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Chapter 7

Sand Control Sand production is usually associated with wells which are completed at shallow depths of tertiary age. It can also be associated with unconsolidated formations encountered in depths of 16,000 feet or deeper. The appropriate selection of well completion practices is critical in formations that have a tendency towards sand production. Very often problems of sand production are created due to inadequate completion practices. According to Allen and Roberts, 1989 and DePriester, 1972 producing sands are solids that form part of a mechanical structure in formations. Loosely attached fines on the pore surface are always produced and are beneficial because the more they are free to move, the less likely they are to plug the pore by forming bridges across the pore channels. Most sandstone reservoirs can be described as:

Solids smaller than 90 percentile are interstitial fines and are produced when production is initiated.

Solids between 90 and 75 percentile range bear the smallest formation load and are likely to be produced.

Solids larger than 75 percentile are load bearing solids. If a significant part of solids are produced (50-75 percentile) they will certainly present production problems.

From past experience industry has developed a basic strategy for controlling sand production. For example, for oil wells in the Gulf Coast, the limit of sand production is considered to be 0.1 percent or 900 lbs/1000 bbl (28gm/m3). In wells with high GOR (>10,000 scf/bbl) the limit is set at around 10lbs/100bbl (28gm/m3). In areas where costs of well operation is high, such as the North Sea, this limit is set at as low as 5 lbs/1000 bbl (14 gm/m3). There has been no norm for gas wells as it has been difficult to estimate a sand production limit. Historically, the limit for gas wells was determined based on the erosion in chokes, probes etc in flow lines. Sand production causes reduction in hydrocarbon production and reduces life of subsurface and the surface equipment which significantly reduces the economics of the project. In addition to this, sands need to be separated before the hydrocarbon can be transported to the sale point and environmentally safely disposed. This requires expensive facilities, in particular in offshore environment. Some specific problems associated with sand production can be summarised as follows:

Production interruptions are caused by sand plugging of casing, tubing, flow lines or separator. This involves additional expenses with “clean out” workovers to return wells to production.

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Collapse of casing due to changes in overburden stresses within the formation. Casing may buckle due to void space created by the sand production (unsupported casing). Also, casing or liner is subjected to an additional compressive loading as subsidence occurs, which may lead to casing buckling or collapse

Downhole and surface equipment can be damaged due to erosion, thus costing time and money for equipment replacements, spills, clean ups or even a dangerous uncontrolled blow-out.

Sand accumulation in surface lines and equipment leading to abrasive wear on surface controls, valves and pipes.

Sand production causes very serious problems with subsurface safety valves by eroding or jamming them. Operating subsurface safety valves are mandatory under government regulations.

Lost revenue due to restricted or shut-in production. Disposal of produced sand is costly.

In this chapter first the causes of sand production and different techniques used to reduce and or prevent sand production are discussed. Finally, design and implementation of gravel pack technique with Chapter objectives are as follows:

Causes of sand production Different techniques use to reduce and to prevent sand production Design and Implementation of a Gravel Pack (including examples) examples are

presented.

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7.1 Causes of Sand Production Sands and formations fines are produced with formation fluids (oil/water/gas) due to the lack of grain bonding or consolidation. Miocene (tertiary or 5-23 million years before the present) and later sands lack consolidation due to young age. Due to the young age and shallow deposition these sands are prone to movement when disturbed by the produced fluids. Thus, loosely consolidated or unconsolidated sands move when subjected to stresses caused by:

fluid pressure drop, fluid friction and overburden stress.

When the net effect of the above conditions exceeds the formation restraining forces, sand grains and formations fines may be produced. Laboratory studies and field experience suggest that sand production may take place by one or more combination of the following mechanisms: The simplest mechanism is the grain by grain movement away from the formation

face at low fluid viscosities under low pressure. At high fluid flow rates small masses of sand break away leading to rapid failure of

the formation. In cases, where high overburden stress combined with high fluid flow rates exist

formation become fluidised, resulting in the gross flow of sands with the produced fluid.

Semi-competent formations can initially produce fines followed by sands that are loosely bonded. The cause of this sand production is primarily due to fluid friction (drag force). As the reservoir pressure declines the individual sand grains carry more and more over burden stress which eventually causes the bonding (cementing between the sand grains) to fail, leading to sand production.

Changes in fluid phases, such as oil to water and/or gas result in the reduction of intergrain cementing /bonding due to change in interfacial tension. Water has the highest interfacial tension (72 dyne/cm). This phenomenon of sand production (water wet situation) has been observed frequently when the well starts to produce water. Fluidisation of formation may take place in unconsolidated formations (tertiary age) which have excessive overburden stress combined with high pressure drawdown and fluid velocity.

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Among the factors discussed previously fluid velocity plays a critical role in sand production problem as this causes a drag force where separated grains are transported with the flowing fluid. Particle fluid drag force is a function of: particle geometry and diameter, particle density, fluid velocity, interfacial tension, fluid density. The two equations most commonly used to determine the critical flow at which sands of a given geometry and diameter begins to move with the producing fluid are the:

critical transport velocity for oil/water wells and critical transport velocity for gas wells.

CRITICAL TRANSPORT VELOCITY FOR OIL/WATER WELLS

2 23 (9 . . )( )(0.015476 0.19841. ) /( (0.