessful in uniquely identifying optimum gravel packing procedures.

Consequently this article can only:

·Provide a summary of current "best" practices.

·Give acceptable ranges for the main design parameters.

·Identify procedures and methods that offer the best scope for productivity enhancement.

·Encourage operators to challenge established practices and field test in structured way new approaches to

sand control.

1.2 Formation sand analysis

1.2.1 Sampling

Samples should be representative of the interval under consideration. The number of samples required is a

function of the formation heterogeneity. The field geologist should be consulted to establish to what extent grain

size distributions obtained from samples in a well can be extrapolated to other parts of the field.

Formation samples can be obtained from the following sources:

1. The best samples are obtained by coring the reservoir. Good core recovery can be achieved in

unconsolidated sand with rubber or fiberglass sleeved coring barrels, although some material will be washed

away and lost when cutting the core. The number of sample points on a core should be established after a

proper core description. Unlike many other core properties the sand grain distribution will not change during the

aging of the core, hence old core material can be used.

2. Side wall samples can provide acceptable formation samples although they are small and often contaminated

by mud cake. Care must also be taken with the crushing effect of this sampling method which may cause a bias

towards smaller grain sizes.

3. Samples from drilling cuttings, bailer runs or separators should be treated with care as they may not be

representative. Bailer samples will generally include the larger grain fractions and samples recovered at surface

will be biased towards the smaller, more easily transportable sand grains. The origin of these samples in the

reservoir is obviously unknown.

Formation samples need to be cleaned, thoroughly dried and separated into individual sand grains. This must be

done carefully for partially consolidated sands to avoid breaking the individual sand grains and to ensure that

only individual sand grains are present.

1.2.2 Grain size distribution

1.2.2.1 Sieve analysis

The sieve analysis of a sand sample consists in passing the sample through a series of sieves, allowing the

sand grains smaller than each sieve's openings to fall to the next sieve.

The results of the sieve analysis are generally plotted as cumulative weight precent versus grain size (sieve

opening) on a linear or logarithmic scale. This plot typically yields an S shaped curve.

An alternative plotting technique used by sedimentologists is to plot the cumulative weight percentages against

the sieve opening size.

The advantage of this method is that a sample with a Gaussian distribution of sand particles will plot as a

straight line (Fig. 681). This makes interpolation between data points easier.

1.2.2.2 Grain size distribution statistics

The main parameters to be derived from the sieve analysis are the median grain size and the uniformity

coefficient.

a. Median

The median grain size (D50) is the particle size corresponding to the 50 percentile of the cumulative weight

distribution from the sieve analysis. This parameter is directly used to select the gravel used for sand control.

b. Uniformity coefficient

The uniformity coefficient C is defined by the ratio of the 40 percentile grain diameter to the 90 percentile grain

diameter.

C = D40/D90

This parameter gives a measure of the spread in the grain size distribution, in other words the sorting of the

sample and is commonly used when selecting sand control measures. Schwartz used the following

classification:

·C < 3: Uniform sand

·C > 5: Non uniform sand

The more poorly the formation sand is sorted, the more difficult it is to efficiently control sand production.

Geologists use different parameters to characterise sorting but the meaning is the same

1.3 Gravel considerations

1.3.1 Gravel sizing

Several gravel sizing criteria have been used in the past and it is interesting to review some of these concepts to

see how the modern sizing criteria have evolved.

1.3.1.1 Coberly criterion

Coberly formulated one of the first gravel sizing criteria. He observed that spherical grains could effectively

bridge over openings twice as large as the grain diameter. It can be shown that an hexagonal packing of uniform

spherical grains leaves a pore diameter 6.5 times smaller than the sphere diameter. Coberly further reasoned

that if gravel is sized to bridge the largest 10 % of the formation sand grains, then the remaining formation sand

will bridge on these larger grains. Hence Coberly recommended that the gravel should be selected to be 13

(6.5´2) times larger than the 10 percentile formation sand grain size.

In practice this criterion was found unsatisfactory. Experience has shown that the bridging process is unreliable

and allows for too much sand invasion into the gravel which severely restricts the permeability of the pack. Sand

bridges will collapse under disturbed flow conditions caused by production fluctuations or interruptions.

Several authors elaborated on Coberly's original concept and the so-called 10 % rule was proposed, i.e. the

gravel should be 5 to 8 times the 10 percentile formation sand grain size.

1.3.1.2 Schwartz criterion

Schwartz used the formation sand uniformity coefficient and velocity considerations to select the gravel size. He

assumed that sand bridges collapse at high flow velocities.

For most formation sands, Schwartz's and Saucier's method will select the same gravel size especially for

uniform sands. For poorly sorted sands, Schwartz's criterion will advise very small gravel sizes, which may be

less practical and more susceptible to impairment.

1.3.1.3 Saucier criterion

Saucier carried out laboratory experiments to determine the optimal gravel to sand size ratio [924]. His work

proved that it was more effective to stop the formation sand at the gravel/sand interface.

Saucier used a linear flow test cell with gravel on one side and formation sand on the other. The pressure drop

through the cell was measured under various flow rates and a range of gravel to sand ratios were tested. He

showed that under disturbed flow conditions (i.e. by alternating the flow direction) and for certain gravel/sand

size ratios, bridges break down and formation sand invades the gravel pack resulting in impairment at the

gravel/sand interface. Hence the effective pack permeability will only be a fraction of the initial permeability.

The permeability ratio shown on this generalised curve is the ratio of the effective pack permeability (i.e. after

exposure to disturbed flow conditions) to the initial pack permeability.

Above a ratio of 10, high pack permeability’s are obtained but the formation sand can flow through the gravel

and sand control is not achieved.

Saucier also calculated the ratio of the effective pack permeability to the formation sand permeability.

Hence the optimal gravel to formation grain size ratio i.e. the ratio which maintains the highest pack to formation

permeability ratio and avoids sand production is between 5 and 6.

1.3.1.4 Comments on Saucier's criterion

Saucier's gravel sizing criterion has set the trend towards small gravel sizes and has become the industry

standard. The following important comments on Saucier's experiments should be noted:

·For a reasonably uniform sand, Saucier's results are logical as the median grain size is theoretically smaller

than the pore throats of the gravel pack (assuming a hexagonal packing mode). Hence the gravel acts as a filter

for the formation sand.

·The gravel invasion process by formation sand should be a function of the gravel packing quality or tightness.

·Even under controlled laboratory conditions, small amounts of formation sand were produced continually,

independently of the gravel size used. This observation indicates that finite sand production is inherent in gravel

packs and this is confirmed by field experience.

·Saucier's criterion leaves room to be conservative if required. Selection of a smaller gravel size than indicated

by the 5 to 6 ratio will reduce the ratio of effective pack permeability to formation permeability. However even

when selecting one size smaller gravel, the permeability contrast should remain adequate for all practical

purposes. It should be further noted that gravels smaller than 40/60 are susceptible to plugging by formation

fines clays and wellbore contaminants.

1.3.1.5 Gravel sizing guideline

a. Uniform formation sands

Before proceeding with the selection of the gravel size, it is important to check the quality of the formation sand

samples. A wide spread of the data points in the grain size distribution plots indicates poor quality or non-

representative samples. Such samples should be treated with caution as they may lead to inefficient sand

control:

For each sample the permeability should be measured, and it should be determined whether or not the sample

is from an interval that is likely to be produced.

For each producing sample, the gravel size should be selected according to Saucier's rule i.e. select a gravel

median diameter 5 to 6 times bigger than the formation sand median diameter:

D50, gravel = 5 to 6´D50, sand

The gravel selected for each producing interval sample should be the next smallest API recognised size or

commercially available size.

The gravel selected for packing the hole interval should then be the smallest gravel size chosen for all the

producing interval samples.

b. Non-uniform formation sands

If most samples obtained for the interval under consideration are poorly sorted then there is a higher risk that the

gravel pack will not effectively control formation sand and will become impaired due to fine sand invading the

pack. This poor sorting may be due to the sampling quality (e.g. few samples of poor quality) and if possible

better samples should be obtained.

In the confirmed case of poorly sorted sands or when better samples cannot be obtained then a more

conservative gravel selection criterion should be adopted.

c. Fine formation sands

In practice, gravel sizes smaller than 40/60are rarely used because this fine gravel is more susceptible to

impairment and more difficult to retain in place (smaller screen slots required). Field experience indicates that

40/60size gravel is small enough to control most "productive" formation sands. However in some cases 50/70or

even 80/100gravel has been used to minimise fines production.

According to Saucier's criterion, the finest formation sand that can be controlled with 40/60gravel ( D50, gravel =

340 mm) has a median grain size in the range of 68 to 57 microns. Referring to Table 70, a sand with a grain

diameter less than 60 microns is classified as silt!

1.3.2 API recognised gravel sizes

Gravel can be ordered by specifying two mesh sizes. A selected number of gravel sizes for sand control

applications were recognised by the API. Following API specifications, 96 % of the gravel should pass the

coarse designated sieve and be retained on the fine designated sieve (see also Section 1.3.4, Gravel quality

specifications).

The nominal median diameter is the arithmetic average of the two mesh sizes given by the gravel designation.

It is recommended that the API recognised gravel sizes are used whenever possible. Other gravel sizes are

commercially available but may be more difficult and more costly to obtain. Also API gravel quality control

specifications are fully defined for the recognised sizes given above.

For logistical reasons, operators with large gravel packing operations will tend to standardize on a minimum

number of gravel sizes.

1.3.3 Why not always the smallest gravel?

Small gravel sizes provide adequate permeability contrast with formation sand. For example, the permeability of

40/60gravel is some 50 Darcies, much larger than the permeability of most reservoirs. Hence one could argue

that small gravel sizes could be used in all cases without significant effect on the well deliverability. However

small gravel sizes are more prone to impairment through contamination by dirty fluids or poor placement

techniques. Smaller screen slots can plug-up more easily and the manufacturing of these smaller screen slots is

more difficult. In practice 40/60gravel is the smallest gravel size routinely used as experience indicates that it is

small enough to control most "producible" formation sands.

1.3.4 Gravel quality specifications

The use of high quality gravel is an important factor for placing unimpaired gravel packs. Gravel characteristics

which can result in reduced pack and formation permeability’s include excessive fines and clay content,

excessive oversized grain content, excessive fines generation under load and pumping conditions. High quality

gravel should consist almost exclusively of quartz which is a hard mineral very resistant to crushing and attack

by acids. High quality gravel normally contains a minimum of 98 % quartz. Impurities are an indication of a

weaker gravel and also indicate that the gravel will be more soluble in acid, steam and even in water. These are

important considerations for water or steam injectors. Grain multi-crystallinity contributes to fines generation

when gravel is subjected to load. Well rounded gravel is preferred as it is less subject to grain breakage and

gives more permeable packs.

Efforts to raise the quality of gravel pack supplies were started by the publication of proposed gravel pack sand

specifications. Some problems were recognised with the original Shell gravel specifications, i.e. roundness and

sphericity tests were probably subjective, the "visual appearance" test was unsatisfactory and no strength or

abrasion resistance tests were available. The latter test should measure the gravel's ability to resist fines

production during storage, transport and pumping into the well.

Discussions on this subject have been carried out throughout the industry under the auspices of the API

subcommittee on the evaluation of gravel packing materials.

Full quality control of gravel is generally made at the production chemistry laboratory. It must also be ensured

that the proper gravel is delivered at the wellsite and that it has not been contaminated with foreign material.

If gravel is used with a silt and clay content near the API 1% limit then formation impairment is a substantial risk.

It is recommended to check this parameter on batches of gravel. A new specification has not been established

but as a working guide, a level of 0.1 % silt and clay content should be maximum acceptable.

1.3.5 Effects of pumping equipment on gravel

Gravel slurries are normally pumped with positive displacement pumps. Roll has found that an insignificant

amount of fines is generated when pumping gravel with viscous or low-viscous carrier fluids. He observed that a

general reduction in the gravel grain size due to grain breakage can occur if poor quality gravel is used.

1.3.6 Miscellaneous data

1.3.6.1 Gravel permeability

Fig. 685 shows gravel permeability data for various sizes of commonly used gravel as reported by Sparlin, and

Gurley. Note that grain shape and gravel compaction are factors that also affect pack permeability.

1.3.6.2 Effect of mixing of gravel and formation sand

Sparlin has shown that the permeability of gravel can be significantly reduced if mixing with formation sand

1.3.6.3 General data

Absolute sand density: 2650 kg/m3 (164 lb/ft3)

Bulk gravel density (loose pack): 1620 kg/m3 (100 lb/ft3)

One gravel sack 100 lb (50 kg): 1 ft3(0.028 m3)

1.3.6.4 Low density gravel

In many cases low productivity from internal gravel packs, particularly in highly deviated completions, has been

attributed to poor perforation tunnel fill. Many researchers have associated this with poor rheological properties

and leak off characteristics of the gravel pack carrier fluid. Much of the research and development work in this

area has therefore concentrated on the development of improved viscosifiers.

1.4 Screen considerations

Slotted liners and screens come in many forms and their use demands a correct evaluation of the slot width

necessary to retain the gravel. This equipment should also satisfy a number of quality specifications.

1.4.1 Slot sizing

The slot width of a screen is sized in accordance with the gravel size used to control the formation sand. A

bridging criterion is totally inadequate here as experience shows that bridges collapse under disturbed flow

conditions. Any production of gravel will jeopardise the success of the treatment by creating voids in the pack or

erosion of the slots. An absolute stoppage criterion has to be used i.e. the slot width or wire spacing must be

smaller than the smallest gravel grain size used.

The term gauge refers to the slot width or the spacing between wires as measured in thousands of an inch. For

example a "20 gauge" screen has a slot width of 0.020 inch. This can easily be checked with a feeler gauge.

Because of the relationship between formation sand grain sizes, gravel grain size and slot width; it is obvious

that formation sand can flow through the screen openings if it is not completely covered by gravel. In critical

gravel pack applications (e.g. long, highly deviated intervals) where it is difficult to ensure that the screen is fully

covered with a gravel sheath, a prepacked screen can be used as a further insurance against sand production

1.4.2 Screen types

Four basic types of screens are currently available i.e. slotted liners (SL), wire wrapped screens (WWS),

prepacked screens (PPS), and sintered pack screens (SPS). This section briefly discusses the basic

characteristics of each. Other recent developments in screen technology are also covered.

The configuration of the openings in all screens is very important. If the sides of the slots are parallel, plugging

will occur as small particles can bridge inside the slot. Openings with non-parallel sides and with the narrowest

width on the outside are less susceptible to plugging

1.4.2.1 Slotted liners (SL)

Slotted liners can be manufactured from oil field tubulars which are slotted with a precision saw or mill. Slots

should be cut longitudinally so as not to weaken the pipe in tension. Various vertical slotting patterns can be

used depending on the desired inflow area and retained strength of the base pipe. Vertical staggered slots as

shown in Fig. 690 are most commonly used. The base pipe material should be selected according to the

downhole conditions. In a corrosive environment, erosion/corrosion of the slots may lead to plugging of the

screen.

Undercut slots are not recommended for slotted liners as they are difficult to saw and in practice still tend to plug

because of the thickness of the pipe and the poor undercuts generally obtained. Undercut slots may also present

feathered edges which are susceptible to much more rapid erosion/corrosion than square cut slots. It is difficult

(i.e. expensive) to cut small slots with the necessary degree of quality control. The smallest practical slot size is

about 0.3 mm (0.012 in) straightcut or 0.51 mm (0.020 in) undercut.

Slotted pipe screens have relatively low initial cost but have their limitations, i.e. limited inlet flow area and

susceptibility to plugging and corrison/erosion. Manufacturers have been able to create undercut slots using

laser cutting technology. Slot sizes as small as 0.006" have been cut. Rapid development of this technique is

expected if costs drop significantly below the cost of a comparable wire wrapped screen.

1.4.2.2 Wire wrapped screens (WWS)

Wire wrapped screens are constructed by wrapping a wire around a perforated or slotted base pipe. The wire is

spaced to give the required slot width and a self-cleaning slot is obtained by using a "keystone" shaped wire.

The "all welded" wire wrapped screen type (Fig. 691) has superseded all other wire wrapped screen types used

in the past (wrapped-on pipe, grooved type, ribbed type.

All welded screens are generally manufactured as follows. The jacket is first manufactured by wrapping a

"keystone" shaped wire on longitudinal ribs. The wire is welded at each contact point with the ribs by induction

welding and this process is continuous and automated, giving good manufacturing control of the slot width. The

longitudinal ribs are required to manufacture the jacket but they also increase the flow capacity by providing an

annular space between the wire wrapping and the pipe base. This jacket is then slipped over and welded to a

perforated base pipe.

A major advantage of the all welded construction technique is that the wire is well secured to the jacket. A

common problem with earlier screen designs is that the wire wrapping could unwrap from the base pipe when

attempting to run or retrieve the screen. Induction welding also allows the use of narrow wrapping wire thereby

maximising the slot flow area.

All welded screens with a slot size of 0.002 inch have been successfully manufactured for special applications.

However small slot sizes result in reduced inlet flow area and increased plugging risks. For gravel packing

applications, the smallest slot size used in practice is 0.006 in with 50-70 gravel.

Stainless steel wire (304 or 316 grade) is generally used because of its superior abrasion and corrosion

resistance. The wire is usually wrapped on normal grade carbon steel tubulars for non- corrosive environments.

The manufacturing of wire wrapped screens is a semi-automated process. Hence screen design can be adapted

to suit particular needs.

1.4.2.3 Comparison of wire wrapped screens and SL

The slot size that can be economically cut for a SL is an obvious limitation. For the same slot size, a SL has a

smaller inlet flow area compared to a wire wrapped screen. This may create an excessive pressure loss

depending on flow rates and fluids produced although an adequate flow area is still provided for most

applications. However fines production and high flow velocities may lead to rapid erosion of the slots.

It is claimed that SL are more susceptible to plugging because of the slot configuration and the smaller inlet flow

area. Slot plugging may be caused by hydrocarbon or scale precipitation or even corrosion. SL is stronger and

cheaper than wire wrapped screen but is more susceptible to erosion/corrosion of the slots which may lead to a

sand failure. SL can be manufactured in corrosion resistant alloys.

Because of their lower costs, slotted liners are the preferred option for marginal, low rate wells. However, where

corrosion resistant alloys are required the incremental cost of wire wrapped screen compared to slotted liner

becomes marginal, hence the use of wire wrapped screen in such cases is generally preferred.

1.4.2.4 Prepacked screens (PPS)

The primary use for prepacked screens is generally as a substitute for wire wrapped screen when:

1.gravel packing highly deviated zones, or

2.as a primary form of sand control in horizontal wells.

Prepacked screens are basically an assembly of concentric screens packed with gravel. The gravel should

always be consolidated to prevent or losing gravel when handling the liner. The design criteria is basically the

same as for gravel packing i.e. the gravel should be sized to stop the formation sand (Saucier). The 40/60and

20/40U.S. mesh gravels commonly used in these screens have average pore throats of 50 and 100mm, much

smaller than wire spacing of conventional screens, perhaps explaining why they plug more easily.

Furthermore, mechanical damage to prepacked screens has been widely reported. This is thought to be

attributed to:

1.manufacturing defects,

2.flexing of prepacked screens during pick up and make up, or

3.while installing as this may cause the brittle thermo-setting plastics used to crack.

Prepacked screens are more expensive and are usually physically larger than conventional screens. By design,

they are less likely to fail because of erosion. However, plugging of the gravel sheath during installation due to

dirty completion fluids or by formation fines during production is a major problem. To help minimise plugging

some operators run the prepacked screen open ended.

a. Types of prepacked screens

Fig. 692 illustrates the main screen types which are marketed by the major sand control service companies and

screen manufacturers.

Single screen prepack - This screen consists of an inner perforated base pipe surrounded by an inner screen.

Only resin coated gravel is used as the filtration medium placed between the inner screen and outer perforated

case. This is probably the most commonly used prepacked screen, and is the most rugged design.

Dual screen prepack - This screen consists of an inner perforated base pipe, surrounded by screens. The gravel

may or may not be resin-coated, and is normally contained by an outer screen. This is a heavy duty premium

grade assembly designed primarily for maximum erosion resistance. Hence it found most of its application in

high rate gas wells.

Thin body prepacked screen - Recently developed, thin bodied prepacked (with gravel or resin coated gravel)

screens are now being used extensively (mainly in the US) as replacements for wire wrapped screen. Because

of the reduced OD and/or increased ID, it allows better optimisation of the radial clearance outside the screen

and washpipe, thereby enhancing gravel placement. A reduction in overall screen OD was made possible by

using a wire mesh as the inner screen to retain the gravel. There are no known guide-lines for the minimum

gravel sheath thickness to be used.

A number of variation are available from the major screen manufacturers:

1.Baker Sand Control's Slim-Pack,

2.Johnston's Thin-Pak,

3.Howard Smith's LOW-PROFILE screen marketed by Otis Sand Control.

Further advantages and disadvantages of the "new" thin bodied prepacked screen are briefly discussed below.

Advantages:

·These types of prepacked screen have the same OD as regular wire wrapped screen, hence increasing

potential applications.

·Less expensive than larger diameter prepacked screen.

·It has a higher flow capacity per unit length of screen than conventional prepacked screens with thicker gravel

bodies.

Disadvantages:

·They are more susceptible to physical damage than conventional prepacked screens.

·The thin gravel layer is less resistant to erosion.

·The even placement of a thin gravel layer is difficult to achieve during fabrication.

In summary, highly permeable slim pre-packed screens seem to present an insurmountable series of

manufacturing and quality control problems, thus making their use difficult to justify as a stand alone primary

means of sand control.

b. Quality control of prepacked screen

Wellsite checks - Since the permeability of gravel inside the screen cannot be directly checked it is

recommended that each joint is thoroughly inspected to ensure the resin coated gravel is not cracked or missing.

Apart from the obvious visual checks, screens can be tested by flowing filtered water through the screen and

monitoring the pressure drop. Squirting jets of water emanating from a localised point usually indicate a screen

defect.

Screen manufactures should in all cases be consulted on recommended quality control checks. Additionally,

service companies will help establish the maximum dog-leg which prepacked screen can endure without causing

irreversible damage or deformation.

1.4.2.5 Sintered metal screens (SMS)

One major screen manufacturers has developed a unique sintered stainless steel gravel pack screen. This

screen consists of an outer sintered sleeve welded onto a perforated base pipe. It is claimed that the mechanical

characteristics of such a screen make them ideally suited for high dogleg, deviated or horizontal wells where, the

performance of prepacked screen is questionable.

One size screen is compatible with all gravel sizes used in gravel packing and is available in different alloys. For

example the 100mm sintered pack screen has a median pore diameter size of 42mm and is suited to control

sand in the 40-100mm range.

Vendors claim that this type of screen has maximum erosion resistance. They are certainly more rugged, and

therefore suitable to pipe handling procedures. The susceptibility to plugging is thought to be greater than that of

prepacked screens. Howard Smith Screen Co. the manufacturers, claim that sintered screens are cleared more

readily by back flushing.

The manufacture of sintered metal screen is a three part process. The first phase is known as isostatic

compaction. A polished steel mandrel forms the ID of the screen, with an outer shell forming the screen OD.

Powdered metal is placed in the "jig", which is sealed and subjected to radial pressure, which directly controls

screen thickness.

The second part of the process is sintering, resulting in a re-crystalisation of the micro-structure, and in inter-

particle bonding. Screens are placed in vacuum ovens and cycled over a range of temperatures for a 24 hour

period.

The third part of the process involves electro-polishing of the screen jacket which increases corrosion resistance

and tends to slightly open pore throats. The jackets are then installed on a perforated pipe and welded into

position.

The maximum OD sintered pack screen that can be manufactured is currently limited by the size of the existing

sintering ovens which allows 11" to be manufactured.

Potential advantages:

·Sintered pack screen design is tougher and more damage resistant than wire wrapped screen and prepacked

screen.

Less flow restriction compared with wire wrapped screen and prepacked screen.

·Flexibility/elasticity to pass high doglegs without causing irreversible damage. Tests carried out by the

manufacturers have demonstrated the ability to pass through tight radius bends with no damage to the screen

jacket.

·There is scope to further reduce screen costs by omitting the base pipe. Manufacturers are investigating

alternative coupling techniques.

Disadvantages:

·Susceptible to fines, scale and wax plugging.

·Certain high nickel alloys are not suited to the manufacturing process, hence their use is presently restricted to

non-hostile environments.

·Sintered screens are more costly than wire wrapped and prepacked screens.

1.4.2.6 Selective Isolation Screens (SIS)

This screen type is a variation of the normal wire wrapped screen. Its development was driven by the need to

improve the success ratio of remedial treatments through gravel packed completions.

The basic design includes inner annulus seals and only a few holes or slots machined in the pipe base. The

inner annulus is restricted by using wire wrapped screens fabricated with seals approximately every 1.5m. The

seals do not need to totally stop fluid flow, but only restrict the movement of fluid in the inner annulus. In all

cased hole completions the screen inflow area (maximum number and size of the holes) should not exceed the

number and sizes of the perforations in the casing.

A number of potential advantages of this type of screen are:

·If for whatever reason the gravel pack has to be washed after placement (with acid, or under saturated brine)

each short section of screen can be somewhat isolated with a wash tool to promote positive injection into each

interval.

·Diversion of acid or solvent can be achieved by means of conventional ball sealers.

·Allows the possibility of isolating bottom water once it breaks through

The main disadvantage with this type of screen is the higher cost as most screen manufacturers make them to

order.

1.4.2.7 Shunt screen

This screen is also a variation of the normal wire wrapped screen.

Poor gravel pack integrity as a result of premature sand bridging is a classical problem associated with the

completion of long, highly deviated intervals. In such applications the risk of bridges forming adjacent to zones of

higher permeability due to preferential leak off is pronounced.

a. System description

The shunt or alternative path gravel pack system was developed and licensed by Mobil Research and

Development, and is presently being marketed by Baker Sand Control (BSC) as the ALLPAKTM gravel pack

system.

Alternative flow paths, called shunts or secondary conduits are attached to the side of a gravel pack screen. The

shunts have small holes drilled every few feet allowing communication to the screen/casing (or hole) annulus

across the entire completion zone. Four rectangular shunts are set a right angles to ensure that at least two are

at the top of the liner. The rectangular geometry has been selected to maximise clearance in the annulus while

providing sufficient flow area. The tubes are open at top of the screen only to facilitate alternate by-pass.

The number and size of holes drilled in the shunt is a critical parameter in order to avoid excessive leak off,

hence low shunt velocity and hole plugging. They are sized and configured with respect to gravel size,

completion configuration and fluid rheology. Since no design guidelines have been published, readers are

advised to consult BSC directly for more details on shunt design criteria.

b. Test results and field trials

The screen has been tested [987] in BSC's large wellbore model which allows visual observations to be made.

The results on a 10m long, highly deviated (> 70°) interval indicated gravel pack efficiencies of 95-100%

compared to 65-80% for conventional procedures. Furthermore over 20 shunt screens have now been installed

by a number of companies operating in the Gulf of Mexico and the North Sea. Results to date demonstrate that

long (>100m) highly deviated (> 70°) wells can be successfully packed.

Opcos interested in evaluating the shunt screen for "difficult" completions are advised to consult their local BSC

representative for more information.

1.4.3 Screen dimensions

The selection of the screen size is mainly a function of the type of gravel pack. In cased hole, screen/casing

annular clearance is critical to ensure proper gravel placement and to allow for fishing operations if the need to

retrieve the screen arises. In multiple completions the engineer must take into account the size of tubulars

needed to drain reservoirs below the current gravel pack. From a productivity point of view, little will be gained

by selecting a screen ID larger than the tubing ID.

1.4.4 Screen material selection

Screen material specifications should be derived from the expected service conditions. The main concern for a

screen is to ensure slot integrity over the desired lifetime. In practice no slot corrosion can be allowed as

tolerances on the slot width is typically a thousandth of an inch. Screens made of special alloys are obviously

more expensive and quality control specifications are more stringent.

Slotted liners can be manufactured from carbon steel or stainless steel. It should be noted that a carbon steel

configuration is very susceptible to corrosion/erosion in the slot area. Wire wrapped screens are generally made

of carbon steel for the base pipe and AISI 304L or AISI 316L stainless steel for the jacket as a minimum

specification. Although corrosion of the base pipe is less critical than for the jacket, this aspect should not be

overlooked. Some general guidelines for materials selection are given below

304L and 316L alloys are good for CO2 service (when < 0.1 bar partial pressure H2S) in producing wells (no

free oxygen present) and up to at least 120°C. Experience suggests there is no upper limit on the CO2 level that

can be tolerated. These austenitic alloys are however susceptible to pitting, crevice and stress corrosion

cracking (SCC) in the presence of chlorides and oxygen (AISI 316L has a higher corrosion resistance than AISI

304L). To minimise the risk of corrosion, the pH of the workover brine should be kept above nine, also when

extended exposure (more than a day) to brine is unavoidable, an oxygen scavenger should be used.

Alloys such as Sanicro 28 or Incoloy 825 are adequate for higher H2S levels (up to 5 bar partial pressure), with

or without CO2 and for the same temperature range.

1.4.5 Mechanical properties

API grade J 55 provides enough mechanical strength for screen base pipes of slotted liner or wire wrapped

screen as these are only subject to limited tensile loads and pressure differentials. The connection type is also

not critical and normal flush API connections are generally specified.

1.4.6 Centralisers

Screen centralisation is critical for good gravel placement, especially for cased hole applications when there is

low clearance between the screen and the casing. Welded, blade type centralisers should be used as positive

centralisation is achieved and they are more reliable than clamp-on types. Spacing will be a function of the

specific wellbore conditions. In open hole completions, bow-type centralisers are normally used. However a

number of companies have used rigid aluminium centralisers in barefoot wells to provide sufficient stand off to

minimise screen damage.

When wire wrapped screen are used in gravel pack operations the length of blank section should be reduced as

much as possible to minimise the risk of bridging.

Local sand control service companies will help design a centralisation programme

1.4.7 Screen quality specifications

Screen quality is as important as gravel quality. Quality control of the slot width is the most critical aspect.

Oversize slots will allow some gravel to be produced through the screen and may jeopardise the success of the

treatment by creation of voids in the pack.

The API Std 11D on Miscellaneous Production Equipment was withdrawn from publication in 1976. The

requirements for screen pipe and slotted pipe openings contained in Std 11D were dropped and are no longer

covered by an API standard.

These tolerances normally do not present a problem except for manufacturing the full weld on type of screen.

The problem is caused by slight irregularities in the dimensions commonly found in the base type. These

irregularities are enough to cause the wire spacing to deviate during manufacture. For this screen type the

tolerance figures shown above may be unrealistic. In such cases +2 and -3 gauge specifications may be

appropriate.

It should also be noted that certain doubts remain within the industry as to whether or not the above tolerances

are realistic in view of base pipe distortion during the manufacturing process

1.5 Gravel packing systems

Gravel placement requires the use of specialised downhole tools. A wide variety of gravel placement techniques

and tools exist and only the basic tools and methods are reviewed in this section. Historically, the first gravel

placement methods used were:

·The wash-down method: gravel is first deposited in the well and the screen assembly is subsequently washed

down through the gravel. The gravel is then left to settle around the screen. Disadvantages of this method are

the difficulty of placing a dense pack around the screen, the stirring of gravel inside the wellbore and the risk of

sticking the work string when washing out the gravel.

·The reverse circulation method: gravel laden fluid is circulated down the annulus, gravel is retained by the

slotted pipe and the fluid returns through the work string. The equipment and procedures are fairly simple but the

main disadvantage is that the gravel is subject to contamination as the slurry is pumped down the annulus.

These methods have been completely superseded by the crossover circulation method.

1.5.1 Crossover circulation method

The standard technique for gravel packing either in cased hole or open hole relies on a packer (or a simple

annulus pack-of device in the simpler systems) and cross-over tool combination for routing the gravel slurry from

inside the work string to the screen-casing annulus.

After the screen and liner are run and positioned, the packer is set and the gravel pack port is opened to allow

communication behind the screen and below the packer. A washpipe is used to force the slurry to the bottom of

the screen/casing annulus. By choking the returns or by the use of a multi position gravel pack tool, the slurry

can be circulated behind the screen and partially squeezed into the formation. After the gravel pack has been

placed, the work string with the crossover is pulled and the well is completed by running a production tubing with

a seal extension, landed in the packer. The main advantages of the crossover technique are:

·The slurry is pumped through a work string which is kept clean to avoid contamination of the slurry.

·Treatment pressures are confined to the completion interval below the gravel pack packer.

·Multi-position gravel pack tools allow for accurate control of the different phases of a gravel packing job i.e.

circulating behind the screen, squeezing into the formation and reversing out excess gravel. Additionally, all

gravel packing operations can be carried out in one trip.

There are many different types of gravel packing tools, each developed for specific applications. A thorough

discussion of all these variants would be beyond the scope of this manual and detailed technical information can

be provided by vendors upon request.

1.5.2 Multi-position crossover tool

Gravel pack tools which allow the gravel pack liner assembly to be run and all gravel placement operations to be

carried out in one trip have been developed by all reputable gravel packing contractors. It is the recommended

method for gravel packing in cased or open hole as it allows to work clean, is faster, gives better control over the

job and provides better capabilities for good gravel placement.

The heart of the system is the multi-position crossover tool. Different gravel placement modes are obtained by

manipulation of the work string.

·Running position: when running the gravel pack liner assembly into the well, circulation can be achieved

through the crossover port and up the work string annulus.

·In the squeeze position, no fluid returns are possible and the fluids are pumped into the formation. This position

is used either to carry out injectivity tests prior to pumping the slurry or to dehydrate the slurry into the formation

exclusively.

·In the upper-circulating position the slurry is circulated through the gravel pack port and behind the main screen.

Fluid can leak-off into the formation or circulate through the screen and return through the washpipe.

·A lower telltale screen can be added at the bottom of the completion string. A seal bore is then incorporated

between the two screens. The end of the washpipe seals into the sealbore in the lower circulating position and

fluid is forced to circulate through the lower telltale screen.

·In the reverse circulation position fluid can be circulated above the packer from the annulus to the work string.

This position is used after screen-out to reverse circulate excess slurry out of the work string. It can be also used

to circulate slurry batches to the crossover port when attempting a top-up.

The different positions of the crossover tool have to be located and tested prior to carrying out gravel packing

operations.

All the capabilities discussed above may not be available for a particular tool. It is important to be aware of the

design and capabilities of the specific tool which is used. Vendors will provide detailed documentation upon

request.

1.5.3 Gravel packing systems for special applications

Many variants of the one trip gravel packing tools have been developed for use in special applications e.g.:

·Systems enabling several zones to be gravel packed in one trip, i.e. Baker Sand Control "Beta" system for Shell

Oil's Beta field in the U.S. [932], [933].

·Systems tailored for deep water offshore applications i.e. the four position long stroke gravel pack tool for deep

water offshore operations [934].

1.5.4 Low cost gravel packing system

Low cost gravel packing systems generally feature a crossover tool fitted with a temporary annular pack-off

device i.e. rubber cups. After gravel packing, the crossover and liner section above a back-off sub are pulled

from the well. If required, the liner extension can be sealed with a low cost lead drive-over seal. This gravel

packing system is applicable to low pressure, economically marginal wells. One disadvantage is that many

round trips are required in order to complete the well. Many variants on this completion technique are possible.

Detailed information can be obtained from vendors.

1.5.5 Through tubing gravel packing

There are systems available for gravel packing through tubing which can be attractive for remedial sand control

work in economically marginal operations. Very little experience is available within the Group with this system.

Coiled tubing is used to clean out the hole and to spot gravel to cover the perforations. A small diameter screen

and liner is then run and washed down to the desired depth. After the liner is placed, the work string is removed

and a wireline pack-off may be run to seal the top of the liner.

1.5.6 One trip perforate and gravel pack technique

Integrated one trip perforating-gravel packing systems have been available to the industry since the early

1970's. These systems are often preferred by many operators to minimise exposure to losses, and also to

reduce operating costs. The original concept was first developed by Otis, and since then all major sand control

service companies have developed equivalent one trip systems (eg Baker's SCP system, Dowell's PERFPAC).

The basic design is similar and based on TCPs being positioned below a gravel packing assembly, separated by

a retrievable "perforation" packer. The installation procedures are also comparable for each of the marketed

systems, and are summarised as follows:

1.TCPs are positioned on depth and the "perforation" packer set.

2.The well is perforated and allowed to clean up.

3.The well is killed by reverse circulating. Note the TCPs are normally dropped into the well sump, but are

sometimes left hanging below the 'perforation packer'.

4.The "perforation packer" is unset and the assembly moved down until the screens straddle the completion

interval.

5.The 'perforating packer' is then re-set. The gravel pack packer is then set and gravel pack operations

commence.

A ceramic flapper-knock out isolation valve is normally incorporated into the gravel pack string which will help

reduce losses when the inner wash pipe is withdrawn, hence losses during tripping are minimised. Once the final

completion string has been run and tested, the flapper valve can be shattered using the completion assembly

tail, or by the application of pressure.

Although this technique is field proven it is mechanically more complex than conventional systems, thereby

increasing the chance of operational problems. The incremental cost of drilling a well sump should also be taken

into consideration.

It should be noted that prepacking the perforation tunnels prior lowering the assembly is considered a risky

operation and is not generally carried out. This technique is not therefore applicable to long intervals (> 10 m) or

intervals with significant variations in reservoir permeability, as the risk of premature bridging is considered too

high.

1.6 Gravel pack liner assemblies

1.6.1 Work string

The work string can be either tubing or drill pipe. Drill pipe is commonly used and has the advantage of a higher

load rating. The work string should be rigorously cleaned prior to the gravel packing operations. The work string

ID should be small enough to allow proper placement of the slurry (i.e. to prevent U-tubing). Pumping should be

carried out under turbulent flow conditions to minimize mixing of the various fluids.

1.6.2 Gravel pack packers

1.6.2.1 Requirements

A packer is required during gravel packing operations to isolate the treated interval from the wellbore. It is also

required when the well is on production to avoid pack fluidisation by upward flow through the annulus and in

many cases, to seal the annulus as a mandatory completion requirement. However calculations have shown that

fluidisation is theoretically possible only for very high flow rates or small gravel reserves.

In some cases a temporary gravel pack packer is used for the gravel pack operation. The retrievable packer is

subsequently replaced by a cheaper annulus pack-off assembly.

1.6.2.2 Permanent gravel pack packers

Permanent gravel pack packers fulfil the dual role of a gravel pack and production packer. They form part of the

gravel pack liner assembly which is run together with the crossover tool. Some important considerations when

selecting a packer are the setting and crossover release mechanisms.

1.6.2.3 Permanent-retrievable type

Probably the most commonly used gravel pack packer type is the "permanent-retrievable" type. They offer

nearly the same capabilities as a permanent type packer but have a retrievable feature. The disadvantage is that

they are mechanically more complex implying a higher cost and potentially lower reliability than permanent

packers. In a sand control context, retrievability is no longer a primary requirement as gravel pack installation is

generally successful and failure rates are low.

Permanent-retrievable packers can be set mechanically or hydraulically. In a gravel packing context, these

packers are run together with the crossover tool which can also be released either mechanically or hydraulically.

The choice of the setting and crossover tool release mechanism will be a function of the particular wellbore

conditions. Hydraulically set packers are generally preferred in difficult holes where mechanically set packers are

more difficult to run and operate.

The retrieval procedure requires the use of a specialised retrieval tool run on drill pipe. The packer is released by

a straight pull or by rotation. Wireline retrievability is probably effective only in shallow applications. If required

permanent-retrievable packers may also be retrieved using packer milling tools.

a. Permanent type

Permanent type packers may be preferred in critical applications (high pressure and corrosive environment)

because of the higher reliability of their sealing capability. The same considerations as for retrievable packers

apply when selecting a permanent packer.

1.6.2.4 Temporary gravel pack packers

In some areas it is common to use a rented temporary gravel pack packer and crossover tool assembly for the

gravel packing operations. The packer can be either a cup type packer or squeeze type (RTTS) packer. Cup

type packers are more susceptible to damage when running in the well.

After the gravel pack operations are complete, the temporary packer and crossover tool are retrieved and a

cheap (usually lead seal) annulus pack-off is set above the top of the liner, if required. Several trips may be

required to complete the well. This type of equipment is used for low-cost, low pressure completions.

1.6.3 Blank liner sections

A reserve gravel volume, which is placed in the annular volume between the gravel pack port and the top

screen, is required for the following reasons:

·To allow for pack settling (filling of voids, compaction, gravel dissolution in water or steam).

·As a safety margin to cover uncertainties when determining the total gravel volume required i.e. the gravel

volume placed behind casing for IGPs and wellbore volume for EGPs.

In highly deviated wells ( > 60 degrees ), gravity no longer assists the filling of voids in the pack by settling and

compaction and limits the effectiveness of the gravel reserve.

In the absence of an upper tell-tale, the final screen out occurs when a certain length of compacted gravel

covers the top of the wire wrapped screen. The height of this compacted column is a function of the carrier fluid

viscosity, geometry of the completion etc. The amount of gravel contained in the annulus between the top of this

column and the gravel pack port at the time of the screen-out is a function of the gravel mix ratio.

With a 15 lb/gal slurry, nearly 65% of the annular volume will be filled with gravel. Conversely, at low gravel

concentrations only a small gravel volume can be placed after screen out. This is the reason why upper tell tales

are used in water packs. The upper tell tale allows reserve gravel to be placed by providing an additional

circulating path above the main screen. This will only be effective if the deviation is not excessive (< 60

degrees). The reserve gravel volume will therefore differ when an upper tell tale is used (refer also to Section

1.6.5.2: Upper tell-tales).

There is no definite set of rules to determine the blank liner length required and practices vary to fit each

operator's need. For example BSP uses the following design rule for slurry packs: the gravel pack port to top

screen length is 40% of the gravel pack port to sump packer length. When using high gravel concentrations (12 -

15 lb/gal), 60 feet of blank liner is generally adequate for most cases, except very long intervals.

There are cases where the blank liner length has to be restricted e.g. multiple completions where the separation

between intervals is small. Smaller reserve gravel volumes can be tolerated when multiple slurry batches can be

placed with the placement method used. Also the reserve volume can be reduced when the operator is confident

that uncertainties can be minimised i.e. that proper fill at screen-out can be achieved.

1.6.4 Washpipes

A large OD washpipe is the best way to ensure good gravel placement, especially in difficult wells. The favorable

effects of running large OD washpipe in the gravel pack assembly have been demonstrated. A large washpipe

minimises the flow area between the screen and washpipe thereby forcing the fluid to flow preferentially in the

casing/screen annulus. This promotes packing from bottom upwards, prevents bridging in the annulus before

complete gravel fill-up, especially for deviated wells, and generally improves the packing efficiency. The

perceived disadvantage of using relatively large diameter washpipe is the somewhat increased risk of washpipe

sticking inside the screen assembly.

Field experience suggests the following guidelines:

·A washpipe OD to screen ID ratio of at least 0.6 should be used in all cases.

·Perferably and certainly in difficult applications, this ratio should be at least 0.8.

·The washpipe is not required for a single stage squeeze pack (IGP). However this type of gravel pack is

generally not recommended, except for very short intervals.

Difficult applications are defined by one or a combination of the following features:

·Highly deviated zones (> 60 degrees).

·Long intervals (> 50 feet).

·Zones with a high permeability contrast.

1.6.5 Tell-tales

Tell-tales are short (5-10 ft) sections of screen which are included in the liner assembly to assist with the gravel

placement and/or to give a positive indication over the placement of the gravel.

1.6.5.1 Lower tell-tales (LTT)

The lower tell-tale assembly consists of a seal bore sub and a short piece of screen located below the main

screen. The washpipe is sealed into the sealbore and fluid returns are taken through the wash pipe via the lower

tell-tale screen.

Historically, lower tell-tales have been incorporated into gravel pack strings to facilitate good gravel placement

across the entire completion interval. It is also claimed that lower tell-tales allow a positive indication (lower

screen out) when gravel slurry covers the lower screen. However, in practice it is felt that this is not the case.

Although lower tell-tales are in general not recommended they have found applications in specific areas of well

completion design. For example, several Opcos have used graded salt (NaCl) mud systems as an under-

reaming fluid prior to carrying out an EGP. In such cases the incorporation of a lower tell-tale has proven useful

in displacing the wellbore (to remove the filtercake) with an under saturated brine prior to placing gravel

conventionally.

Furthermore, when designing long highly deviated wells, the use of a lower tell-tale has assisted in efficiently

transporting gravel to the bottom of the completion interval.

In summary, the use of lower tell-tales is one of several areas where there appears to be a lack of consensus.

1.6.5.2 Upper tell-tales (UTT)

Upper tell-tales are used exclusively when using low gravel mix ratios. They are positioned immediately below

the gravel pack port with at least one blank joint between the tell-tale and top screen. The upper tell-tale allows

to maintain circulation after the main screen is packed-off. Gravel is thus separated against the screen and

settles along the blank joint to constitute a reserve volume. This approach is cheaper than to increase the gravel

reserve by installing extra joints of wire wrapped screens.

In highly deviated wells (> 60 degrees) the upper telltale is of no use as gravity no longer assists in filling the

annulus above the main screen.

Upper tell-tales should not be used with slurry packs (high gravel concentration, high viscosity slurries) where

enough gravel reserve is provided by settling of the slurry between the gravel pack port and the top of screen

when screen out occurs. The high gravel concentration obviates the need for a tell-tale above the screen. An

upper tell-tale in conjunction with high viscosity slurries may cause bridging because of layering of gravel on the

screen.

1.6.6 Miscellaneous equipment

1.6.6.1 Gravel pack bases

In cased hole, permanent type gravel pack bases are generally preferred as they provide a reliable fixed

platform. Either bridge plugs or sump packers can be used. The advantage of a sump packer is that clean-out

trips can be avoided and the possibility for future logging below the pack is maintained. Bridge plugs are

however generally cheaper.

In multiple completions, the packer of the lower zone can constitute the base for the next zone. In open hole, a

cement plug generally serves as the gravel pack base.

1.6.6.2 Shear safety joint

It is common practice to run a shear safety joint between the gravel pack packer assembly and the blank liner to

facilitate fishing operations. The shear rating of the safety joint should be sufficient to withstand the loads

associated with installation of the pack.

1.6.6.3 Gravel pack port

The gravel pack port provides communication between the work string and the annulus below the packer. It is

part of the gravel pack extension and has to be closed or sealed after placement of the gravel. Different systems

are available:

·Gravel pack extension with sliding sleeve: The housing of the sliding sleeve contains the gravel pack port. The

sleeve is activated by a shifting tool.

·Perforated gravel pack extension: This consists of a perforated extension with a seal bore sub. The length of

the perforated extension is controlled to ensure proper crossover tool positioning and travel. The gravel packing

ports are blanked-off by the production string which seals in the packer and the seal bore sub.

1.6.6.4 Fluid loss control devices

Several mechanical fluid loss control devices can be incorporated in the gravel pack liner assembly or the work

string. These devices minimise fluid loss to the formation prior to and after gravel packing the well.

a. Low bottom hole pressure ball

A low bottom hole pressure ball can be added to the crossover tool and serves as a check valve to prevent the

loss of fluid to the formation while the crossover tool is in the reverse circulation position. When a crossover tool

is in the reverse position, the fluid pumped down the annulus can flow through the bypass ports, down the

washpipe and into the formation. The ball is run in place inside the seal subs below the gravel pack port of the

crossover tool.

b. Knock-out isolation valve

The knock-out isolation valve is a flapper valve (often ceramic) that is located between the gravel pack extension

and the screen and allows isolation of the gravel packed interval once the job is complete. It is used for wells

which experience severe fluid loss.

During gravel packing the valve is held open by the washpipe. When the washpipe is pulled the valve closes

isolating the formation from wellbore fluids. This valve can subsequently be shattered by applying pressure or

with the tail pipe of the production tubing.

c. Developments in surface blending and pumping equipment

The beginning of the 1990's saw many operators return to the technique of conventional water packing. This in

turn has led to the development and introduction of alternative blending and pumping equipment.

Clearly a detailed description of all the available blending and pumping systems is outwith the scope of this

manual. However, a brief review of commonly used equipment which is marketed by several large sand control

service companies is given. For more details readers are advised to consult their local service company

representatives.

Infuser sand mixing pump - During 1989 Baker Sand Control introduced their gravel pack infuser design to

facilitate consistent and efficient blending of low concentration slurries, with the additional benefit of allowing the

variation of pump rate and gravel concentration. The infuser consists of an auger pump having inlets for gravel

and carrier fluids, and an outlet to a triplex gravel-pack pump driven by a variable speed motor. Observations to

date have been positive with a number of operators reporting a number of benefits, including accurate and

consistent gravel concentrations a higher rates.

CLAM system - Otis Sand Control offer a different approach for preparing conventional water slurries. The

CLAM (Constant Level Additive Mixer) system which has evolved from conventional Halliburton hydraulic

fracturing equipment eliminates the necessity to batch mix slurries. This system is capable of continually mixing

uniform slurries (0-22 lb/gal) on the fly. Otis claim that the desired slurry can be controlled to within ±0.1 lb/gal.

The basic components of the system are the CLAM blender, a high rate centrifugal pump and a high rate

pressure pump.

WASP system - The WASP (Water And Sand Proportioner) is a relatively new blender specifically designed by

Dowell Schlumberger to proportion and blend gravel and low viscosity carrier fluids. Controlled and continuous

blending with low viscosity fluids at concentrations up to 9 lb/gal is achievable.

Other comparable systems include BJ's Service Cyclone Blender and Dowell Schlumberger's POD blender.

1.7 Fluid considerations

The main requirements for a gravel packing fluid are:

·To provide a safe overbalance against reservoir pressures.

·To minimise permeability damage by particle plugging, clay swelling and migration, or excessive fluid loss.

·Transport gravel downhole behind the screen and into the perforations in the case of an IGP.

1.7.1 Base brines

Base brines used for gravel packing operations should provide the required formation overbalance and be

compatible with the reservoir rock and fluids. Unless higher brine densities are required, freshwater with 1-3%

KCl or sea water are the most commonly used base brines for gravel packing applications.

The addition of potassium chloride to freshwater will inhibit interaction with water sensitive clays present in most

formations. Clay de-stabilisation does not occur or can be neglected in brines with total salinities in excess of 50

kg/m3. Sea water which is compatible with most reservoirs is the obvious choice for offshore operations.

1.7.2 Fluid compatibility

When designing gravel packs for the first time in a field, the following tests are recommended:

·Core flow tests with the same fluids that will contact the formation to determine potentially damaging effects on

the reservoir rock and to design remedial treatments.

·Compatibility tests with the reservoir fluids such as emulsion and precipitation tests.

Most sandstone reservoirs contain some clay minerals. The types of clay present, the concentration and the

location of the clay minerals should be known. Small formation samples (drilling cuttings, side wall samples) can

be used to establish the clay mineralogy.

All these tests are fairly simple and can be done by most reputable service companies.

1.7.3 Fluid filtration

Reservoir permeability impairment due to dirty fluids can be extremely severe. It is very important to filter all

completion fluids that may contact the producing formation. Additionally, all viscosified brines used for

underreaming, fluid loss control or gravel packing operations should be sheared and filtered to remove

unhydrated polymer lumps ("fisheyes") . WORKING CLEAN IS AN ABSOLUTE PRE-REQUISITE TO ACHIEVE

LOW IMPAIRMENT.

1.7.3.1 Filtration requirements

The level of filtration depends on the formation plugging potential, practical limitations of the filtration equipment

and cost considerations. It is generally accepted that there is a relationship between particle size that is

damaging and pore throat size. As a rule of thumb [939]:

·Particles smaller than 1/7of the average pore throat size can freely pass through the pores and will not affect

the permeability.

·Particles larger than 1/3of the average pore throat size will be filtered off on the formation face.

Ideally, the size of the particles in the completion fluid should be small enough to freely flow through the

formation without plugging the pore openings. Hence the filtering criterion should be to remove all particles

larger than 1/7of the average pore throat size.

The average size of the pore throat openings of a particular formation can be measured directly from core

samples using a scanning electron microscope or can be estimated using various correlations. A rough estimate

of the average pore throat size is given by the Kozeny equation.

The concept presented above can be used to estimate filtration requirements for each well. However this rule is

not an appropriate guideline as such. Other parameters to consider are the total suspended solids, the maximum

particle size at a given solids content and the treatment cost.

Recent work has shown that particles down to 1/14th of the average pore throat size can still create appreciable

impairment at low fluid velocities. For such applications the filtering criterion should be aimed at removing all

particles larger than 1/14th of the average pore throat size.

Many companies have found it possible to filter completion fluids to 2mm absolute without difficulty. This should

be adequate for most situations.

Through cleaning of the well and surface equipment prior to gravel packing, and use of gravel with minimal fines

content [989] and the application of appropriate perforation cleaning is essential to help minimise impairment.

Therefore, a maximum degree of cleaning should be aimed for.

1.7.3.2 Filtration equipment

There are different types of filtration systems available for oil field use namely cartridge type, bag type or

diatomeceous earth (DE) filters. High speed disk stack centrifuges can also be used to remove solids from

brines.

In general the selection of a filtration process will depend on many factors such as quality of influent brine,

required level of filtration, location, flow rate, volume of brine to be filtered and cost aspects.

DE filters are the most cost effective devices for filtering large brine volumes. The DE filters require a cartridge

filter downstream to filter out any DE that bleeds through the filter and to "polish" the completion fluid prior to

pumping it downhole. Viscosified brines should always be filtered to remove all traces of unhydrated polymers.

1.7.3.3 Quality control

Well site quality control of the fluid cleanliness can be difficult. The different methods available to monitor the

solids content of a filtered brine are:

·Gravimetric analysis: a given fluid volume is passed through a fine pre-weighed filter. The filter is dried and re-

weighed to measure the Total Suspended Solids (TSS) concentration.

·Particle size analysers: the most common system available is the Coulter Counter. This equipment performs

well but is delicate, has to be operated by expert personnel and is a non-continuous sensor.

·On-line particle size analysers based on laser optical systems have recently been introduced and allow to

monitor, in real time, the residual solids content as a function of particle size.

·Turbidity metering: turbidity can be defined as the property of a liquid that causes it to scatter or absorb light.

This is usually caused by fine particles suspended in the liquid. Nephelometric Turbidity Units (NTU) are typically

used to measure turbidity. Nephelometry is the technique of beaming light on a sample and measuring the

amount of light scattered at a certain angle, usually 90°. The correlation between NTU readings and solids

content is a function of the particle size distribution. Hence NTU readings will only give an accurate indication of

solids content when the particle size distribution is known and a proper calibration has been carried out.

Calibration of turbidimeters is based on Formazin standard suspensions. Formazin is a polymer of uniform

particle size and shape and is a reliable turbidity indicator when prepared properly. Turbidimeters are supplied

with a reference turbidity standard. Turbidimeters are perfectly adequate for field applications and it is possible

to monitor solids content continuously.

These fluid quality control methods are detailed in the Workover and Completions Fluid Manual

Completion&sol;work-over fluid manual. A solids content specification of less than 10 mg/l of particles larger

than 2 microns is currently the best that can be technically and operationally achieved under field conditions.

This is generally considered to be adequate for completion operations even under conditions of moderate

losses.

When circulating a well clean, a realistic solids content target can be defined by monitoring the fluid returns

cleanliness using one of the methods above. The irreducible minimum can be taken as the solids content value

which can only be marginally improved over lengthy circulating times. In most cases, 20 NTU is a good

yardstick.

1.7.4 Carrier fluid rheology

1.7.4.1 General requirements

Various viscous fluid systems can be used for gravel packing purposes e.g. viscosified oils or water based fluids.

A brine viscosified with a soluble polymer is the most commonly used system as it offers many claimed

advantages over water:

1.High gravel transport capacity.

2.Control over fluid leak-off rate through viscosity variations.

3.Control over total leak-off through gravel loading.

4.Reduces the tendency to erode equipment and tubulars.

5.Leads to less intermixing of gravel and formation sand.

6.Internal chemical breakers can be added to break the viscosity after placement.

7.Less rig time, hence less costly.

8.Better diversion over completion interval.

Soluble polymers such as HEC, XC or Shellflo-S can be used to viscosify brines for gravel packing purposes.

These polymer solutions exhibit a marked non-Newtonian shear thinning character i.e. the apparent viscosity

decreases with increased shear rates all other conditions remaining constant. This is a desirable property for a

carrier fluid as it gives good gravel transport properties and minimises the pressure losses when the slurry is

pumped downhole. The rheology of these solutions follows a power-law model above a certain, generally very

low, shear rate. For a power-law fluid, the shear stress t is related to the shear rate g.

Viscosified polymer solutions are viscoelastic, that is the viscosity is dependant on the instantaneous shear rate

as well as the shear rate history. Viscous polymer solutions are also thermally degradable. Each polymer type

shows its own thermal stability characteristics as shown in the following sections.

The carrier fluid rheology determines three basic properties:

·The gravel transport capacity.

·For an IGP, the fluid leak-off into the formation.

·The pressure gradients within the dynamic hydraulic system.

The relative importance of each of these properties will depend on the specific well conditions. In an IGP for

example, the perforation packing process is chiefly governed by the leak-off properties of the carrier fluid.

Conversely fluid leak-off is not essential to obtain a good gravel pack in open-hole. Gravel packing is a

complicated process which is governed by many different factors. Fluid rheology is only one of the variables that

has to be properly controlled to ensure good gravel pack placement. The optimal rheology of a carrier fluid for a

given set of wellbore conditions is inherently difficult to define.

a. Gravel transport capacity

The gravel settling rate is a measure of the ability of the carrier fluid to transport gravel. Correlations for particle

settling velocities in non-Newtonian fluids are generally modifications of Stokes law (see Section 4). Variables

that play a role are the density difference between gravel and fluid, particle size, gravel concentration and

apparent viscosity which in this case is a function of shear rate, temperature and breaker activity.

Gravel settling in a viscous carrier fluid occurs at very low shear rates which can be estimated by dividing the

particle settling rate by its diameter. However carrier fluid viscosities at very low shear rates are of no value in

measuring particle settling rates under dynamic conditions experienced in gravel pack jobs. Because gravel

packing fluids are highly shear thinning, the apparent viscosity determined at a shear rate equivalent to that

being imposed by fluid motion during placement is more realistic. Viscosity determinations at low shear rates are

of value in determining the gravel settling rates after pumping has stopped and the slurry is in a static state.

b. Fluid leak-off

With viscous polymer solutions, fluid leak-off is controlled by two factors:

·The resistance to flow due to invasion of the near wellbore formation by a shear thinning, viscous fluid.

·The impairment caused by a gel layer build-up on the formation face.

The main controlling variables are the fluid properties at downhole conditions, the formation permeability and the

overbalance exerted on the formation.

As the fluid flows out radially from the wellbore, the velocity and hence the shear rate decreases. Given the

shear-thinning properties of polymer solutions, a decrease in shear rate results in an increase in fluid viscosity.

According to Darcy's law, for a given pressure differential an increase in viscosity decreases the flow rate.

One of the problems of modelling fluid leak-off during gravel pack operations is that the fluid viscosity is a

function of the shear and temperature history and also of the breaker activity. When using strong breakers such

as HCl with HEC solutions, the viscosity may be severely degraded before the carrier fluid even reaches the

formation.

Another uncertainty is the surface build-up of gel. Laboratory tests have shown that significant pressure drops

can be caused by such surface deposits. This effect can be minimised by adhering to strict fluid quality control

procedures, i.e. to shear and filter viscous gels. Carrier fluid leak-off properties have rarely been reported in the

literature.

c. Friction pressure losses

When pumping the fluids the friction pressure drops will be determined by the velocities and shear rates

encountered in the different parts of the hydraulic system.

For gravel packing operations, the slurry viscosity is a function of the carrier fluid viscosity and the gravel

concentration.

1.7.4.2 Viscosifiers

a. HEC polymer

Hydroxy-Ethyl-Cellulose (HEC) is the most widely used polymer in the gravel packing industry and it is currently

the standard against which other polymer viscosifiers are judged. It is beyond the scope of the manual to discuss

the many different HEC brands that are commercially available.

The apparent viscosity of the static fluid is quite high but rapidly decreases with shear rates of only a few sec-1.

At high shear rates, the data fall on straight lines and the solutions follow the "power-law" model.

The main advantages of HEC are that it is non-ionic, which implies that its solubility is not affected by salts and

that it is compatible with most chemical additives, and its viscosity can easily be broken with a variety of

chemical breakers. Very little residue is claimed to be created when the polymer is decomposed by breakers.

Commonly used breakers for HEC solutions are acids (HCl), oxidising agents (sodium persulphate) or enzymes.

Enzymes can be used in wells with bottom-hole temperatures below 50°C.

HEC solutions will also loose their viscosity under prolonged exposure to elevated temperatures, typically a few

days at a temperature of 80°C. Laboratory tests have shown however that flocs consisting of degraded polymer

products may be formed which result in severe formation damage.

b. XC polymer

XC polymer is a water soluble, polysaccharide gum produced by microbial fermentation. Some operators and

service companies have adopted a clarified XC polymer (commercial name Xanvis) as an alternative to HEC.

XC solutions are more shear thinning than HEC solutions. Depending on the relative polymer concentrations, XC

solutions can have a higher apparent viscosity at low shear rates and a lower viscosity at high shear rates than

HEC solutions.

A higher viscosity at low shear rates gives better gravel suspension capacities and lower viscosity at high shear

rates gives lower frictional pressure drops, hence lower pumping pressures.

In principle, because of its pronounced shear thinning character, better leak-off properties can be achieved while

still maintaining good gravel suspension properties. This needs careful selection of the polymer concentration as

a function of the specific borehole conditions. However these requirements have not yet been clearly defined.

Field experience indicates that more gravel can be placed in the perforations when using XC instead of HEC

polymer.

The disadvantages of XC polymer are that:

·It is more expensive than HEC.

·It is more difficult to mix properly.

·It was found to cause more permeability damage in cores than HEC.

·Available breakers are less effective and reliable than is the case with HEC.

c. Shellflo-S

Shellflo-S is a biopolymer viscosifier produced by microbial fermentation and marketed by Shell International

Chemical Company. It is commercialised as a stabilised liquid concentrate. Unlike powdered viscosifiers,

Shellflo-S has a viscosity building rate relatively insensitive to brine salinity, pH and temperature. Furthermore,

provided that sufficient shear mixing is applied, Shellflo-S will disperse more rapidly than powdered polymer.

Like XC polymer solutions, Shellflo-S solutions show a more pronounced shear thinning character than HEC

solutions. Also, by careful selection of the polymer concentration, better leak-off properties can be achieved

while still maintaining good gravel suspension properties. Additional research however is still required to define

the optimal rheology for given wellbore conditions.

Shellflo-S has a unique viscosity/temperature profile. Solutions show only a small decrease in viscosity with

increasing temperature until a sudden loss of viscosity occurs at a critical transition temperature as a result of a

change in molecular structure. Above the transition temperature, the viscosity of Shellflo-S declines to a few cP.

By tailoring the transition temperature to specific downhole conditions, the use of external chemical breakers can

be avoided. This is seen as a major advantage due to the general operational problems and unreliability of

chemical breakers. Hence a Shellflo-S solution with a transition temperature somewhat less than the reservoir

temperature is ideal for gravel packing operations. During gravel packing, the solution temperature is lower than

the static bottom hole temperature. Eventually the solution will reach the reservoir temperature and the viscosity

will drop to several cP. Shellflo-S can then be back-produced with little or no formation impairment.

Shellflo-S's viscosity is not affected by temperatures below the Tm, contrary to HEC which exhibits rapidly

decreasing viscosity with increasing temperature. Hence, in contrast to HEC, Shellflo-S maintains a relatively

constant viscosity as it travels down the well (assuming it experiences a uniform shear regime), as long as its

temperature is below the Tm.

Shellflo-S shows promise for gravel packing applications due to its unique rheological properties and good

temperature degradability. Field trials have confirmed this. A disadvantage of Shellflo-S is that it is more

expensive than HEC, although to a large extent this is offset by its ease of handling and high filterability.

d. Permpac

Dowell-Schlumberger introduced a micellar viscosifying system for gravel packing applications. The fluid is

prepared by mixing a surfactant with brine. The surfactant molecules align with water molecules to form micelles

which gives viscosity to the brine. Dowell Schlumberger claim superior gravel carrying capacity, leak-off

characteristics and absence of fisheyes or microgels. Permpac is also claimed to loose its viscosity when

exposed to temperatures in excess of some 50°C.

Permpac has been evaluated by the production chemistry laboratory. Although the carrying capacity and

filterability properties of Permpac were confirmed, both operators experienced mixing problems during the

preparation of the fluid and found differing viscosity breakback characteristics.

1.7.4.3 Recommended viscosifier concentration

The carrier fluid viscosity must be high enough to transport gravel under surface and downhole conditions but, in

the case of an IGP, must be low enough to allow sufficient leak-off rates to pack the perforations. Downhole

carrier fluid viscosity requirements especially are not well defined. One of the problems is the difficulty to predict

the downhole properties of a polymer solution as a function of breaker activity, shear rate and temperature

history, although this is less of a problem with Shellflo-S.

Research is ongoing to define the optimal carrier fluid rheology for a given set of wellbore conditions.

High gel viscosities are currently specified by the industry to provide adequate gravel carrying capacity on

surface. However much lower viscosities can probably be tolerated downhole or may be beneficial for the gravel

placement process.

Generally polymer concentrations are specified as a function of the slurry gravel mix ratio selected for the

particular job. When HEC is used as a viscosifier, the following polymer concentration is recommended:

1.7.4.4 Breakers

a. Viscosity breakback criterion

Chemical breakers need to be incorporated in brines viscosified with HEC or XC polymer solutions in order to

effectively breakdown the viscosity and prevent formation impairment. Chemical breakers act by acidic

hydrolysis or oxidative breakdown of the polymer chain. The proper type and concentration of breaker for a

gelled brine must be determined by laboratory experiments simulating the temperature profile of the fluid during

the gravel packing operation and by specifying a minimum reduction of initial fluid viscosity within a pre-

determined period. The apparent viscosity breakback time is a function of the breaker type and concentration,

temperature and shear rate at which it is measured. It is essential to establish breaker dosage requirements

prior to each job as breaker performance is dependant on the shelf life of the products and possibly varies

significantly from batch to batch.

Viscosity breakback criteria are not properly defined in the industry. Based on field experience, the

recommended criterion is that the apparent viscosity should be broken to 10% of its original value one hour after

the slurry is placed. Adequate conditions are thus given for the gravel pack to settle and loss of screen out can

be detected immediately after the gravel pack job. Extra gravel can then be placed if necessary.

In practice, viscosified brines will begin losing viscosity immediately after the breaker is added, and continue to

slowly break with time and temperature. This is a complicating factor when trying to mathematically model a

gravel packing process.

With Shellflo-S the use of external chemical breakers can be avoided by tailoring the transition temperature to

the specific downhole conditions. This is seen as a major advantage due to the general operational problems

and unreliability of chemical breakers.

b. Breakers for HEC solutions

The viscosity of an HEC solution can be broken by acids, oxydants or enzymes. An acid breaker is preferred

because control of the breakback time with acid is less sensitive to temperature and concentration than with

other type of breakers. In addition, powdered agents such as persulphates and enzymes may suffer from a

limited shelf life resulting in reduced breaking activity.

The following guidelines should be observed for the selection of a breaker:

·Acid breakers should be preferred to other types of breakers e.g. oxidative breakers and enzyme breakers. Acid

breakers should not be used when gravel packing carbonate rich formations, when using resin coated gravel

systems, and in the case of low reservoir temperatures.

·If an acid breaker is acceptable, HCl breaker of any acid strength can be used. Alternative acid breakers to

consider, in order of decreasing strength are 1M formic acid + 0.1MHCl, 1M formic acid, and 1M acetic acid.

These breakers cover a wide range of applications.

·Enzymes can be used below 50°C and have an optimum activity at 30°C. Further advice can be obtained from

SICC London.

c. Breakers for Shellflo-S

The main advantage of Shellflo-S is that the use of external breakers can be avoided by adjusting the transition

temperature somewhat below the static bottom hole temperature. This transition temperature (Tm) can be

adjusted from 40 to over 100°C by changing the type and concentration of salts added to the solution.

Laboratory experiments backed up by a field trial have shown that it is possible to reduce the Tm by addition of

urea. Starting with a base brine of 2% KCl (Tm = 65°C) each additional 10% w/v of urea reduces the Tm about

5°C. The viscosity of the solution is also reduced almost linearly: each additional 10% w/v of urea reduces the

viscosity of the solution by some 10%. At concentrations beyond 45% w/v this behaviour becomes non-linear,

though still usable. Because of the reduction in viscosity on addition of urea an increased concentration of Shell

Flo-S should be used to ensure the viscosity requirements are met.

1.7.4.5 Formation damage caused by viscous polymer solutions

When first introduced in the oil field, HEC gels were assumed to be totally non-damaging to most reservoirs and

this may be true under laboratory conditions. However, under field conditions, great care is required when

preparing HEC solutions to avoid the formation of small accumulations of partially hydrated polymer which can

have a very broad size range, from microns to centimetres. These microgels or "fisheyes" are almost impossible

to remove (acidise) and will result in permanent formation and pack impairment. It is therefore recommended to

shear and filter all polymer viscosified brines be sheared and filtered in order to remove any particulate matter.

The utmost care needs to be exerted when preparing viscous polymer solutions. Field experience suggests that

exposure of the formation to HEC or other polymer solutions can cause permeability impairment even when they

are adequately mixed, sheared, filtered and include the proper breaker system. Damage caused by HEC can at

best be partially removed. Residues left over from broken polymer are difficult to acidise whilst fisheyes or

microgels are unlikely to be removed by acids or enzymes.

The presence of ferric ions in the wellbore may lead to cross linking of Shellflo-S, giving a sawdust like

precipitate. To avoid this serious problem either sequestering agents, such as citric acid or iron reducing agents,

such as erythorbic acid should be applied.

1.8 Slurry design

This section covers the design of a gravel pack slurry and the following aspects are addressed:

·Slurry composition, i.e. gravel concentration and rheology.

·Slurry volumes.

·Spacer or pad composition and volumes.

The slurry design ultimately depends on the type of gravel pack (IGP or EGP) and the specific downhole

conditions.

1.8.1 Slurry types

1.8.1.1 Non-viscosified carrier fluids (conventional packing)

Over the last two decades the industry has seen a trend towards viscous slurry packing (for IGP's mainly) for a

variety reasons including:

1.Quicker operations, hence less exposure to losses.

2.Better placement into perforation tunnels.

3.Better gravel transport and tighter higher quality packs.

4.Reduce fluid losses.

However recent success with gravel packing high angle intervals has led to the re-surgence of low gravel

concentration, low viscosity gravel packing.

Water or non-viscous brines were the first carrier fluids used for gravel packing, hence the name conventional

water packing. Until recently, the gravel concentration was limited to about 2 lb/gal maximum, but in practice

often limited to about 0.5 lb/gal. This generally required only the use of a gravel pot and injection pump, where

gravel is essentially added and pumped on the fly. Such a set up provides little control over gravel placement

and limited the average gravel concentration. For similar reasons non-viscosified brines are not suited for

transporting gravel into perforation tunnels and hence are normally, only considered for open hole applications,

or packing the annulus after perforation prepacking operations.

a. Recent developments

The increasing application of water based, low viscosity systems has been assisted by:

1.The ability to filter completion brines typically using a combination of diatomaceous earth (DE) in combination

with absolute cartridge filtration systems, and

2.The development of superior equipment for blending and pumping low concentration slurries.

Conventional water packing methods are in various areas, proving to be cost effective alternatives to gravel

packing open (and cased) hole completion intervals:

·Gravel packing low permeability intervals,

·Highly deviated and horizontal wells,

and routinely to carry out conventional "top ups" when i.e. screen-out cannot be re-confirmed.

1.8.1.2 Viscosified carrier fluids (slurry packing)

The concept of "slurry packing" was originally developed by Sparlin in the early 1970s to avoid the permeability

reduction associated with formation sand/gravel mixing. A viscous oil-based fluid was used to transport high

gravel concentrations (up to 15 lb gravel per gallon of fluid) and pump the slurry through the perforations at slow

rates. The "Aquapack" system which uses a brine viscosified with HEC as a carrier fluid was designed by Shell

Oil also in the early seventies.

The use of a viscous, water based carrier fluid with high gravel concentrations has become the most popular

gravel packing method in the industry and the name "slurry packing" was adopted to designate this technique.

The most popular fluid system is brine viscosified with a HEC polymer. Alternative viscosifier systems such as

XC polymers and Shellflo-S are being evaluated and field tested.

Slurry packing was claimed to offer the following advantages over conventional packing:

·The gravel is less likely to mix with formation sand in the perforation tunnels due to the viscosity of the fluid and

the lower transport velocities. As discussed previously, gravel mixing with formation sand can be a major source

of impairment.

·More effective placement of gravel into the perforations.

·Reduced fluid loss to the formation and hence less risk of formation damage.

·The gravel is less susceptible to damage during pumping (crushing in pumps).

·Easier handling of the slurry on surface.

·Jobs are shorter because of the smaller fluid volumes to be pumped.

However field experience, has proven that the slurry packing technique has not exactly lived up to all its

promises.

1.8.2 Guidelines for slurry design

Brine based viscosified carrier fluids have seen wide application for over 15 years. The main parameters for the

slurry design are the gravel concentration and viscosifying polymer concentration.

In the past high gravel concentrations (15 lb/gal or 1800 kg/m3) together with high polymer concentrations (80

lb/Mgal or 9.6 kg/m3 HEC) was the standard slurry design used by many operators. This slurry is extremely

heavy and viscous and in the case of IGPs, is likely to result in poor perforation packing because of restricted

fluid leak-off and poor control of the packing operations.

The recommended standard formulation is an intermediate viscosity carrier fluid and gravel loading i.e. 65

lb/Mgal (7.8 kg/m3) HEC and 10-12 lb/gal (1200-1440 kg/m3) gravel mix ratio. Beside being less heavy and

viscous, this slurry formulation presents less impairment potential due to the lower polymer concentrations.

Field experience and full scale gravel pack models have shown that this standard slurry design must be further

tailored to the specific wellbore conditions. This is particularly true for "difficult" applications i.e. when any of the

following conditions are met:

·Highly deviated zones (> 50-60 degrees)

·Long intervals (> 50 feet)

·Zones with a high permeability contrast

Studies have shown that satisfactory gravel transport and improved packing can be achieved in difficult wells

with lower gravel concentrations and lower carrier viscosity fluids.

Gravel concentration recommendations for the different types of gravel packs are given in Section 2: Internal

Gravel Packing and Section 3: External Gravel Packing.

1.8.3 Slurry properties

1.8.3.1 Slurry density

The gravel loading has a significant impact on the slurry density. This parameter governs the overbalance on the

formation when pumping the slurry. High gravel concentrations may lead to a large overbalance and loss of

circulation which may lead to poor pack placement.

1.8.3.2 Slurry viscosity

It is very difficult to predict the viscosity of HEC based slurries as it is a function of fluid composition, flow

characteristics, temperature, gravel concentration and breaker activity. This is a major problem when trying to

model the gravel packing process.

For Shellflo-S based slurries the temperature and breaker activity do not substantially effect the viscosity until

the gravel is placed and modelling is much simpler leading to better job control.

Flow loop experiments have shown that the slurry viscosity can be significantly higher than the original carrier

fluid viscosity, especially for gravel concentrations higher than 10 lb/gal. Experimental correlations describing the

effect of gravel concentration on slurry viscosity are discussed in the well stimulation manual.

1.8.4 Spacer or pad design

Viscous spacers or pads are generally pumped ahead of the gravel slurry to prevent roping of the slurry through

the completion brine. This allows for better volumetric control of the placement operations. Another function of

the pad can be to control the injectivity profile of the completion interval, but this aspect is rarely documented by

operators.

Unless used to control the injectivity of the well prior to gravel packing (e.g. to obtain circulation) the pad volume

should be kept to a minimum, i.e. 300 feet of work string capacity. A large viscous prepad may be required in

reservoirs prone to total losses to ensure that controllable partial returns are obtained during gravel placement.

Pads should be non-damaging and the same requirements as for the carrier fluid are applicable. It is

operationally convenient to keep a small batch of the carrier fluid for use as a spacer. The pad should be made

more viscous than the carrier fluid by adding some additional polymer.

1.9 General gravel packing procedures

This section provides examples of recommended general procedures for gravel packing operations. Operators

should tailor these programmes to their specific needs. Procedures

1.9.1 Well preparation before perforating or underreaming

Wellbore cleaning before completion fluids can contact the formation is a critical aspect to minimise formation

damage. Any source of damage that can be avoided should be avoided.

Wellbore cleaning is usually carried out by displacement of the drilling mud in the well by seawater or clean

completion fluid and using a variety of pills in between. The casing should be scraped prior to perforating or

underreaming

1.9.2 Assembling and running the tools

1.9.2.1 Gravel pack liner assembly

·Assembling the gravel pack tools is the contractor's responsibility but he should be given sufficient time to carry

out the job properly.

·All tools should be thoroughly cleaned with e.g. a steam cleaner.

·No painted tools should be allowed to enter the wellbore. Paint flakes can very effectively plug perforations.

·Pipe dope must not be used in permanent connections, i.e. anything that stays in the hole.

1.9.2.2 Work string cleanliness

A critical requirement is to ensure that the work string is thoroughly cleaned prior to gravel packing. The best

way is to clean the string joint by joint with a steam cleaner prior to running in the hole, followed by an acid wash

(pickling). Pickling the work string prior to gravel packing is an efficient method for removing pipe dope, scale,

rust or deposits from inside the work string. Aromatic solvents work best for removing pipe dope but create

safety and environmental problems. The optimum way of pickling a work string consists in using an acid pill in

combination with gravel.

Pipe dope is a perennial problem in gravel pack operations; it is an efficient impairment agent and is very difficult

to remove by remedial methods. Limited application of pipe dope is a difficult operation to control, even when the

rig crew has been provided with only toothbrushes! The requirement for pipe dope can be eliminated by

employing a dedicated work string with special metallic thread coating to prevent galling. Additional rig time is

however required to pick-up a dedicated work string. If pipe dope cannot be avoided, then automated pipe dope

applicators are strongly recommended in order to minimise the total quantity used.

2 Internal gravel packing

2.1 Perforating considerations

Field experience and laboratory studies indicate that perforations are a primary source of well performance

problems, especially in gravel packed completions. Under certain conditions perforation tunnels can cause a

very high pressure drop for produced or injected fluids. In comparison, the pressure drop through the screen and

the gravel sheath in the screen/casing annulus is relatively insignificant.

2.1.1 Pressure drop in a gravel packed perforation tunnel

A gravel packed perforation tunnel can be modelled as a linear flow cell filled with porous material. The exact

geometry of the flow cell is difficult to ascertain as the flow pattern in the perforations and the nearby formation is

a combination of linear and radial flow. The length of the linear flow cell is a matter of judgement but the

minimum length should be the sum of the casing wall and cement sheath thickness and a term related to

formation damage

·Turbulent flow effects can cause a significant pressure drop in addition to the laminar (Darcy) pressure drop

component.

·Formation sand can effectively plug a perforation tunnel.

·Perforation diameter and flow rate are the main controlling variables for the pressure drop.

·Gravel size plays a less significant role due to the high permeability contrast with formation sand.

Although the calculations above consider turbulent flow, they are still conservative because they do not consider

multiphase flow, wettablity and relative permeability effects.

Mixing of gravel and formation sand leads to reduced permeabilities and impaired gravel packs.

2.1.2 Perforation geometry

The four basic perforation design variables have the following general order of importance for cased hole gravel

packs:

·Perforation diameter (provided a threshold penetration is exceeded).

·Effective shot density.

·Penetration depth beyond a damage defined threshold level.

·Gun phasing.

The benefits that can be obtained from a reduction of the flow velocity in a perforation by using large diameter

perforations and high shot densities. Modern perforating guns enable perforations to be made with an entry hole

diameter up to 1.25 inch and a density up to 12 shots per foot. Some guns allow for even higher shot densities.

However the perforation diameter can only be maximised at the expense of penetration depth as there is a limit

to the energy available in a shaped charge. The other constraint is of course the cost of the perforating

operation.

2.1.2.1 Penetration depth

Perforation penetration beyond the damaged zone i.e. the zone invaded and damaged by the drilling or

completion fluid, is of particular importance for productivity. As a rule, deep penetrating charges should be

selected whenever deep formation damage is suspected (indicated by mud losses when drilling through the

reservoir) or when the borehole has been severely washed out. In all other cases, the perforation diameter

should be maximised at the expense of total penetration.

2.1.2.2 Perforation diameter

Whenever possible the perforation diameter should be maximised to reduce the pressure drop in the gravel

packed perforation tunnels when the well is on production. Entry hole sizes up to 1.25 inch can be obtained with

modern perforating guns. When the specific production conditions are known, a pressure drop analysis can

assist in the selection of the optimal entry hole size.

2.1.2.3 Effective shot density

In theory, a non-gravel packed completion with four perforations per foot should give the same productivity as an

open-hole completion. In a cased-hole gravel packed completion however, only a fraction of the perforations

may be open to flow with the rest being impaired, plugged with formation sand or a mix of gravel and sand. To

overcome these effects, the highest possible shot density should be considered. Field experience should

establish whether the maximum perforation density is justified in view of the additional costs involved. In any

case, intervals to be gravel packed should be perforated with a minimum of 6-9 shots per foot.

2.1.2.4 Gun phasing

For normal applications (deviation < 60 degrees), the perforation pattern should cover the complete

circumference of the borehole. This minimises the geometrical skin component and ensures homogeneous

placement of the pack as dehydration of the slurry occurs in all directions around the casing/screen annulus.

In highly deviated and especially in horizontal wells, it is debatable whether perforating should be limited to a

fraction of the lower hemicycle. The main argument in favour of this practice is the difficulty to ensure complete

packing of the annulus and hence to achieve effective sand control. Low side perforation minimises the risk of

gun sticking due to sand ingress in the wellbore when shooting but reduces the chance of tightly packing the

high side of the annulus.

2.1.3 Perforating method

The major concern when perforating prior to gravel packing is to effectively clean the perforation tunnels i.e. to

evacuate all perforation debris and to remove the "crushed" zone. Gravel packing will trap any debris and

damaged reservoir rock which is left behind after perforation cleaning. This results in permanent impairment as it

is very difficult to remove this material.

Perforating guns can either be tubing conveyed or wireline conveyed. The superiority of one or the other method

has not yet been demonstrated in terms of well productivity, especially in a gravel packing context. Hence the

choice of the perforating method should be based on operational and cost considerations in addition to technical

considerations.

2.1.3.1 Tubing Conveyed Perforating (TCP)

Tubing conveyed perforating enables long intervals to be shot with underbalance in one run and to backsurge

and backflow immediately after perforating in one run, thus eliminating the need for subsequent perforation

washing. TCP also enables the inlet flow area to be maximised by using high shot densities (up to 12 spf)

together with large entry hole sizes that minimise pressure drops across gravel filled perforations. TCP also

enables the well to be back flowed and killed through the same work string.

The cleaning efficiency of underbalance perforating depends on the surge profile which governs the rate and

volume of fluid flow through the perforations. The surge profile is controlled by several factors: reservoir

parameters, wellbore geometry, underbalance pressure differential and underbalancing method.

It is recommended that underbalance is created by using air, nitrogen or natural gas instead of a liquid e.g.

diesel. A compressible wellbore system results in a much longer flow period exposed to the underbalance

pressure before equalisation and increases the effectiveness of perforation surging.

2.1.3.2 Wireline perforating

Wireline perforating can be performed with the wellbore fluid over or under balancing the formation pressure.

When perforating with overbalance, there is no mechanism for removing debris generated during the perforating

process. In wells which do not require sand exclusion, much of this material is removed during the early

production life of the well. Prior to gravel packing, perforations need to be cleaned by washing or backsurging.

a. Perforation washing

The objective of perforation washing is to break down and remove the formation between perforations. A cavity

can be created behind the casing which can then be packed with highly permeable gravel. However invasion of

fines generated when washing and fluid losses increase the risk of permeability impairment. Perforation washing

tools rely on opposed swab cups to isolate sets of perforations and establish a circulation path behind the

casing.

The efficiency of the washing operation can be measured by the perforation wash factor which is defined as the

volume of sand removed per unit length of perforated interval. Wash factors measured in the field can vary

considerably, from 0.02 up to 1 to 4 ft3/ft (or 0.002 to 0.4 m3/m).

The main controlling variables for perforation washing are:

·Pump rates: The amount of formation sand removed by perforation washing is largely dictated by the wash

rates that can be achieved. Rates of up to 8-10 bbl/min may be required for washing to be effective. The flow

rate required to breakdown the near wellbore formation and to create a cavity is a function of the specific

wellbore conditions and has to be established locally. If this flowrate cannot be achieved then acidisation should

be considered.

·Fluids: Brines are more effective in washing perforations than viscosified fluids. Fluids should be clean and non-

damaging to the formation.

·Washing pressure: It is important to know the pressure drop through the tool in order to calculate the pressure

to which the formation is exposed.

b. Perforation backsurging

Backsurging is a perforation cleaning method favoured by various operators especially in the Gulf of Mexico.

The method uses a sudden pressure underbalance to move fluids into the wellbore and flush debris from

perforation cavities and tunnels. The potential for plugging is theoretically less than with washing.

The "backsurge" tool consists of a packer and a backsurge chamber isolated by two valves. The tool is operated

by rapidly opening the bottom valve to expose the perforated interval to near atmospheric pressure. Penberthy

has shown in large scale model testing that perforation surging is capable of removing about the same quantity

of formation sand per foot as perforation washing. However perforation surging may not open all the perforations

and the amount of formation material removed per foot is erratic.

The main controlling variables for perforation surging are:

·Chamber volume: The efficiency of the backsurging operation is mainly a function of the chamber volume used.

Typical Operator practices can vary from 1 to 10 ft3 per foot of perforations (0.1 to 1 m3 per meter).

·Pressure differential: Backsurging utilises atmospheric pressure and high pressure differentials can result.

Excessive pressure differentials can collapse perforation tunnels.

2.1.3.3 Recommended perforating method

Underbalance perforating with a tubing conveyed gun is generally the preferred method of preparing a cased

hole for gravel packing as it is often the quickest method (depending on the length of the interval) and

supposedly gives the best results. Research and field studies indicate that underbalanced perforating is more

effective in cleaning perforations than backsurging or washing, but this may be a function of the type of reservoir

rock. Underbalance perforating is also possible with wireline guns.

Perforating overbalance has to be followed by perforation washing or surging. Perforation washing can

potentially remove more formation damage from the wellbore vicinity in an unconsolidated sand than

underbalanced perforating or surging. However washing causes contamination of the completion fluid and may

cause impairment with clays and fines.

Underbalancing at the instant of perforation appears to be the critical difference between underbalanced

perforating and perforation surging. The true merits of each method are still a matter of debate and local

experience is essential for predicting the effects, if any, on productivity. With lack of proven technical superiority

of a particular method, the economics of perforating are an important consideration when selecting a perforation

method. Cost comparisons between different perforating methods should take into account the rig time spent for

washing or surging operations in addition to the time spent for perforating. Tubing conveyed perforating tends to

be more expensive than wireline perforating and washing, unless the intervals happen to be relatively long and

deep.

2.2 Gravel placement considerations

2.2.1 The ideal IGP

The requirements for a successful internal gravel pack are:

·To tightly pack the perforations with gravel. This is the most important requirement as it prevents formation sand

movement and creates a highly permeable flow path for the produced fluids. A perforation tunnel that is not

packed may collapse when the well is on production. This perforation will then be impaired due to the high

pressure drop caused by the presence of formation sand.

·To maintain a clean gravel/sand interface to avoid permeability impairment associated with mixing of gravel and

sand.

·To place a dense homogeneous gravel sheath around the screen in order to prevent backflow of the gravel

placed in the perforations and control formation sand opposite poorly packed perforations.

·To tightly pack the perforations with gravel. This is the most important requirement as it prevents formation sand

movement and creates a highly permeable flow path for the produced fluids. A perforation tunnel that is not

packed may collapse when the well is on production. This perforation will then be impaired due to the high

pressure drop caused by the presence of formation sand.

·To maintain a clean gravel/sand interface to avoid permeability impairment associated with mixing of gravel and

sand.

·To place a dense homogeneous gravel sheath around the screen in order to prevent backflow of the gravel

placed in the perforations and control formation sand opposite poorly packed perforations.

·To ensure that the near wellbore formation remains unimpaired by drilling and gravel packing operations.

Good gravel placement can be ensured by proper control of all of the following variables:

·Mechanical variables i.e. equipment selection: screen size, washpipe dimensions.

·Hydraulic variables i.e. the slurry design: rheology, gravel concentration, viscosity breakback.

·Operational variables i.e. circulating or squeezing, leak-off rate, pump rates and pressures.

All of these variables have to be given the same level of attention!

2.2.2 Fluid flowpaths

The slurry can flow through the annulus and into the perforations. The carrier fluid can leak-off into the formation

or through the screen and flow the washpipe/screen annulus. The flow distribution is governed by the frictional

pressure losses in the dynamic hydraulic system.

2.2.3 Factors affecting gravel placement

2.2.3.1 Factors affecting gravel placement in perforations

Gravel is deposited as the slurry flows into the perforations and dehydrates into the formation. The amount of

gravel placed in the perforations is proportional to the volume of slurry pumped into the perforations. Fluid leak-

off is controlled by the following factors:

·The overbalance on the reservoir pressure which is a function of the slurry hydrostatic head and operational

variables such as the pump rate and the control or choking of fluid returns.

·The formation and perforation zone permeability which can vary significantly over the perforated interval.

·The geometry of the perforation tunnels which governs velocities and frictional pressure drops.

·The carrier fluid rheology. Low viscosity promotes fluid leak-off but a minimum viscosity is required to avoid

gravel drop-out over the perforation tunnel length.

The perforation packing efficiency can be defined as the amount of gravel placed in the perforation per unit

volume of slurry pumped in the perforation and is a function of the following variables.

·The fluid carrying capacity which determines how efficiently gravel is transported "round the bend" into the

perforation tunnel. This is mainly a function of viscosity, the difference between gravel and fluid density and the

gravel grain size.

·The gravel mix ratio.

·The inclination of the perforation tunnel. In highly deviated wells, gravitational forces will impede the placement

of gravel in perforations situated on the high side of the wellbore.

A high perforation packing efficiency is desirable in order to reduce the fluid volume lost in the formation and

thus minimise formation damage. The importance of minimising fluid loss is a function of the impairment

potential of the carrier fluid.

Fluid rheology plays a dual conflicting role (leak-off and carrying capacity) in the perforation packing process. An

optimal rheology must exist for a given set of wellbore conditions to pack the perforations while minimising

impairment. This optimum has not been defined yet and further research is required.

Modelling the gravel placement process is a complex problem because many factors are difficult to predict. The

slurry rheology of HEC and XC polymer solutions under downhole conditions is a function of temperature and

shear rate history and the breaker activity. These problems are much reduced with Shell Flo-S. Similarly, the

effective formation permeability is difficult to determine as it is a function of the perforating process and the

perforation cleaning method used. The flow of a viscous fluid into the formation also influences the leak-off

process.

Tightly packed perforation tunnels are crucial for the productivity of the completion. The main obstacle to

adequate perforation packing may however be the interaction with the annular packing process.

The volume of gravel placed behind casing can thus be considered as a gravel pack quality indicator although in

general the average cavity will not be well characterised. In practice, this volume can only be indirectly

determined. Gravel pack logging tools are inadequate for this purpose and the volumetric balance methods are

generally inaccurate.

2.2.3.2 Factors affecting gravel placement in the annulus

Gravel transport in the annulus is mainly a function of carrier fluid rheology, flow velocities and wellbore

inclination.

Deviation has a major impact on the gravel placement process in the annulus. In vertical and moderately

deviated wells (< 60 degrees), gravity helps in packing the annulus from bottom-up. Full annular gravel

placement is relatively easily achieved. If bridges are formed in the annulus, these are likely to collapse as

pumping stops and the differential pressure is removed. Gravel settling will then prevent formation of voids if

adequate gravel reserve is available. In these wells, fluid rheology and gravel concentration are not critical

parameters. It is generally accepted that high gravel concentrations and high viscosity fluids can substantially

increase the risk of bridging especially in a small annulus.

In highly deviated wells (higher than about 60 degrees) the gravel transport mode changes significantly. Gravel

will fall out of the slurry as it exits the crossover port and will form a bed. The equilibrium bed height is a function

of fluid properties, gravel concentration and fluid velocity. This bed will progress along the wellbore if adequate

circulation rates and flow paths are maintained along the annulus and bridging does not occur. When the end of

the screen is reached, a dune propagating in the opposite direction will fill the annular void on the high side of

the hole. Slurry design and flow parameters are critical factors in these wells and there is a high risk of not

achieving a complete annular pack. Voids formed in the screen/casing annulus below a bridge will subsist after

pumping stops because friction between the gravel grains prevents downwards settling of gravel.

The gravel pack in the annulus can be qualitatively evaluated with the help of logging tools which can detect top

gravel and voids, depending on their size and shape.

2.2.3.3 Interaction between annular and perforation packing

Since we are generally successful in packing the annulus, attention must focus on properly packing the

perforation tunnels. The problem is that the annular packing process interferes with the placement of gravel in

the perforations. In fact it is probably very difficult, if not impossible, to achieve complete packing of all

perforations and the annulus simultaneously using current gravel packing procedures.

If the annulus is allowed to pack too rapidly, then the gravel will pack-off the perforation entrance before the

tunnel is fully packed.

All of these variables have to be given the same level of attention!

2.2.4 Fluid flowpaths

Different fluid flowpaths are available when gravel packing. The slurry can flow through the annulus and into the

perforations. The carrier fluid can leak-off into the formation or through the screen and flow the washpipe/screen

annulus. The flow distribution is governed by the frictional pressure losses in the dynamic hydraulic system.

Lower tell-tales (LTT) are commonly used to force slurry circulation to the bottom of the annulus. Flow must

enter through the LTT screen as the lower end of the washpipe is sealed into a sealbore. There is a more subtle

fluid flowpath that may defeat the effectiveness of a LTT. The carrier fluid can bypass into the washpipe/screen

annulus, re-enter at the bottom of the casing/screen annulus to flow through the LTT. The most practical way to

ensure that slurry flow is forced to the bottom of the annulus is to minimise the screen/washpipe clearance.

2.2.5 Factors affecting gravel placement

2.2.5.1 Factors affecting gravel placement in perforations

Gravel is deposited as the slurry flows into the perforations and dehydrates into the formation. The amount of

gravel placed in the perforations is proportional to the volume of slurry pumped into the perforations. Fluid leak-

off is controlled by the following factors:

·The overbalance on the reservoir pressure which is a function of the slurry hydrostatic head and operational

variables such as the pump rate and the control or choking of fluid returns.

·The formation and perforation zone permeability which can vary significantly over the perforated interval.

·The geometry of the perforation tunnels which governs velocities and frictional pressure drops.

·The carrier fluid rheology. Low viscosity promotes fluid leak-off but a minimum viscosity is required to avoid

gravel drop-out over the perforation tunnel length.

The perforation packing efficiency can be defined as the amount of gravel placed in the perforation per unit

volume of slurry pumped in the perforation and is a function of the following variables:

·The fluid carrying capacity which determines how efficiently gravel is transported "round the bend" into the

perforation tunnel. This is mainly a function of viscosity, the difference between gravel and fluid density and the

gravel grain size.

·The gravel mix ratio.

·The inclination of the perforation tunnel. In highly deviated wells, gravitational forces will impede the placement

of gravel in perforations situated on the high side of the wellbore.

A high perforation packing efficiency is desirable in order to reduce the fluid volume lost in the formation and

thus minimise formation damage. The importance of minimising fluid loss is a function of the impairment

potential of the carrier fluid.

Fluid rheology plays a dual conflicting role (leak-off and carrying capacity) in the perforation packing process. An

optimal rheology must exist for a given set of wellbore conditions to pack the perforations while minimising

impairment. This optimum has not been defined yet and further research is required.

Modelling the gravel placement process is a complex problem because many factors are difficult to predict. The

slurry rheology of HEC and XC polymer solutions under downhole conditions is a function of temperature and

shear rate history and the breaker activity. These problems are much reduced with Shell Flo-S. Similarly, the

effective formation permeability is difficult to determine as it is a function of the perforating process and the

perforation cleaning method used. The flow of a viscous fluid into the formation also influences the leak-off

process.

Tightly packed perforation tunnels are crucial for the productivity of the completion. The main obstacle to

adequate perforation packing may however be the interaction with the annular packing process.

The volume of gravel placed behind casing can thus be considered as a gravel pack quality indicator although in

general the average cavity will not be well characterised. In practice, this volume can only be indirectly

determined. Gravel pack logging tools are inadequate for this purpose and the volumetric balance methods are

generally inaccurate.

2.2.5.2 Factors affecting gravel placement in the annulus

Gravel transport in the annulus is mainly a function of carrier fluid rheology, flow velocities and wellbore

inclination.

Deviation has a major impact on the gravel placement process in the annulus. In vertical and moderately

deviated wells (< 60 degrees), gravity helps in packing the annulus from bottom-up. Full annular gravel

placement is relatively easily achieved. If bridges are formed in the annulus, these are likely to collapse as

pumping stops and the differential pressure is removed. Gravel settling will then prevent formation of voids if

adequate gravel reserve is available. In these wells, fluid rheology and gravel concentration are not critical

parameters. It is generally accepted that high gravel concentrations and high viscosity fluids can substantially

increase the risk of bridging especially in a small annulus.

In highly deviated wells (higher than about 60 degrees) the gravel transport mode changes significantly. Gravel

will fall out of the slurry as it exits the crossover port and will form a bed. The equilibrium bed height is a function

of fluid properties, gravel concentration and fluid velocity. This bed will progress along the wellbore if adequate

circulation rates and flow paths are maintained along the annulus and bridging does not occur. When the end of

the screen is reached, a dune propagating in the opposite direction will fill the annular void on the high side of

the hole. Slurry design and flow parameters are critical factors in these wells and there is a high risk of not

achieving a complete annular pack. Voids formed in the screen/casing annulus below a bridge will subsist after

pumping stops because friction between the gravel grains prevents downwards settling of gravel.

The gravel pack in the annulus can be qualitatively evaluated with the help of logging tools which can detect top

gravel and voids, depending on their size and shape.

2.2.5.3 Interaction between annular and perforation packing

Since we are generally successful in packing the annulus, attention must focus on properly packing the

perforation tunnels. The problem is that the annular packing process interferes with the placement of gravel in

the perforations. In fact it is probably very difficult, if not impossible, to achieve complete packing of all

perforations and the annulus simultaneously using current gravel packing procedures.

If the annulus is allowed to pack too rapidly, then the gravel will pack-off the perforation entrance before the

tunnel is fully packed

The leak-off rate must be high enough to ensure that the perforations are packed before the screen/casing

annulus is filled. Conversely, if excessive leak-off occurs, the gravel may prematurely bridge in the annulus

especially across high permeability streaks. Premature bridging is not always obvious in vertical wells because

the bridge may collapse as soon as pumping ceases. Perforations below the bridge will however remain

unpacked.

The correct fluid leak-off to fluid returns ratio is a matter of debate. Surprisingly this critical topic has never been

properly researched and no specific guidelines can be given. Ideally, all perforations should be fully packed

before gravel in the annulus screens out the perforation entrance. In a vertical well, the annulus normally packs

from bottom-up but the perforation packing order will be a function of the effective leak-off in each perforation

and this is a self diverting process. Progress has been hampered by the lack of reliable logging methods to

measure cavity geometry and gravel volume behind the casing, and the lack of realistic gravel packing numerical

and physical simulators.

An optimal pumping schedule would aim to preferentially fill the perforations in the early stages of the job (while

still maintaining slurry flow to the bottom of the annulus) and complete the annular pack at the end of the job. In

other words, one would aim for a high leak-off rate at the beginning of the job while gradually increasing fluid

returns as the job progresses. In practice it is difficult to effectively control operational variables. When high

gravel/gel mix ratios are used (> 12 lb/gal) the pack may be complete in a few minutes time, leaving little

opportunity to adjust operational variables. Furthermore, significant time lags are inherent to the hydraulic

system. Hence optimal gravel pack placement is only seen possible through automation of the gravel placement

process. This requires a step change in oil-field pumping equipment.

Alternatively, the perforation packing process can be separated from the annular packing process i.e.

prepacking. This can be achieved by squeezing gravel into the perforations prior to running the screen

assembly. Prepacking techniques are considered to offer the best scope for improving perforation packing

efficiency.

2.2.5.4 Effect of interval length and deviation

The maximum length of interval that can be gravel packed will vary with the wellbore conditions. In general, as

the deviation angle increases it becomes more difficult to pack a long interval. Reservoir heterogeneity increases

the difficulty of gravel packing long intervals. Moreover perforation cleaning of a long interval by under balanced

perforating may be ineffective.

Although the amount of gravel placed behind perforations cannot be measured accurately, field experience

suggests that perforation packing effectiveness declines rapidly with interval length. Some authors go so far as

to suggest that perforation packing is very poor for intervals longer than 10 feet, even in vertical wells as shown.

The design of a gravel pack for long intervals, especially when highly deviated, is typically chosen to promote

annular fill at the expense of perforation fill. The most efficient way to obtain good annular packing is to size the

washpipe correctly. The ratio of washpipe OD to screen ID should be at least 0.8 in long and/or highly deviated

wells. However, as the screen/casing annulus packs from bottom-up the carrier fluid is forced to travel down the

washpipe/screen annulus. The pressure required to flow down this annulus increases as the job progresses and

may become excessive in long intervals. Hence a long zone may be required to be broken down in shorter

intervals. There are however no specific guidelines as to what is the maximum allowable length.

As the deviation increases, gravity will prevent filling of voids left in the annulus after the job and it becomes

essential to avoid bridging. Hence annular packing should be promoted at the expense of perforation packing. If

necessary, remedial action should be taken following a gravel pack log.

2.2.6 Gravel placement methods

2.2.6.1 Squeeze pack

In a squeeze pack, the slurry is forced to dehydrate in the perforation tunnels thereby ensuring gravel placement

in cavities behind the casing. This can be achieved either by placing the gravel pack tool in the squeeze position

or by closing-off the returns.

A squeeze pack promotes a radial build-up of the gravel pack inside the screen/casing annulus as gravel nodes

build-up on the packed perforation entrances, with a consequent high risk of bridging. Non uniform injectivity

profiles increase the risk of bridging as the slurry preferentially dehydrates opposite high permeability zones.

Bridges may not be so problematic in vertical wells as they may collapse as soon as pumping stops. However

the perforations below the bridge run a high risk of being poorly packed. In deviated wells and/or long intervals

however, neither the perforations nor the annulus below the bridge will be adequately packed.

Also in a squeeze pack, it is not possible to dehydrate slurry below the lowest perforation. This will create a void

which may be subsequently filled when the pack settles with the risk of leaving voids elsewhere in the pack.

Squeeze packs are generally not recommended and should in any case be restricted to gravel packing very

short intervals i.e. less than 5 meters

2.2.6.2 Circulation-squeeze pack

Lower tell-tales are frequently used to force a self-induced squeeze pack. The crossover tool is left in the lower

circulating position for the entire gravel placement operation.

The pressure drop for returns to flow through the telltale increases dramatically as the annulus packs-off and the

circulating pack effectively becomes a squeeze pack, which is undesirable in most cases. This method, which

does not require any operator intervention during the job, has however been adopted by many operators

because of its operational convenience .

2.2.6.3 Circulation pack

The objective of a circulation pack is to fill the annulus and the perforations from the bottom up. The wash pipe

must be spaced out so that return circulation occurs as close as possible to the bottom of the screen (Fig. 723).

As the gravel pack begins, slurry fills the annular space outside the screen and the slurry starts to dehydrate at

the bottom of the screen opposite the end of the washpipe. As packed gravel accumulates around the bottom of

the screen, a higher pressure drop is required for the fluid to enter the screen and to flow in the screen/washpipe

annulus to the end of the washpipe, thereby inducing fluid leak-off through the perforations. It is important to

correctly size the washpipe OD to enhance this effect.

Fluid leak-off can also be controlled by action on the fluid returns. However, when high gravel concentrations are

used, the pack can be completed in a few minutes and little time is available to effectively control the operations.

More elaborated instrumentation than that currently available on the wellsite today is required to allow effective

control of gravel placement i.e. real time measurement of operational variables like fluid returns. Ideally the

gravel placement process should be completely automated.

2.2.6.4 Perforation prepacking

Conceptually, the ideal cased hole gravel packing method is to separate the perforation and annular packing

process, i.e. to prepack the perforations. Prepacking methods are claimed by other operators to be very effective

at packing perforations, controlling fluid loss and to result in high well productivity. Gravel can be squeezed into

the perforations prior to running the gravel pack liner assembly. This method is to some extent a revival of the

old "washdown" gravel packing technique. If it is necessary to control fluid losses after squeezing gravel behind

the casing the perforations can be sealed with fluid loss control pills.

As previously discussed severe formation damage can occur during the installation of an IGP. This is thought to

be due to combination of losses during completion operations and inadequate tunnel fill. It is well known that

standard gravel packing operations result in a conflicting process of tunnel versus annular fill, and with such a

scenario, preferential leak off can occur and in many instances this will result in the formation of bridges over

certain perforations.

a. Prepacking

Prepacking - the concept of splitting the process of perforation packing from gravel packing the annulus is not

new, and has been carried out routinely by a number of operators.

Flling the perforation tunnels immediately after perforating is thought to reduce the likelihood of tunnel damage

by:

·Minimising the possibility of tunnel/cavity collapse.

·Invasion of incompatible (possibly unclean) fluids.

Furthermore the chance of filling the perforations is increased as the annular packing stage often results in

additional packing of perforations.

Recent work has been in support of an extensive field test programme. Although reports to date indicate

prepacking to be an operational success, no clear gains in productivity have been demonstrated.

Nevertheless prepacking followed by circulation packing is the currently recommended technique for difficult

wells: long highly deviated zones, reservoirs which exhibit high permeability contrasts, and zones where

excessive losses are anticipated.

b. Types of prepacking

The following is a summary of the basic prepacking techniques:

Prepacking without acidising - commonly referred to as the BP technique. This involves perforating the zone

underbalance, pulling guns above the completion interval and packing the perforations with low concentration

slurry. Perforation tunnels are then seal using a LCM system. Thereafter the annulus is conventionally packed. If

required an additional LCM pill is spotted inside the wire wrapped screen prior to pulling the work string.

This techniques has been used extensively by BP in the North Sea where high permeability reservoirs dictate

the use of particulate LCM.

Prepacking while acidising - as above this operation can be carried out before or after running the gravel pack

liner. However in order to minimise losses and the risk of tunnel collapse it is preferred to pack perforations prior

to pulling the perforating guns. Again LCM can be spotted where required.

Uniform treatment is difficult as acid will preferentially flow into the first set of open perforations or into high

permeability zones. Possible diversion techniques include gelled or foamed acids. A relatively new but promising

method is to use viscous pills loaded with gravel to divert acid stages and simultaneously prepack perforations

A typical gravel diverted acidisation procedure is as follows:

·The treatment should be carried out in stages with gelled prepack diverter slurry placed between stages. The

diverter pill containing the gravel required to pack the perforations opened by the preceding stage. Each acid

stage should be over flushed far enough into the formation to reduce the potential for damage due acidisation

side effects.

·The final acid stage should be followed by the gravel pack slurry required to complete the pack.

A typical design practice is to target between 10-20ft of perforations, with gravel concentrations in the range of 1-

2 lb/gal.

Chevron have reported a significant increase in productivity in both oil and gas wells completed with this method

compared to conventional gravel packs.

Auger system - this system involves spotting gravel across the completion interval, and "screwing" a specially

designed screen into the gravel bed. It is claimed that this technique promotes tighter more effective packing of

perforations.

c. Prepacking fluids

Gravel laden water, HEC and Shellflo-S slurries where tested under deviated well conditions at various pump

rates and sand concentrations.

·The use of high viscosity slurries (100 cP) at a shear rate of 100 sec-1) leads to filling of perforations from the

top to the bottom of the interval, node formation and risk of premature bridging, leading to incomplete filling of

perforation at the bottom of the interval. High viscosity slurries are not recommended.

·The use of medium viscosity slurries (40 cP at a shear rate of 100 sec-1) leads to simultaneous filling of

perforations, along the entire interval, node formation, tightly packed perforations, but with a substantial risk of

premature bridging due to a weak diverting effect. With this slurry a pumping rate of 2 bpm and gravel loading of

3 ppg seems ideal.

·The use of low viscosity slurries (20cP of a shear rate of 100 sec-1) leads to filling of perforation from bottom to

top of the interval. Gravel is swept into the perforations from the top of the gravel bed that develops in the

casing. Provided leak off rate is sufficient, tightly packed perforations can be achieved, also for deviated wells.

Because the perforations are treated layer by layer, there is not much risk of premature bridging in the casing.

This option is very attractive and should be tried out in the field.

Based on the above studies the use of water as a prepack carrier fluid is not recommended, mainly because

very high leak off rates are required to fill perforations with gravel. However the study did confirm that water is

ideal for gravel packing the annulus.

2.3 Slurry design for IGPs

2.3.1 Gravel loading and polymer concentration

A gravel concentration of 15 pounds per gallon of fluid is commonly used by many operators as a standard slurry

recipe for IGPs. This probably represents the maximum gravel concentration that can be pumped under field

conditions. In many cases however, the use of lower gravel concentrations together with lower polymer

concentrations may offer clear advantages. For an IGP, the pro's and con's of low, medium or high gravel

concentrations are as follows:

Low gravel and zero polymer concentration (0.2-2 lb/gal gravel)

·Dune formation in highly deviated and horizontal wells leading to dense annular pack.

·Large fluid volume required to pack perforations.

·More intermixing compared to viscosified slurries.

·Minimises bridging in the case of restricted screen-casing clearance.

·High risk of screen plugging in the case of circulation packing.

·Ineffective placement of gravel in upward facing perforations in high deviated wells.

Low gravel and low polymer concentration (1-2 lb/gal gravel)

(< 58 lb/1000 gal HEC, <0.2 w/w % active Shell Flo-S)

·Is essentially an enhanced water slurry to promote better transport of gravel into perforations.

Medium gravel and polymer concentrations (3-6 lb/gal gravel mix ratio)

(58 - 62 lb/1000 gal HEC, 0.2 - 0.4 w/w % active Shellflo-S)

·Generally helps to prevent bridging, especially when the casing/screen annulus clearance is small.

·Compared to high gravel and polymer concentration increases the volume of slurry to be dehydrated in the

perforations. The importance of this aspect is a function of the impairment potential of the fluids used.

·Compared to high gravel and polymer concentration placement times are longer. Although rig time increases,

more time is available to control the gravel placement operations, i.e. to ensure that the adequate balance

between losses and returns is achieved during placement.

·Is beneficial for gravel annular placement in highly deviated wells.

·Should result in better placement over entire interval in the case of prepacking.

High gravel and polymer concentrations (> 7 lb/gal gravel mix ratio):

(> 62 lb/1000 gal HEC, > 0.4 w/w % active Shellflo-S)

·Causes significant increases in slurry viscosity and density. The resulting increase in hydrostatic head and

friction pressure losses may prevent circulation of fluids to surface.

·Increases the risk of bridging especially in long intervals and when a highly heterogeneous formation is being

packed. High fluid viscosity and high polymer concentrations promote layering on the screen surface and the

build-up of nodes at perforation entrance. This may cause bridging especially across high permeability intervals.

·Minimises the fluid volumes to be squeezed and the time required for placement.

·May cause "U-tubing" of the wellbore fluids when pumping the slurry, with consequent loss of volumetric

control.

·Less risk of plugging the screen in circulation pack.

·Diverts fluid over interval for medium permeability contrasts.

Slurries with high gravel and polymer concentrations have generally resulted in gravel packs that effectively

control sand. However when applied indiscriminately, this recipe may contribute to poor gravel placement in the

perforation tunnels and hence restricted well productivity. The inability to circulate or the poor operational control

of gravel placement (it may take a few minutes only) is most likely the cause of poor gravel placement in many

cases. Impairment may also be caused by the high polymer concentrations required to suspend the gravel.

High gravel and polymer concentrations

(i.e. > 12 lb/gal gravel mix ratio) should be avoided in the following cases:

·Non-uniform formation injectivity profile, especially if high permeabilities are encountered at the top of the

interval.

·With low screen/casing clearance (one inch or less).

·Long intervals (> 50 feet) and/or highly deviated (> 60 degrees) wells.

·If no circulation can be achieved or poor control of placement operations is expected.

2.3.2 Recommended placement procedures

Prepacking methods, where the process of perforation tunnel fill is separated from the annulus packing

operation offers the best scope for improving pack factors and hence increased productivity.

Circulation squeeze packs are commonly carried out using a lower tell-tale and leaving the crossover tool in the

lower circulating position. The pack essentially becomes a squeeze pack after the lower tell-tale is covered with

gravel. This method is generally not recommended for intervals longer than 15 feet.

Field experience and experimental investigations allow placement recommendations for specific conditions, for

example:

1.Low angle wells (< 30°) and/or short intervals (< 15 feet)

The circulation pack method is recommended.

2.Intermediate angle wells (30°-60°) and/or long intervals and/or intervals with high permeability contrasts.

Prepacking followed by circulation packing is recommended in this case. Low or medium gravel and polymer

concentrations are recommended for the prepacking phase, low or medium gravel/polymer concentrations are

recommended for the annular pack.

The use of low density gravel is expected to further increase pack factors, but this has still to be tested within the

Group.

3.High angle and horizontal wells (> 60°)

The circulation pack method is the preferred method in this case, where low gravel concentration with water or

brine is applied. Flow experiments in Shell Oil have shown that a minimum fluid velocity in the screen-casing

annulus must be sustained to achieve successful annular packs in such cases.

2.3.3 Gravel volume

The total gravel volume required is the sum of:

·The annular volume from sump packer to top screen,

·The gravel volume expected to be placed behind casing,

·The reserve gravel volume

Because of the uncertainty over the expected gravel volume behind the casing, a safety margin is

recommended, e.g. 10 % of the total volume calculated. Using an excess should not itself cause problems since

circulating out excess slurry can be usually be easily accomplished.

The amount of gravel that can be placed behind casing is a function of the perforation design and the perforation

cleaning method used as well as the formation type. Larger cavities may exist behind the casing when

perforations are washed instead of backsurged. In practice it is difficult to estimate the void volume behind

casing. This can be done by measuring the volume of formation material recovered on surface, if any, during

washing or backflowing operations and/or by measuring the level of fill within the casing before and after these

operations.

The gravel volume effectively placed also depends on the perforation packing efficiency of the gravel placement

procedures used. Hence the gravel volume to be placed behind casing should be derived from local field

experience. When no field experience is available, use a minimum of 0.3 ft3 per foot perforated interval in the

case of a new well.

In the case of wells that have produced significant sand quantities prior to gravel packing, (cumulative sand

production data is rarely reliable) a gravel volume of 1 ft3 per foot perforated interval can be taken as a

guideline. If the slurry volume proves too small, additional batches of slurry can be placed.

2.3.4 Example of gravel volume calculation

2.4 Gravel pack liner assembly

2.4.1 Screen considerations for IGPs

2.4.1.1 Screen size

The following considerations apply for the selection of the screen size:

·A minimum radial clearance of one inch (between the largest screen/pipe OD and the casing ID) is required to

allow for washing over the screen.

·A larger annulus clearance reduces the chance of bridging especially in the case of difficult packing conditions

i.e.: long intervals (> 50 feet) and/or non homogeneous injectivity profile and/or highly deviated zones (> 60

degrees). However too large clearances could affect the placement of gravel in the annulus if flow velocities

become too low, especially in highly deviated wells. In practice a clearance of 1-2 inch is recommended.

·A screen ID larger than the tubing ID offers little advantage.

·For multiple completions, the screen internal diameter is governed by the size of the production tubing required

to reach a lower zone.

2.4.1.2 Screen overlap

A minimum screen overlap over both top and bottom perforations of 5 feet should be used. A larger overlap

provides some flexibility in the screen design as the length can be rounded of to a number of whole joints. The

overlap also dictates a minimum gravel height in the annulus as screen out occurs only when the entire screen is

covered. This may be important to ensure that the entire perforated interval is covered, especially in highly

deviated wells.

Screen overlap is not a critical design consideration and a length of 10 to 20 feet above top perforations is often

used.

2.4.2 Lower tell-tale (LTT)

A lower tell-tale is supposed to give a positive indication of when the slurry reaches the bottom of the annulus

and to enhance pack quality (fewer voids. It is the subject of continuing industry debate as to its best use and

effectiveness. In general however the use of LTTs cannot be recommended for the following reasons:

·The use of LTTs makes the screen assembly and gravel pack tool (slightly) more complex and expensive.

·Discussions with vendors have shown that the use of LTTs gives little proven benefits. There is ample

speculation within the service industry that an LTT improves pack quality but the reason for enhancement is not

entirely clear. In low deviation wells, an LTT is of little use as gravel settling promotes complete annular packing.

In highly deviated or long intervals, gravel pack models have shown that the most efficient method of maintaining

flow in the screen/casing annulus and to ensure good packing is to minimise the washpipe/screen annulus and

to set the washpipe end at the bottom of the screen.

·The LTT is supposed to provide a displacement bench-mark for the gravel packing procedure. However in

many cases there is hardly any surface pressure indication of when the slurry reaches the LTT. If U-tubing has

occured when pumping the slurry, a sudden surface pressure increment may only occur when the displacement

fluid catches up with the slurry, long after the slurry has screened out on the LTT.

·The gravel pack tool should be moved to the upper circulating position after screen out on the LTT to avoid

packing under squeezing conditions. However in many cases this is largely ineffective as it is difficult to detect

the right moment to do so.

·Operators often leave the gravel pack tool in the lower circulating position during the whole packing operation.

Once compacted gravel covers the tell-tale the resistance to flow becomes higher and forces fluid leak-off into

the formation. Gravel packs executed in this manner effectively become squeeze packs, which is undesirable in

most cases.

In summary, lower tell-tales should be generally be avoided as they are costly, increase the mechanical

complexity of the liner assembly and are of little proven help in ensuring good placement. The best method to

ensure good gravel placement is to correctly size the washpipe and closely monitor the progress of the packing

operations.

2.5 Procedures

2.5.1 Killing the well

The manner in which the well is killed after underbalanced perforating and backflowing will affect both the

placement of gravel and the ultimate well productivity. The same level of precautions as taken during gravel

packing operations should be applied when killing the well. There are two methods for killing the well after

backflow operations: bullheading or reverse circulating.

Bullheading is unacceptable as formation and gun debris will be pumped back into the perforations, defeating

the whole object of backsurging. This method is however operationally more convenient as hydrocarbons are

kept from reaching surface.

Specialised downhole accessories (e.g. Halliburton OMNI valve) allow isolation of the formation downhole and

enable the well to be reverse circulated with zero fluid loss to the formation. This avoids squeezing formation or

gun debris back into the perforations. These downhole tools are typically used for Drill Stem Testing (DST) and

more information on these tools can be found in the DST manual, EP 89-0490.

Reverse circulating through a DST valve above the TCP packer will remove hydrocarbons in the work string

above this level. A significant volume of hydrocarbons may however remain trapped below the packer and will

be released when unseating the packer. Bullheading all hydrocarbons below the DST valve should be

considered as it allows:

·Positive prevention of brine contamination and associated problems (well control).

·Fluid loss agents can be placed immediately thus preventing any losses.

·The amount of debris pumped back into the perforations should be limited if the well is closed in for some time

after backflowing to allow debris to settle.

2.5.2 Fluid loss control after perforating

Fluid loss control may become necessary after perforating operations as it is not always possible to avoid

significant losses while maintaining the minimum operationally required overbalance (usually 100 - 200 psi). This

is a function of formation properties, fluid overbalance at the formation face and perforation efficiency.

The maximum acceptable fluid loss rate above which fluid loss control systems are necessary must be

established. If the completion brine is non damaging to the formation the main concern from a well productivity

point of view is that fluid loss control systems are difficult to remove efficiently even by acidisation due to

diversion problems. After an acid treatment, many perforations can effectively remain plugged, resulting in poor

gravel placement and poor well productivity. Hence, if fluids are non-damaging (i.e. filtration is up to scratch), the

limit is the maximum fluid loss rate operationally acceptable and is mainly a well safety issue. With proper

planning and under certain conditions, some operators may allow up to 50 bbl per hour losses to the formation.

Conventional fluid loss control systems can be grouped into two categories: soluble particle systems and viscous

fluid systems. A relatively new method of fluid loss control consists of prepacking the perforation tunnels with

gravel. Field experience should assist in selecting the system which allows to control fluid loss rates and result in

minimum residual impairment.

2.5.2.1 Soluble particle systems

Particulate fluid Loss Control Material (LCM) include acid, oil or water soluble material of selected grain size (for

details refer to the Workover and Completion Fluids Manual. These systems can be very effective in controlling

fluid loss by the build-up of a thin, impermeable layer of particulate matter on the formation face. Subsequent

removal of this layer requires placement of a solubilising fluid i.e. under saturated brine for soluble salts or

hydrochloric acid for calcium carbonate material. Removal efficiency is a function of solubility and contact time

but fluid diversion is the main problem. Incomplete removal of LCM will prevent proper placement of gravel in

perforation tunnels. Removing LCM after gravel packing may result in voids which may jeopardise the success

of the gravel pack.

The use of calcium carbonate bridging agent is not recommended in the context of gravel packing as it requires

the use of acid washes for removal. Formation collapse has been observed as a result of exposure to acid just

prior to gravel packing. The use of oil soluble resins is not recommended either as they are difficult to dissolve

and require hydrocarbon flow from rather than into the perforations.

A technique for controlling fluid loss is prepacking perforations with gravel after perforating. Typically the TCP

string is used to squeeze gravel into the perforations after the backsurge operations. The packed perforations

can further be sealed with a graded salt system prior to round tripping. The graded salt is removed by flushing

under saturated brine prior to packing the annulus. This method is considered to offer much scope for obtaining

unimpaired, gravel packed perforations. If perforations are not fully packed, graded salt will enter the perforation

tunnels and will be extremely difficult to remove, which will lead to productivity loss. If there is any doubt that the

perforation tunnels are poorly prepacked i.e. low pack factors, it is considered prudent to avoid the use of graded

salt and opt for the use of a viscous fluid system. If the bottom hole temperature is appropriate Shell Flo-S is

recommended.

2.5.2.2 Viscous fluid systems

Fluid loss can be controlled using high viscosity fluids, generally prepared with HEC or XC polymers. Rather

than forming an impermeable filter cake on the formation face, the viscous fluid penetrates the formation. As the

fluid flows out radially from the wellbore, the velocity and consequently the shear rate decreases. Given the

shear-thinning properties of polymer solutions, a decrease in shear rate results in an increase in fluid viscosity.

According to Darcy's law, for a given pressure differential an increase in viscosity decreases the rate. This effect

can be calculated using a power law fluid model. Effective fluid loss control will depend on the volume of the pill.

Typically, fluid loss is not eliminated but can be reduced to an acceptable rate.

Typical polymer concentrations for fluid loss control pills are the range of 100 to 120 lb. HEC per 1000 gal. of

fluid. A classic pitfall in using viscous pills is improper gel preparation due to a shortage of time and lack of

planning. High viscosity pills must be sheared and filtered in order to remove any particulate matter.

2.5.3 Prepack acidisation

2.5.3.1 When to acidise

Adequate leak-off is essential to obtain good gravel placement in the perforation tunnels. In case fluid loss

control material has been used to kill the well, it is mandatory to remove it from the perforations, with a water,

solvent or acid treatment. Acidisation must be considered if any potential for formation damage exists prior to

gravel packing. Acidisation may follow an injection test performed prior to gravel packing to ensure that the

perforated interval has enough injectivity.

There is no fixed guideline for what is an acceptable injectivity index. Operators should determine which criteria

best suits their specific conditions. The following should be considered:

·Most gravel packs are placed at a pump rate of 2 to 4 bpm. As a rough rule of thumb, 50 to 70 % of the slurry

needs to be diverted into the perforations during placement giving the injectivity range required.

·The bottom hole pressure when injecting or packing must be kept below the formation fracturing pressure.

The injectivity test is normally carried out with the crossover tool in the squeeze position, after setting the packer

and checking the crossover tool positions. One or two rates are enough to establish the injectivity index. The

work string must be cleaned prior to injecting any fluids into the formation.

2.5.3.2 Design guidelines

The design of a prepack acid job is critical as spent fluids cannot be back produced and a poorly designed job

can do more harm than good. The following guidelines should be followed:

·If not already done, the work string must be pickled prior to the treatment to remove pipe dope, rust, scale.

·The acid types, concentrations and additives employed should be based on the formation mineralogy, type of

damage expected, etc.

·Low strength acid should be preferred to reduce the possibility of precipitating acid by-products.

·Acid volumes should be kept to a minimum, 10-20 gal/ft is enough to target the perforation area and higher

volumes e.g. 50-100 gal/ft perforation should be used if deeper damage is suspected.

·The treatment should be over flushed to minimise the potential for formation impairment due to precipitation of

by-products. Typical practice: 150-200 gal/ft perforations.

·Pumping should be continuous throughout the acidisation and gravel packing operation to avoid the collapse of

perforation tunnels weakened by acidisation.

2.5.4 Gravel packing

2.5.4.1 Pumping the slurry

As the progress of a gravel pack job is monitored by the volumes of fluids pumped, it is essential that the slurry

arrives at the crossover tool in one homogeneous slug. Several authors recommend a velocity of 500 ft/min to

prevent premature sand outs or roping of the slurry during the pumping operations [935], but the evidence

supporting this criterion is weak. According to this criterion the minimum pump rate required in a 3 1/2inch work

string is some 4.5 bbl/min. Further experimental and theoretical work is being progressed.

In many cases the slurry will free-fall as soon as it enters the work string. This is due to the difference in density

between the slurry and the completion brine which is generally high especially with high gravel loadings. As the

fluids in the wellbore are unbalanced the system "U-tubes" i.e. the fluid return rate is higher than the pumping

rate; in other words voids are generated in the work string. If "U-tubing" occurs then the slurry may reach the

cross-over port long before expected and volumetric control of the placement operations is lost. This will prevent

proper gravel placement in the case of an IGP. One practical method to ensure that U-tubing does not occur is

to keep the pump rate high enough to maintain positive pressure at the wellhead (the pump pressure may only

indicate the friction pressure loss in the lines from the pump to the wellhead).

An alternative is to spot the slurry with the crossover tool in the reverse circulating position. If a low bottom hole

pressure ball is used in the crossover tool then the formation is completely isolated from circulating pressures

while spotting the slurry. U-tubing of the wellbore fluids can be prevented by choking the fluid returns. This

method has one drawback as great care has to be taken to shift the crossover tool in the circulating position

before slurry reaches the crossover port (use a healthy safety factor e.g. 5 barrels). Failure to do so may result in

a stuck gravel pack string because of gravel in the annulus above the packer.

When the pad is within 5 barrels of the GP port, the pump rate can be slowed and the crossover tool lowered

into the circulating position. The pump operator should react quickly to any unexpected sharp pressure increase.

This may be caused by bridging of the gravel in the casing/screen annulus. The blank liner can easily be

collapsed under excessive pressures and an expensive fishing job will result.

2.5.4.2 Placing the gravel (circulation pack)

The pump rate affects the annular and perforation gravel packing efficiency. In highly deviated wells, circulation

rates should be kept high (2-3 bbl/min) to prevent premature bridging. In less deviated wells, circulating rates of

1/2to 2 bbl/min are adequate and allow more time to for the perforations to fill.

Circulation packs are in a sense a gradual squeeze. As the gravel pack begins, slurry fills the annular space

outside the screen. As gravel accumulates at the bottom of the screen opposite the end of the washpipe, the

pressure drop required for the fluid to enter the washpipe increases. Hence leak-off increases while fewer

perforations remain unpacked.

Returns must be monitored during placement of the pack to ensure that adequate leak-off occurs. Measuring

fluid lost after the job only serves the purpose of filling a number in a job report form. As gravel placement may

be over very quickly, a real time measurement of the fluid returns (positive displacement flowmeter) is required

to be able to adjust the balance between fluid returns and leak-off. There is no fixed guideline for the ratio of fluid

leak-off to fluid returns. An acceptable range is 50 to 70 %.

2.5.4.3 Screen out

When screen-out occurs, pump pressures will increase sharply. The screen out pressure is supported by the

gravel fill in the casing/screen annulus. This pressure is transmitted only a short distance within the pack and the

formation is in principle not exposed to screen-out pressures. Hence formation breakdown pressure

considerations should not be used as the criterion for screen out pressures.

Screen out pressures should not exceed 1000 to 1500 psi over the initial circulating pressure for the following

reasons:

·It is unlikely that more gravel can be placed as the pack cannot be compacted in this fashion.

·Excessive pressures can damage downhole equipment. Incremental pressures must stay well within the blank

liner collapse pressure.

Once screen-out occurs, pumping is stopped and the bleed-off rate should be observed. A slow bleed-off is a

sign of a good pack as it indicates that carrier fluid leak-off is impeded by the presence of gravel in the annulus

and the perforation tunnels.

A screen-out should be re-confirmed immediately, e.g. a few minutes after the initial screen-out. The reason

being that if a premature bridge has formed, it often collapses as soon as pumping stops and packing can then

be resumed without unnecessary time loss.

2.5.4.4 Reversing excess slurry

If the screen-out is confirmed, the annulus should be pressured-up to 500-1000 psi and the crossover tool

picked-up to the reverse circulating position. Pressurising the annulus avoids having slurry entering the annulus

as this may result in a stuck crossover tool. Connections should be spaced out to avoid stripping a connection

through the Hydrill. To avoid bridging in the tubing do not wait too long before reversing the excess slurry.

Circulate 1 1/2to 2 tubing volumes at high rates. It is best to have a check valve (low bottom hole pressure tool)

in the crossover tool to isolate the pack from circulation pressures.

In order to estimate the volume of gravel placed outside the casing the volume of gravel returns should be

measured. It is however difficult to measure accurately return volumes. The following procedure, developed by

Baker, is recommended.

Returns are diverted to the blenders when gravel is spotted. With the blenders thoroughly mixing the return

slurry, several samples of the fluid are taken. Using a centrifuge or a graduated cylinder, an average volume

percentage of gravel is determined. The blenders can be shut down momentarily to determine the total volume

of return slurry. Multiplying the total volume by the volume fraction of gravel reversed out gives a reasonably

accurate value of the total volume of gravel reversed out of the well. The amount of gravel placed in the

perforations can then be estimated.

2.5.5 Repacking

After the excess slurry has been circulated out, the pack should be left to settle. If a viscosified carrier fluid was

used, then the pack should be left undisturbed for the time corresponding to the design viscosity break back

time, usually an hour.

Screen-out should then be re-confirmed by an injectivity test. The "confirmed screen-out" criterion is then based

on the circulation rate and/or pump rate achieved during the injectivity test. There are no firm guidelines to give

here. Each operator should rely on field experience to establish threshold values.

If an additional batch of slurry is required, then a batch of no more than half of the original slurry volume should

be mixed. Consideration should be given to reduce gravel concentration and viscosity to ease secondary

placement operations. Slurry for re-packing can best be placed with the gravel pack tool in the reverse position.

2.5.6 Reporting

Concise and accurate job reporting is essential to allow for subsequent analysis and review of gravel packing

operations. Suitable job report forms are provided in Section 22.

The Perforation Pack Factor (PPF) or amount of gravel placed behind casing is a qualitative indicator of the

success of an IGP. This can be estimated if the volume of gravel reversed out of the well is known.

It should be noted that it is notoriously difficult to accurately measure the volume of the gravel returns,

nevertheless it is recommended that as accurate measurements as possible are made to enable the quality of

the technique and operation to be evaluated.

2.5.7 Gravel Pack Simulators

The lack of firm guidelines for gravel packing is partially due to the fact that large scale physical gravel pack

simulators do not model the gravel pack process realistically (linear leak-off, no diversion effects, low perforation

cavity volume). Furthermore, data gathered in the field are inaccurate and incomplete, for example the void

space behind the casing prior to gravel packing is unknown, the uniformity of perforation filling along the interval

is unknown, leading to evaluation difficulties.

An improvement in the understanding of the gravel pack process is expected to yield better job design and

control. This can be realised by numerical simulation where process sensitivities can be tested and validated

experimentally if necessary.

3 External gravel packing

Two types of External Gravel Packs (EGP) can be distinguished, the Open Hole Gravel Pack (OHGP) and Milled

Casing Underreamed Gravel Pack (MCUGP). For both types, the completion interval is underreamed to enlarge

the wellbore diameter and remove the zone of damaged permeability due to drilling operations. Drilling and

casing milling operations are beyond the scope of this manual, readers are advised to consult local service

companies for more information.

Most of the concepts applied in cased hole gravel packs are also valid for open hole gravel packs. Hence only

procedures and equipment requirements that differ will be addressed in this section.

3.1 Underreaming

3.1.1 Effect of underreaming on well productivity

EGPs should theoretically provide a higher productivity than open hole completions or cased hole gravel packs

because the restrictive casing perforations are eliminated and the underreamed borehole is filled with highly

permeable gravel which improves radial flow into the well.

In theory, an 8 inch hole underreamed to 16 inch and then gravel packed should have 15 % higher productivity

than an open hole completion. In practice, this effect is rarely seen because of the various impairing

mechanisms that may be caused by the underreaming and gravel packing operations. For example shale

streaks in the reservoir may release clay cuttings which can severely impair the formation.Nevertheless EGPs

generally show a better productivity than comparable IGPs. This is due to the avoidance of restrictive perforation

tunnels, making an EGP more forgiving with respect to impairment mechanisms.

3.1.2 Underreaming operations

Underreamers come in many different types and sizes. Expandable underreamers have arms that expand when

hydraulic pressure is applied and retract in the tool body when the pumps are shut-down. A variety of cutters can

be attached to the arms i.e. roller cones, PDC cutters or hardened cutter blades. Blade type underreamers are

only suitable for soft unconsolidated formations. The maximum diameter to which a hole can be underreamed is

a function of the tool used and the reservoir rock strength. Conventional tools allow to underream up to twice the

tool diameter in medium hard to soft formations.

3.1.3 Underreaming fluids

Great care must be taken to prevent formation impairment when underreaming as any damage will subsequently

be locked behind the gravel pack. Impairment can be caused by several mechanisms: by build-up of an

impermeable filter cake with formation debris, by clay swelling due to fluid incompatibility or by reservoir invasion

of particulate matter. Formation damage can be minimised by:

·Thoroughly cleaning the wellbore prior to underreaming.

·Minimising the overbalance on the reservoir.

·Using filtered, compatible fluids.

·Using efficient solids removal equipment.

·Using a properly sized acid or water degradable particulate system to control losses if required.

Viscosified clear brines are commonly used as underreaming fluids to minimise impairment. Viscosity is required

to provide solids transport capacity and will help to control fluid loss. HEC or HEC/XC polymers are generally

used as viscosifying agents. Shellflo-S should be seriously considered in view of its better carrying capacity and

degradability.

Fluid cleanliness is extremely important for underreaming operations. Viscous underreaming fluids should be

prepared with the same precautions as for a gravel pack carrier fluid i.e. polymer solutions should be sheared,

filtered and mixed with a viscosity breaker, if required, prior to pumping downhole. When volumes are excessive,

viscous fluids are often sheared only. An efficient solids removal equipment is mandatory. If price and brine

availability allows, part of the circulating system should be frequently changed out in order to reduce the

suspended solids content.

Losses are inevitable when underreaming with clear brines. In very permeable or depleted reservoirs, a

degradable particulate system will be required to maintain hole stability and minimise formation damage.

Bridging solids shape and size distribution should be such that a thin cake forms quickly [938]. Calcium

carbonate or salt particle systems can be used. Subsequent removal of the bridging cake is however difficult and

often incomplete. The use of graded salt particles is a recent development. The advantage is that the bridging

cake can be dissolved by circulating under-saturated brine. Graded salt systems have recently been used to

successfully drill horizontal wells in poorly consolidated sandstone reservoir.

3.2 Gravel placement considerations

3.2.1 Factors affecting gravel placement

EGP completions only require that the annular space between the screen and the underreamed hole be packed

with gravel. Hence fluid leak-off into the formation is not essential to place the pack but contributes to

compacting the pack by slurry dehydration.

Basically, the factors affecting gravel placement in the casing/screen annulus for an IGP also apply here. The

main difference is that the annular clearance is much larger in the case of an EGP.

Especially with thin fluids, there is an inherent risk of mixing gravel with sand as the slurry is being circulated

downhole. Hence circulation rates are generally limited to about 4 barrels per minute in normal applications,

where fluid velocity in the annulus is not a critical factor for gravel placement. In highly deviated intervals fluid

velocity in the annulus should be designed with a view to optimise gravel transport in the annulus.

As with an IGP, gravel placement is increasingly difficult with increasing deviation, interval length and fluid leak-

off variations along the interval.

3.2.2 Gravel placement methods

The one trip crossover circulation method is normally used to place an EGP. The cheaper option is to use the

"over the top" system together with low viscosity fluids (conventional packing).

3.3 Slurry design for EGP's

3.3.1 Gravel loading and polymer concentration

Slurry design is generally less critical for an EGP than for an IGP because there are no perforations to pack.

For normal applications (deviation < 60 degrees), both low viscosity and high viscosity carrier fluids have been

used with equal success. The fluid carrying capacity is not a critical factor as gravity helps to pack the annulus

and packing occurs from the bottom of the well upwards. In the context of open hole gravel packing, merits of

slurry and conventional packing are as follows:

Conventional packing (non-viscosified brines, 0.2 to 2 lb/gal gravel concentration):

·Cheaper with respect to fluid costs, may however require long pumping times and hence generally more costly

overall.

·More likely to de-stabilise the formation sand and cause impairment by mixing of gravel and sand.

·Significant fluid losses may occur.

·Prolonged exposure of the formation to completion fluid.

Slurry packing (up to 80 lb/Mgal HEC, 15 lb/gal gravel concentration):

·Short pumping times.

·Less likely to cause gravel/sand mixing because of the lower pump rates, higher viscosity and reduced

exposure times.

·More expensive than conventional packing

3.3.2 Gravel volume

The gravel volume should be calculated using a similar approach as for cased hole gravel packing. A calliper log

should be considered prior to running the screen to insure the hole is open and to provide an indication of the

quantity of gravel to be placed. 20 to 25 % excess should be added to the calculated amount of gravel. Field

experience may indicate that a different value is more appropriate.

3.4 Gravel pack liner assembly

3.4.1 Screen dimensions

It is recommended that the same screen sizes be used in open hole as for cased hole completions.

3.4.2 Tell-tale

As for an IGP, the use of a Lower Tell Tale (LTT) when slurry packing is not recommended. The benefits of a

LTT are even more doubtful here as the screen/openhole annulus is many times larger than the

screen/washpipe annulus. However the washpipe should be properly sized.

Upper Tell-Tales (UTT) should be used only when conventional packing (packing with non viscosified brines) is

carried out.

3.4.3 Blank liner

Part of the gravel reserve should be in the open hole portion of the completion. More reserve gravel will be

available because of the relatively larger annular volume in the screen/open hole annulus. Hence the blank pipe

should extend 10 feet below the casing shoe. Another reason is that a void can easily develop just below the

casing shoe if the screen extends into the casing and if there is a small screen/casing clearance. A bridge may

form as the gravel in the open hole settles and a void is created below the casing shoe.

3.5 Procedures

3.5.1 Acidising

Prepack acidisation must be carried out if acid degradable loss control material has been used when

underreaming.

3.5.2 Gravel placement

4 Gravel packing in special applications

4.1 Completing horizontal wells through unconsolidated sands

Most common applications:

1.To promote cone suppression.

2.To laterally connect multiple reservoirs and/or high permeability features.

3.To increase productivity through increased exposure to the formation.

The vast majority of conventional vertical/deviated wells employ gravel packs when sand control is necessary.

However, in horizontal wells the technical difficulties and additional cost make this traditional option difficult to

justify in the majority of cases. Clearly, completion costs become disproportionate with long lateral drain holes,

particularly when simple completions are not feasible.

4.1.1 Wellbore stability

There is a common mis-conception that openhole horizontal wells are inherently more stable than their vertical

counterparts. From a rock mechanical point of view this is not the case for a normally tectonically stressed area,

as the horizontal wellbore will be perpendicular to the principal (vertical) stresses. In fact by extending the above

further, it can be clearly shown that from a theoretical point of view a cased horizontal hole, perforated in the

vertical plane only is the most stable horizontal completion configuration. However with such a configuration

massive failure will occur more suddenly as post failure stabilisation is not possible.

This can be partially explained by considering the following:

·The drawdown imposed on the formation is comparatively less due to the increased inflow area. This is thought

to greatly offset the effect of hole angle and to a lesser extent orientation.

·Fluid velocity, hence drag forces are much lower in horizontal wells. This together with reduced drawdown

pressure may in some cases help substantiate less stringent sand control requirements in horizontal wells.

·Changes in wellbore fluid composition frequently lead to increased sand production. As mentioned above,

horizontal wells may help defer water (gas) breakthrough and hence indirectly promote sand exclusion.

·In cased hole completions, although the hole is horizontal perforations might be shot in the vertical plane.

·In uncased horizontal holes the formation may collapse on the liner or screen, creating a new stable situation

without excessive damage and with little sand production.

Regardless of the above, most of the horizontal wells drilled to date have been justified on the basis of

optimising production, reservoir sweep, etc. With time and experience, sand exclusion requirements may prove

to be less stringent - offering potential to reduce completion costs compared to vertical gravel packs. Clearly

horizontal wells offer a way of offsetting problems associated with conventional wells and traditional sand control

practices.

In summary, open hole horizontal wells through normally stressed reservoirs are more likely to be unstable

compared to vertical wells drilled through similar formations. Nevertheless, Group experience to date indicates

that neither wellbore stability nor sand control are insurmountable problems when planning horizontal wells

through poorly consolidated reservoirs. However, long term production history from existing horizontal

developments may prove otherwise.

4.1.2 Completion types

Unconsolidated formations are generally completed with a variety of screen designs: including dual screen

prepacks, perforated outside prepacks, slotted liners and pre-drilled liners or appropriate combinations. Although

attempts have been made to gravel pack highly deviated wells, to date no attempt has been made within the

Group to gravel pack a truly (90°) horizontal well.

Within the industry the most common completion type for horizontal wells through unconsolidated sands is the

installation of a liner or screen. This section describes the pro's and con's of the most widely used completion

techniques.

4.1.2.1 Liners and Screens

In cases where large grain, well sorted reservoirs are being developed, screens used without gravel packing

have proven effective in controlling sand production without a significant drop in short to medium term

production. Furthermore, in many low rate oil well applications, slotted liners or conventional wire wrapped

screens in most cases turn out to be the most "cost effective" options. For higher rate oil (and gas) wells, or in

areas where the cost of remedial action is prohibitive (eg. subsea wells) prepacked screens are normally

recommended to provide added insurance against screen erosion. In short or medium radius wells where

aggressive build rates are required, and/or in applications through poorer quality, finer sands the use of pre-

packed screens should be considered with some caution due to concerns about mechanical integrity, quality

control, and plugging potential.

a. Pre-drilled liners

Completions using pre-drilled uncemented liners offer nothing more than a future logging conduit. They prevent

a unconsolidated formation completely filling the wellbore. Pre-drilled liners although commonly used by

numerous Group companies are not viewed as sand control.

b. Slotted liners

Presently the least expensive form of sand control. Slots are cut parallel to the longitudinal axis and uniformly

distributed around the circumference of the liner whilst retaining the mechanical integrity of the liner.

The disadvantages commonly quoted are:

1.Smaller effective slot inlet area.

2.Slots tend to plug easily with formation fines, corrosion products, and precipitates during installation and during

production.

3.In most applications the required slot sizes, below say 300mm, are difficult to achieve using conventional

machine tools, hence exclusion of formation sand cannot be guaranteed.

4.Slots may erode through fines production.

Slot plugging is not, apparently, adversely affecting short to medium term production performance of such

horizontal well completions. However, in view of the long completion lengths it is possible that plugging may not

be noticeable, or indeed, stable bridges may be formed across the slots. Nevertheless, to reduce plugging

potential the inflow area of the liner should ideally be maximised without reducing the mechanical integrity of the

liner. This however has a direct impact on machining costs, twice as many slots cost nearly twice as much.

Industry guidelines recommend that a minimum of 2% inflow area should be provided.

c. Wire-wrapped screens (WWS)

WWS have around 20 times the inflow area compared to an equivalent slotted liner. Additionally, manufacturing

methods are available which are capable of mass producing WWS with 50mm slots. The main drawback with

WWS for such applications is that they are more costly, which in view of long completion intervals results in

significant up front expenditure. As with slotted liners the use of WWS only should be limited to low erosion risk

applications, which is frequently the case with horizontal wells.

d. Pre-packed screens (PPS)

Success stories on the use of PPS alone as a primary form of sand control in horizontal wells have been

reported. Judging the performance of such sand control measures is always difficult, there is no doubt however

that PPS have been successfully used in unconsolidated reservoirs containing clean, well sorted, large grain

sands and gravels.

As with other forms of screen, the application of PPS in poorer quality reservoirs must however be viewed with

caution as formation slump, or natural packing will lead to an area of low permeability around the screen. This

reduction in permeability will be a function of grain size, sorting and clay content. As with any completion design

which uses screens alone, this effect can be reduced by minimising the clearance between the screen and

wellbore.

By design PPS are less likely to fail because of erosion. However, plugging of the gravel sheath during

installation due to dirty completion fluids or by formation fines during production is a major problem.

Another major design consideration is the high risk of cracking the brittle resin consolidation when running

screens through tight dog-legs, or while flexing during handling and make up. If the resin is cracked while

installing the screens (or before) this can lead to an erosive type failure at a later date, with potentially serious

ramifications, especially in higher rate gas wells.

Design limitations for various screen types marketed by the major service companies are readily available.

Several of the major screen companies have subjected standard (both WWS and PPS and slotted pipe) screen

designs to tests that involve pulling screens through casing strings shaped to represent a medium - short radius

horizontal wells. Readers are advised to consult local screen vendors for more information.

The additional expense (3x WWS), availability and quality control problems are the other main disadvantages of

pre-packed screens.

e. Gravel packing

As previously mentioned most horizontal wells within the industry where sand production problems were

anticipated have been completed with liners or screens in open hole. Although the problem of formation damage

was generally thought to be less important in horizontal wells, work conducted has concluded that formation

damage is just as important in horizontal wells. A number of operators and authors advocate gravel packing

prepacked screens to minimise plugging potential, and therefore maintain economic production rates through

extended well life. Although this philosophy may lead to the technically optimal solution, it may, in many cases

be less attractive due to the incremental cost of gravel packing.

Over the last decade great research effort has focussed on defining key parameters involved in the process of

gravel packing highly deviated or horizontal wells. Numerous organisations and researchers have constructed

full size physical models to observe the process.

In near vertical wells, gravity assists in overcoming placement failures in gravel packed wells due to settling. In

highly deviated wells ( > 60° deviation) the opposite is true, gravity tends to exacerbate placement difficulties

due to duning. In the above models using brine researchers have observed two waves of gravel flow: firstly the

alpha wave - which deposits gravel on the lower side of the hole, and secondly the beta wave which propagates

back towards the ports filling the gap between the alpha and top of the top side of the hole. Although a detailed

description of the process is outside the scope of this review, the technology related to gravel packing high angle

wells can essentially be directly applied to horizontal wells.

If there is a clear need to gravel pack a horizontal well and the project can support the incremental cost then a

number of key design parameters have to be considered:

1.Sand transport efficiency - the ability of the carrier fluid to suspend gravel:

-Carrier fluid rheology.

-Gravel loading etc.

2.Pump rate:

-Fluid flowrate (from 2-4 bbl/min).

3.Tool and screen configuration:

-Washpipe OD screen ID ratio > 0.8.

-Centralization and clearance (0.75-1.0").

-Elimination of blank sections.

-Continuous washpipe to bottom.

4.2 Water injection wells

The requirement for sand control in a water injection well is debatable and no unique technique can be

recommended. Sand control may be required in view of the following:

·Injection water tends to slowly reduce the the formation strength by dissolution of the cementing material and

this is aggravated if the well is acidised.

·Backflow may occur when injection stops and crossflow between layers that have been differentially

pressurised may occur. Significant quantities of sand may then accumulate in the wellbore resulting in

impairment.

·Backflow flow may be required to clean out impairing material (gaslift often being provided for this purpose.

Laboratory tests have shown that significant amounts of silica can be dissolved from gravel pack and formation

sand when large quantities of water or steam are injected into a sandstone reservoir. The rate of dissolution is

dependent on the flow rate of water through the gravel pack and the formation, the pH of the injected fluid and

the temperature

Consolidated gravel packs are used where conventional gravel packs tend to give only short lived protection

against sand inflow because of dissolution of the gravel and formation sand by the injection fluid and fluidisation

of the pack when backflushing the wells. Consolidated gravel is gravel pack sand which is coated with a resin

consolidation system. The more widely used system is gravel pre-coated with a partially cured thermo-setting

resin.

5 Post gravel packing and remedial operations

5.1 Bean-up procedures

The objective of a bean-up policy is to is to reach well potential within the shortest period of time without

jeopardising ultimate well integrity and productivity.

During sand control workshops, bean-up procedures were found to vary greatly and were not always clearly

justified. Some Opcos have however established that rapid bean-up times are detrimental to the long term

performance of their wells.

5.2 Gravel pack logging

5.2.1 Objectives

The objective of gravel pack logging is to inspect the quality of the pack, either immediately after it has been set,

or after some production has taken place. Gravel pack logging is not required on a routine basis after gravel

packing. Recent experience is that most gravel packs are successful from a sand exclusion point of view. Gravel

pack logging may be needed in the following circumstances:

·When premature screen-out is observed or when no screen out is possible.

·When difficult or non-routine gravel packs are carried out, e.g. long (> 50 ft) or highly deviated (> 60 degrees)

hole sections.

·To detect pack slumping with time, e.g. slumping caused by gravel dissolution in steam injection wells.

If an incomplete pack is indicated, secondary packing operations can be employed.The logging technique should

thus be capable of determining the top of the gravel pack and the presence of voids in the pack. By comparing

logs taken at different moments in time, pack slumping with time and the development of cavities in or beyond

the pack can be monitored.

Owing to the presence of metal screens and casing, only nuclear techniques are suitable to fulfil all these

requirements. Two different types of logging devices are suitable, the gamma-gamma (photon) and dual spaced

neutron (compensated neutron) tools. Both provide qualitative indications of changes in the completion interval

i.e. the sum of effects caused by wellbore fluid, screen or liner assembly, gravel, cement and formation lithology

to some extent. Because fewer variables affect its performance and interpretation, the photon tool is preferred

for gravel pack evaluation although the FDC/CNL combination has in many cases also resulted in satisfactory

logs.

5.2.2 Gamma-gamma tools

5.2.2.1 Principle

Like the usual density tools, gamma-gamma tools emit gamma rays into the formation around the tool and detect

the intensity of backscattered radiation. The detector count rate decreases with increasing average bulk density

(more absorption) in the volume investigated. This volume is roughly given by the source-to-detector spacing

and extends radially up to a few inches in the formation beyond the gravel pack. The tool response is thus a

function of the amount of gravel, but also of the amount of steel, fluid, cement and formation density. The relative

importance of these components, however, depends on many parameters, but in particular on the source-to-

detector spacing. This feature is explicitly used in dual detector tools.

5.2.2.2 Available tools

a. Dual detector tools

The Gravel Pack Porosity Tool GPPT (Schlumberger) and the Dual Photon log (Western Atlas) are specially

designed production logging tools with a diameter of 4.3 cm (111/16"). Both tools have two detectors, set at

different spacings from the source. Since the depth of investigation increases with increasing spacing, the two

detectors have a different sensitivity to the presence of gravel in the annular space. This configuration allows a

better qualitative assessment of the gravel sheath.

b. Single detector tools

There is a variety of tools with a single detector, which are either modifications of tools made for a different

purpose, such as the Nuclear Fluid Density Meter tool NFD (Schlumberger), or dedicated such as the Photon log

(Atlas Wireline). Because these tools give less information, they are more difficult to interpret, unless a baselog

is available as a Reference.

5.2.2.3 Interpretation

The standard interpretation technique for dual detector tools is to plot the count rates of the near and far

detectors on linear scales. The scales are chosen such that the logs overlay in good quality gravel (usually at the

bottom of the pack). At places where the pack is poor, the count rate at both detectors will increase, but at

different rates, and a separation is then seen.

The log readings at full fill and no fill may be used as calibration points, provided casing and cement thickness

are the same. Log readings in between these values may be linearly interpolated in terms of percentage of fill.

The resulting quantitative evaluation is of little practical value due to the broad range of variables affecting the

measurement. The minimum detectable change under favourable conditions is some 10%, which corresponds to

one to two litres (0.04 - 0.08 cf) of gravel in a 7" casing, 4" screen IGP.

Single detector logs can readily be interpreted when used in time-lapse mode, e.g. before and after a re-pack

operation. The interpretation of a stand-alone run is much helped by the availability of a schematic drawing of

the hardware (because density tools are very sensitive to hardware effects) and the open hole density log to

recognise features that are not related to gravel pack quality.

5.2.3 Compensated neutron tools

5.2.3.1 Principle

Neutron tools emit high energy neutrons and detect those that have slowed down to thermal energy. Since the

slowing down process is dominated by hydrogen, the detector output is a measure of the amount of hydrogen in

the vicinity of the tool. In the application for gravel pack logging, the count rate increases with decreasing

average void ("porosity") in the volume investigated, provided that the hole is liquid filled. The investigated

volume is roughly given by the source-to-detector spacing and may extend up to several inches into the

formation beyond the gravel pack. In gas-filled holes, the hydrogen content is too low to obtain an interpretable

log.

5.2.3.2 Available tools

All current (open hole) neutron tools may be used. They have a dual detector system and are omni-directional.

Tools are available in different diameters, down to about 7 cm. (2 3/4inch) (e.g. CNT, Schlumberger and NEUT,

Western Atlas).

5.2.3.3 Interpretation

The standard interpretation technique is to plot the count rates of the near and far detector on linear scales. The

scales are chosen such that the curves overlay in places with a good gravel pack. In intervals with a poor pack

the count rates will decrease, but, because of the different depths of investigation, by a different factor and a

separation will be seen. Variations in the formation properties are often reflected on the count rates, in particular

when the gravel is homogeneously packed. The open hole neutron log (count rates) may help to explain such

variations.

5.2.4 Other nuclear techniques

Pulsed neutron capture (PNC) logs, such as the TDT or PDK, may be used, but there is little experience of using

them within the group. These tools emit neutrons, but detect gamma radiation. Consequently, the near and far

count rates react to the presence of gravel in a similar way as neutron logs, but some effect of the hardware may

be seen.

Recently, a method has been proposed measuring silicon activation with pulsed neutron logging. This method is

valuable when very heavy completion fluids are used such that the density contrast between completion fluid

and gravel is too low to be detectable with gamma-gamma tools. It would appear, however, that in such a case

PNC or neutron logs would also be successful.

5.2.6 Recommendations

1.Dual detector gamma-gamma tools should be run in preference to neutron tools and may be interpretated

semi-quantitatively. Single detector tools may be run when the completion scheme is simple, or in time lapse

mode (e.g. before and after a repack job).

2.Near and far count rates should be overlayed over intervals with a good gravel pack.

3.Poor gravel pack will then show as a separation of the curves while both count rates change; a decrease in the

case of neutron logs and an increase in the case of gamma-gamma logs.

4.A CCL should be run with the gravel pack log for proper depth match.

5.Changes in hardware (in particular collars) are clearly visible on the gamma-gamma logs.

6.Neutron logs may reflect changes in formation properties; this can be recognised from correlation with open

hole neutron logs.

7.Neutron logs are not interpretable in gas-filled wells.

8.Completion scheme should be drawn on the plot.

9.Open hole logs (far detector count rate) should also be displayed.

5.2.7 Developments in gravel pack logging

Recent gravel pack development work has focused on optimising the process of gravel placement into

perforation tunnels. This work has led to the introduction of alternative carrier fluids and better defined

operational procedures, with a number of ongoing field trials. Evaluation of these trials and the effectiveness of

routine gravel packs requires a quantitative assessment of perforation tunnel fill. With conventional logging

equipment this is not possible.

Halliburton Logging Services (HLS) have however recently introduced two new tools:

1.TracerScan and

2.RotaScan, which have been developed to assist in assessing gravel placement.

These tools detect radioactive (gamma ray emitting) materials placed in the wellbore and near wellbore region.

By coating gravel uniformly with a radioactive label these tools can be applied for evaluation of gravel inside and

outside casing. Tagging with different isotopes facilitates the selective detection of carrier fluid, prepacked gravel

and gravel placed in the screen-casing annulus. In collaboration with HLS, KSEPL at the beginning of 1992

initiated a study to calibrate tool response of both tools with screens placed around the tool, and for various

(gravel filled) perforation cavity sizes and geometry. The remainder of this sections discusses the capabilities

and limitations of these tools.

TracerScan is a spectroscopic gamma ray tool that can be used for the detection of radioactively labelled gravel

placed in the annulus and/or perforation tunnels. The tool has a vertical resolution of 30 cm and a depth of

investigation of approximately 20 cm. The tool facilitates 360° detection. The measured signal gives information

on both the depth and density of placed gravel.

The TracerScan tool can measure:

1.Perforation tunnel fill along the perforated interval. However, it must be noted that only a qualitative

assessment can be made.

2.Filling of the annulus. Voids in the annular pack, as well as top and bottom of the annular fill can be detected.

RotaScan is a total energy, directional gamma ray tool. The main component is a sodium iodide scintillation

detector within a rotating tungsten shield containing a slotted apperture. The tool detects a circumferential

section of 40°. A three axis accelerometer is used to determine the inclination and azimuthal orientation of the

tool. Vertical resolution is 30cm and depth of investigation is approximately 50cm behind casing.

The tool shows potential for evaluating gravel placement in highly deviated wells. For instance placement in high

and low side perforation tunnels may be compared.

5.2.7.1 Operational considerations

·When using the above mentioned tools it is recommended to tag the pre-pack gravel and annular pack with

different isotopes.

·Half-life of the tracer should be long enough to ensure that, if the operation is delayed, no extra quantities of

tagged material are required. Tracers having half-lives of 90 days are often used. Tagged gravel is normally

prepared and transported to location by a specialised company.

5.3 Production logging in gravel packs

The presence of a gravel pack will necessarily affect the resolution of production logging tools. For this reason

they are likely to have a limited usefulness in short intervals, except as a flow/no flow indicator.

For intervals longer than 10 ft or in reservoirs which are clearly stratified (e.g. large shale breaks) it is quite

possible to obtain information on production contribution from different intervals. This is of interest for reservoir

management purposes or as feedback to aid in the design of future gravel pack procedures. It may also provide

information which is required for calculating gravel pack skins (length of zone contributing to production).

5.4 Gravel pack stimulation

This section addresses the design of stimulation treatments for impaired gravel packed wells. A diagnostic of

well impairment should first be established and the stimulation treatment must be tailored to the type of damage

identified. It is important to realise that some types of damage are difficult or even impossible to remove and

impairment prevention is always more effective than damage removal. Some of the concepts presented here

need further research and development to arrive at optimal damage removal.

5.4.1 Impairment diagnostic

The first step is to carry out a production test and BHP survey to measure the well impairment. This data is

required to establish the severity of the problem and, by comparison with post-treatment data, to quantify the

benefits of the stimulation treatment.

There are many potential sources of impairment in a gravel packed completion e.g. screen plugging, gravel

contamination with formation sand or debris, plugged perforations, etc. Skin data, which is generally a more

reliable indicator of well impairment than PI data, does not allow however to properly identify the impairment

mechanisms apart from geometry and turbulence effects. It is difficult to discriminate between true formation

damage and gravel pack impairment as there are simply too many variables in the problem.

The following approaches may help in identifying the source of impairment:

·A systematic well history (starting at the drilling phase!) and gravel pack job review should be established.

Production engineers may often have only a portion of the well history available and may not be aware of

important events like severe losses when drilling the reservoir. Hence some detective work is usually required.

·The computer program STIMSEL under the ICEPE portfolio is designed to help in pinpointing the extent and

nature of impairment. The use of this tool requires caution however as it uses theoretical correlations to describe

the productivity of gravel packed completions. This may result in overestimating the true formation damage as

the effects of e.g. gravel/sand mixing or perforation plugging are included in the skin due to damage.

·When a large database has to be analysed, the use of statistical methods such as discriminant analysis should

be considered.

·Production logging can also help in the diagnosis of well impairment by establishing the production profile in a

well. KSEPL is currently developing software to aid in the interpretation of such profile with respect to

impairment.

5.4.2 Design of stimulation fluid

The type of acids, the concentrations and additives employed should be based on the mineralogy of the

formation, the type of damage presumed and specific wellbore conditions (e.g. BHT).

As the exact source of damage is rarely known, both laboratory and pilot tests should be carried out to identify

the optimal stimulation treatment. Similarly, stimulation job reviews are an essential part of the optimisation

process.

Ideally one should aim at developing stimulation treatments that can simultaneously remove different types of

damage. Combinations of different treatments can be made e.g. two different damage sources could be targeted

by pumping larger stimulation fluid volumes or by applying a longer soaking time. However tackling more than

one problem at a time may cause undesirable side effects and may require a complicated treatment which

becomes difficult to handle.

5.4.2.1 Screen cleaning

At the installation phase, screens may become plugged with mud, paint from downhole tools, pipe dope, and any

material collected from the casing wall. When the well is producing, screens may become plugged with sand,

wax or scale deposits.

Unless the plugging material is of organic nature (e.g. wax) the obvious choice is HCl acid or mud acid. Mud acid

is normally preceded by an HCl preflush, as a general measure to minimise secondary precipitates.

If organic deposits are suspected, solvents such as alcohols blended with surfactants, mutual solvents or plain

hydrocarbons like kerosene or xylene can be applied. Before using such chemicals, laboratory tests to select the

the optimum solvent are essential.

The preferred placement method is to spot the acid with coiled tubing at the bottom of the well and subsequently

pull the coiled tubing while pumping. This will ensure coverage of the entire screen if the fluids remain in the

wellbore.

Mechanical methods such as jetting or washing are not recommended as they can potentially fluidise the pack or

inject foreign matter into the gravel pack and cause further impairment.

5.4.2.2 Gravel cleaning

Gravel contamination with foreign material leads to impairment of the pack. The contaminating material can have

many different origins:

·Pipe dope, paint debris,etc...

·Formation debris e.g. shale, feldspars...

·Precipitates due to incompatibility between formation and completion fluids.

·Mud remnants.

·Polymer residues.

Acid solubility is an important gravel quality specification item. The lower the quartz content, the higher the acid

solubility as impurities (e.g. clay and feldspar) are more soluble in HCL and HF acid than quartz.

Incompatible fluids can cause impairment by precipitate deposition at the formation interface. The effect will be

intensified if the fluids contain solids or if the viscosifier leaves a residue behind e.g. when poorly mixed,

unsheared and unfiltered, partially broken HEC was used. If the impairment is due to precipitates then 15% HCl

is the suggested treatment.

If however the problem is related to HEC residue, lower acid concentrations can be used. Laboratory and field

work suggested that HEC breakdown by strong acids might create more insoluble residue, specifically at higher

temperatures. However, when properly mixed, breakdown of HEC with acid presents no additional impairment

problem.

Alternatively, treatments with enzymes (if the reservoir temperature is 65°C or lower) or hypochlorite may be

applied. Both types of treatment have shown positive effects.

The volume of the treatment should be restricted to the volume of gravel to be treated, e.g. 1.1 times the volume

of the pack. A diverting agent should be used to promote full coverage of the pack.

5.4.2.3 Near wellbore formation

The near wellbore formation may have been impaired during drilling, cementing, perforating, underreaming or

gravel packing operations. As such gravel packed wells do not differ from perforated wells except for the access

to the formation face. It is even more difficult to direct the stimulation fluids to the impaired region in gravel

packed wells than it is in conventionally completed wells.

5.4.3 Diversion

It is difficult to achieve effective diversion in gravel packed wells, because there are many routes for the acid to

by-pass the targeted zone.

Mechanical techniques such as selective placement tools, or inflatable packers can be used to isolate a

particular zone for injection or stimulation of fluids. Ball sealers are generally not suitable for gravel packed wells

due to the configuration of the screen.

Graded solid particles (e.g. benzoic acid or oil soluble resins) can be used to create a thin, low permeability cake

to plug off high injectivity zones.

Examples of diverters are:

·MMOWG (Halliburton), a solution of ammonium benzoate from which finely divided benzoic acid will precipitate

when mixed with acid.

·J363 (DS), a sodium benzoate that with proper surfactants will rapidly recrystallise to fine grained benzoic acid.

·DIV III (Nowsco) similar to Halliburton's MMOWG.

·Oil soluble resins.

In practice it may take months to fully dissolve the diverters in crude oil. Attempts to speed up the dissolution by

applying solvents, such as methanol (benzoic acid) are usually not successful or may even have a negative

effect, e.g. a diesel overflush to dissolve oil soluble resin will lead to the formation of sticky, impermeable

residues. A solvent preflush preceding the stimulation treatment may be considered in such cases.

Diversion is also possible with viscous pills e.g. polymer solutions injected ahead of the stimulation acids. The

diversion mechanism relies on the shear thinning properties of polymer solutions. The effectiveness of this

method will be a function of the volume of polymer placed. One disadvantage is that viscous pills, may

themselves cause impairment.

A typical foamed acid stimulation pumping schedule may read as follows:

1.Prepare the well and well site for the treatment by rigging up the coiled tubing unit and pumping equipment.

Pressure test the surface lines the blow out preventer, and tubulars to the required pressure.

2.Run the coiled tubing in the hole and clean with the hole with foam. Spot 0.12 m3/m (10 gal/ft) of foamed 10%

w/w HCl, while pulling the coiled tubing up at a velocity of 1 m/min. Maintain pumprate at 159 l/min (1 bbl/min).

3.Run the coiled tubing back to original depth. Spot 0.25 m3/m (20 gal/ft) of foamed mud acid. Pull the coiled

tubing at 0.5 m/min while pumping at 159 l/min (1 bbl/min).

4.Inject 0.12 m3/m (10 gal/ft)of unfoamed 3% NH4Cl brine, while lowering the coiled tubing at 1 m/min and

pumping at 159 l/min (1 bbl/min). An additional similar volume of NH3Cl brine should be injected after running in

the coiled tubing to total depth.

5.Displace the tubing/coiled tubing annulus and while pulling the coiled tubing, the tubing with filtered water,

suitable brine, oil, etc. Keep the well full with displacement fluid to avoid swabbing acid foam into the well bore

while pulling.

Shell Oil also reported good results with viscosifying and/or foaming separate diverter stages in between acid

stages, i.e. using a combination of foam and polymer (HEC). The exact mechanism of such diverted treatments

is not fully understood.

5.4.4 Additives

Additives are required to combat the side effects inherent to the use of acid, i.e.:

·Corrosion of completion items.

·Precipitation of secondary reaction products, e.g. ferric hydroxide.

·Formation desintegration and fines release.

·Formation of emulsions.

·Clean-up problems.

This topic is discussed in the Production handbook and the Well Stimulation Manual [970]. As a general rule, the

recipe should include a corrosion inhibitor and a sequestering agent to prevent iron precipitation.

5.4.4.1 Corrosion inhibition

In the context of gravel packing, special attention should be given to corrosion aspects. Screens are often

manufactured from high alloy steels and the effect of acid on these alloys can be devastating, especially in view

of the close tolerances imposed on screen slot width. Current corrosion inhibitors are chiefly designed to protect

carbon steels, although most contractors claim that they can effectively protect high alloy steels with higher

inhibitor concentrations. A number of service companies were requested to recommend corrosion inhibitors for

these high alloy steels.

5.4.4.2 Sequestering

In case normal HCl is used, it is recommended to add a minimum of 10 kg/m3 citric acid (sequestering agent) to

avoid precipitation of ferric hydroxide. If mud acid is used, the sequestering agent can usually be left out. More

specific recommendations can be obtained by using SEQUES, a program to calculate sequestering agent

requirements which is available under ICEPE.

Other additives should only be used after tests have demonstrated the need for them. For example a silt

suspending agent should only be used if tests show that a significant amount of fines will be generated. Likewise

a demulsifier should not be applied if the crude oil/spent acid does not show an emulsification tendency.

5.5 Remedial sand control

Remedial sand control in the context of this section is defined as a treatment designed to reduce sand

production to tolerable levels in existing wells with or without sand exclusion previously installed.

As discussed in the introduction to this manual an increasing number of operators and Opcos are now willing to

manage the risk of sand failure. The requirement for reliable, cost effective remedial forms of sand control is

becoming increasingly important.

5.5.1 Industry developments

Well completion and maintenance costs can be significantly reduced if specified pumping and or coiled tubing

units are deployed for workover operations. In many areas, rigless workovers and completions are becoming

more popular with numerous operators (including several Opcos) reporting higher success ratios with through

tubing remedial operations. The major sand control service companies are now actively marketing a suite of

remedial sand control services.

Remedial sand exclusion includes many different techniques from beaning down production to reduce drawdown

to sand bailing. There are several basic through tubing methods methods which can be considered for remedial

operations. Method selection is generally driven by the mechanical configuration of the well (casing size, tubing

size, completion accessories, length of completion interval, formation type etc).

5.5.2 Through tubing sand control

Historically through tubing sand control was considered for marginal wells were a major rig re-entry could not be

justified. However, field experience clearly indicates that the chance of successful remedial sand control is

substantially reduced if significant quantities of sand have been produced. This is generally attributed to the

presence of cavities (which leads to poor placement) and additional problem of excessive losses exacerbated by

reservoir depletion.

Due to new procedures advanced in line with recent developments in coiled tubing technology, through tubing

sand control is becoming an increasingly used technique.

5.5.2.1 Mechanical techniques

The deployment of through tubing screens (WWS, PPS) inside tubing, casing or inside failed gravel pack liners,

conveyed by wireline or coiled tubing conveyed systems has been widely applied by a number operators.

With such techniques screens alone are used or gravel can be placed either before or after running the screens.

With the those techniques general gravel pack design considerations normally apply. The following being

particularly important to note:

·Screen overlap of 3m above and below the completion intervals.

·Screen OD will be restricted by smallest completion accessory.

·The wellbore must be clear opposite the completion zone.

·Clean completion practices is, as normal, a job prerequisite.

·Prepacking existing and new perforations is generally recommended.

Although such methods are potentially cost effective solutions, they should be viewed with some caution, as

"cluttering" the wellbore with addition hardware may well present a host of additional problems. Mechanical

through tubing sand control solutions are therefore generally considered as a last resort. Experience within the

Group with this type of workover is limited.

5.5.2.2 Chemical systems

The most attractive advantage of chemical consolidated completions is that they lend themselves to overall

completion flexibility - and eliminate the need for major rig re-entries.

Chemical through tubing sand control techniques employ chemical resins or resin coated gravel (or a

combination) injected into the formation to provide in-situ grain to grain bonding.

a. Chemical consolidation

A follow up on chemical consolidation techniques has led to the successful application of several new chemical

consolidation systems suitable for both initial and remedial through tubing sand control. Chemical consolidation

is routinely carried out in SPDC (and to a lesser extent in Shell Gabon) where it has proven to be an efficient,

cost effective alternative to gravel packing.

Design considerations - The success of such treatments is primarily dependent on the quality of the formation

and effective coverage over the completion zone. As discussed in Section 13 the operational limitations of

chemical consolidation are presently : BHST temperature (35-110°C) range, reservoir quality (< 20% clay) and

interval length (typically 3m)

One of the main design considerations with chemical through tubing sand control is the volume and geometry of

possible voids behind casing created by excessive sand production. At present these cannot be quantified, nor

can produced volumes be sensibly estimated. This often leads to difficulties with optimal placement design. This

design difficulty is "overcome" in several areas by assuming that general dilation of wellbore material occurs. For

example under certain conditions of rock strength and in-situ stress levels the produced sand volumes may

largely be compensated by a general loosening - redistribution of the rock matrix. Nevertheless through tubing

chemical consolidation is often designed on the basis of pumping excess volumes to allow for inaccuracies in

pore volume or assumed cavity size.

Overflush systems are generally prefered for such applications due to the possible presence of cavities behind

the casing, maintaining completion equipment resin free, operational simplicity and shorter curing times. If a

phase separation system is used, the resin could precipitate out in the cavity, falling to the low side, plugging the

formation and reducing productivity.

Placement methods - Chemical consolidation can directly be injected through the production tubing, a dedicated

work string, coiled tubing or snubbing unit. However as previously discussed in Section 13 the risk of pumping

overflush systems directly through production tubing, and its effect on the future performance of subsurface

equipment (SSD's, TRSSSV's) should be properly addressed.

The following factors should be carefully considered when evaluating chemical consolidation as a remedial tool.

1.Remedial chemical consolidation should be applied through a dedicated work string: coiled tubing or snubbing

unit. Where possible the job design should avoid killing the well. In rare cases where several intervals are

required to be treated sequentially the selective placement tool may be considered

2.Volume to be pumped is a function of the length of interval to be treated and pump rate restrictions. Note that

excessive pressure drop may limit coiled tubing applications to 1 bbl/min (or less).

3.In the event of operational problems contingency must be incorporated to still allow displacement of the work

string.

4.Depth control and wellbore deviation.

Utilising coiled tubing may present some limitations with respect to depth control when employing treating

packers. For such applications above 60° deviation "stiff" work strings (eg snubbing strings) may be more

appropriate.

5.Repeated consolidation attempts are generally not recommended as an excessive drop in permeability will

occur.

6.Unless an existing zone is expected to suffer high skin values, re-perforation of the completion interval should

not be considered.

7.Once a massive sand failure has occured, wells repaired with chemical through tubing sand control are

statistically likely to fail on average after 3-6 years. Additionally, the time to second failure is generally much less

than the time to first failure.

8.Treatment of an interval with chemical through tubing sand control before massive failure occurs generally

leads to longer sand "free" production.

9.For longer treatment intervals it is possible to carry out chemical consolidation by modifying the chemical

systems to facilitate self diverting.

b. Resin coated gravel

This technique evolved due to the lack of success of early chemical consolidation systems. A typical operation

involves cleaning out existing perforations (and the wellbore) and squeezing sufficient quantities of resin coated

(applied on the fly or during the manufacturing phase) gravel through a dedicated work string into perforation

tunnels. Essentially this type of operation is a one step pre-pack and consolidation.

Most of the commercially available systems are run in water based carrier fluids. The slurry is typically batch

mixed and squeezed through perforations until screen out occurs. After curing, surplus gravel is drilled and

cleaned out prior to putting the well back on production. For a successful treatment sand must be placed behind

every perforation, which is an inherently more difficult operation than for example uniformly placing epoxy resins.

5.5.3 Planning remedial sand control

Despite the fact that the most effective sand control techniques are those implemented early in the life of a well

before sand production occurs, there remains great economic incentive to delay the installation of downhole

sand exclusion (Section 3). However with this development strategy (and subsequent operations philosophy) in

mind, remedial sand control must be "designed" into initial completions to facilitate well management and

maintenance.

In the past, through conventional type completions, the success of remedial treatments was dependant on the

performance of high expansion ratio packers. To date results with such service tools are poor. In this respect

experience has shown that conventional completions using a host of completion accessories restrict access and

future wellbore operations, and ultimately increase operating costs. However, in combination with full bore

completions (i.e. monobore) such tools present opportunities for more reliable, less costly through tubing

workovers.

In summary, if sand exclusion is likely to be a requirement during a wells life, the required functionality should be

specified during the design stage, and not as an after thought.

5.5.4 Remedial operations: liner vibration and rotation

There are a number of remedial operations which may assist in removing pack bridges and voids after gravel

pack installation. This section discusses some systems commonly available.

The civil engineering industry has been using vibration tools to remove voids in setting cement (concrete) for

many years. Since the late 1970's similar tools have been developed for the oil and gas industry and have in

some cases shown to be practical in promoting a tight gravel pack, with claims that vibration reduces the degree

of fluid loss required to fill perforation tunnels and increases perforation pack factors. This section reviews two

commercial systems available from Solum and Dresser Atlas, the former being a work string conveyed systems

the latter wireline conveyed.

5.5.4.1 Solum system

Gravel packing with liner vibration has been adopted as suitable means of improving gravel placement in highly

deviated wells. Rotating the work string throughout the gravel packing operation in high angle wells prevents

gravel from building up on the lower side of the hole reducing the risk of early bridging. A number of workers

have investigated the effects of vibration on the degree of compaction and rate of fluid loss to the formation

during the gravel placement process.

The Solum Vibra-Pak is a rotary compactor liner vibration system which claims to attain maximum gravel

compaction while gravel packing. The tool is designed to utilise rotary power to transmit vibration inside the

gravel pack liner. A tailpipe rotation of 60rpm results in 3 Hz exitation, which is claimed to be effective 10m either

side of the tool.

Vibration maximises compaction by eliminating bridging and promoting hexagonal packing as opposed to the

less dense cubic pack. The tool has also been used to excite casing during critical casing cementations. To date

the tool has been run in over 300 wells (mainly in the US).

However, two spring centralisers placed either side of the tool where severely damaged with the top centraliser

losing 1 blade, and the bottom one losing three. On the basis of this (limited) yard test the following conclusions

where made:

·The tool should be field tested during actual gravel packing operations to fully evaluate its performance.

·Centralisers should not be run with the vibrator.

5.5.4.2 Dresser Atlas

A combined wireline system utilising a nonfocussed gamma -gamma density device coupled with a mechanical

(eccentric-cam) vibrator is available from Dresser Atlas for the identification and correction of gravel pack

deficiencies.

When pack voids are indicated on a first pass log, an eccentric weight located in a vibrator housing is rotated at

high frequencies (typically 3000 rpm) which transmits mechanical energy through the well bore fluids to the

"packed" annulus. The housing protects high speed collision of the weight and liner. Vibration induced

turbulence within the pack combined with gravitational forces disperse sand bridges and increase pack. It should

be noted that several passes are normally required.

6 Other mechanical sand control methods

6.1 Non-gravel packed screen

6.1.1 Scope for application

Non-gravel packed screens (SL, WWS or PPS) are probably the oldest form of sand control and success stories

have been claimed by operators in a variety of instances. Some operators report the successful use of non-

gravel packed screens in shallow oil and water wells where the formation sand is typically non-uniform, medium

to coarse grained.

·Formation sand will eventually move into the wellbore causing a productivity decline due to the reduction in

effective borehole diameter. This may be further aggravated by mixing of different sand sizes, clays, shale

debris,etc.

·Slots are susceptible to plugging during installation of the screen and during production.

·Because a bridging criterion is generally used, the slots can erode before stable sand control is achieved.

It has been generally accepted that non-gravel packed screens eventually will cause significant impairment in a

cased hole completion because of high pressure drop in sand filled perforation tunnels. In open hole

completions, this effect is of lesser magnitude due to the absence of perforation tunnels. However some

impairment should result from the collapse of the formation sand around the screen. Another problem is that the

retrieval of a failed screen will be more problematic than in cased hole.

Field experience has generally shown that non-gravel packed screens have limited application. Nevertheless,

this method should certainly be considered in the following cases:

·When more efficient sand control methods are not technically or economically feasible.

·For production testing of exploration/appraisal wells.

·Horizontal wells

6.1.2 Screen dimensions

The OD of the screen should be as large as possible to minimise formation sand movement. A minimum casing

radial clearance should be maintained to allow for wash-over operations if required.

6.1.3 Slot sizing

Recent Group experience with horizontal completions has encouraged the application of slotted liners as low

cost completion technique. It is therefore anticipated that an increase in the use of slotted pipe will be observed,

especially in marginal developments

6.1.3.1 Slot sizing criteria

For gravel pack applications an absolute stoppage criterion has to be used to retain high permeability gravel in

place.

For non gravel pack applications applying the same criterion has not been previously possible since machining

slots smaller than 0.012" proved impractical for conventional machine tools, due to high tool breakage costs.

Hence wire wrapped screens have been used almost exclusively.

To overcome this problem Coberly investigated the possibility of formulating a bridging criteria. This work carried

out in the late 1930's concluded that stable sand bridges would form over slot widths twice as large as the 10

percentile formation sand grain diameter. Field experience however, proved that too much sand may be

produced prior to the formation of bridges. Hence the generally (industry) accepted criterion is to size slots equal

to, or smaller than the 10 percentile sand grain size.

With solid state laser technology high quality slots (0.006) are achievable in all commonly used materials for oil

tubulars, including stainless steels.

6.1.3.2 Required inflow area

For all applications the ideal inflow area should be maximised. In other words the largest number of uniformly

distributed slots should be aimed for, without affecting the mechanical integrity of the liner, bearing in mind the

mechanical loading during installation (dog leg severity etc) and operation (packer setting forces etc). However,

there is a clear economic incentive to minimise the number of slots to be cut, as costs are proportional to the

number of slots.

6.1.4 Prepacked screens (PPS)

The concept of using PPS alone for sand exclusion is not new. Successful reports of this technology were first

documented by the water production industry during the 1930's. The main application was then viewed to be in

shallow, relatively coarse, clean reservoirs where formation slump, it was claimed resulted in a "reasonably"

uniform pack.

More recently, offshore (and onshore) oil and gas developments have required highly deviated and horizontal

wells which have posed major gravel placement problems. In search of insurance against the increased

possibility of pack voids and ultimate gravel pack failure, many operators led the resurgence of PPS during the

early eighties. Essentially the PPS was used to replace the WWS in such applications, installed to act as the last

line of defence against sand production and possibly the loss of the well or costly remedial action.

There are a number of PPS designs marketed by major screen manufactures and service companies. Generally

they are constructed either as a screen on which gravel is bonded or as concentric screens packed with gravel.

6.1.4.1 Industry experience with PPS

A number of authors have observed numerous problem areas associated with the use of PPS. Initially such

screens suffered from damage (eg cracking) to the gravel coating while running into the well. To some extent

however this can be alleviated , if catered for in the well design (minimising doglegs etc) and proper screen

protection through adequate centralisation.

A potentially more serious problem is that PPS are very susceptible to plugging (fines,scale, pipe dope etc) while

running in hole and/or during circulation and during production due to fines mobilisation. Hence, it is

recommended to run PPS open ended to reduce the risk of screen impairment. Proponents of PPS claim good

completion practices (eg. clean wellbores, filtered completion fluids etc) help overcome this problem.

Nevertheless, industry studies have shown that very small volumes of fines in the completion fluid result in major

plugging problems.

General observations

Success stories on the use of PPS alone as a primary form of sand control have been reported. However it

should be re-iterated that judging the performance of such sand control measures is always difficult, as in such

cases the need for PPS as the main form of sand control is debatable. Nevertheless, there is no doubt that PPS

have been successfully used in unconsolidated reservoirs containing clean, well sorted, large grain sands and

gravels. Their application in poorer quality reservoirs must however be viewed with caution as formation slump,

or natural packing will lead to an area of low permeability around the screen. This reduction in permeability will of

course be a function of grain size, sorting and clay content. It should be noted that this effect can be reduced by

minimising the clearance between the screen and wellbore. In general PPS application should be viewed in

conjunction with gravel packing where the risk of failure due to an incomplete pack, and the cost of an

unplanned workover is relatively high.

6.2 External casing packers

A novel method, claimed to prevent sand production, was published in the middle seventies, and is based on the

use of inflatable External Casing Packers, (ECP's), positioned across reservoir intervals. The concept behind

this idea is that inflation of an ECP with cement, up to a pressure that approaches the fracture breakdown

pressure, would impose high radial stresses on the borehole wall, and improve sand stability. The cement is

allowed to harden, and the interval is then perforated through the and packer to establish production.

The use of ECP's is well known as an efficient method of zonal isolation. The packer will only improve

perforation stability if, under given field conditions, it causes a reduction in the value of the perforation hoop

stress. This would lead to a lower differential stress at the wall of a perforation, and therefore improve perforation

stability. The packer's influence will depend on the wellbore inclination, the initial stresses and the packer

inflating pressure.

In conclusion:

·Inflatable external casing packers will have an initially favourable effect on perforation stability, except for

vertical or near vertical wells in extensional tectonic/ tectonically relaxed environments. In these cases the

packer has an unfavourable effect.

·For (rare) areas where it can be assumed that all three principal effective stresses are equal, the packer has a

favourable effect on perforation stability.

·For horizontal holes, the packer will have a favourable effect on perforation stability.

·It is possible that the positive effects of the packer pressure may diminish with time, owing to formation creep.

·The favourable effects only delay the onset of sand production. After sand production is initiated, the packer will

have no further influence on preventing continuous sand influx, or massive sand failure.

·The packer cannot be relied upon as an effective long term method for preventing sand influx.

Some of the above conclusions were subsequently confirmed in a high risk (but potentially highly rewarding)

field test with a 40 feet ECP in the Seria field, Brunei. The ECP was installed in a reservoir with a known history

of sand production (nearby wells were all completed with gravel packs).

Following perforation (under drawdown) through the cemented ECP, the well produced initially sand free at a

rate double that of a gravel packed well on the same reservoir. After 3 months of production, water cut started,

and this was followed immediately by massive sand failure. The well was subsequently worked over and a

gravel pack installed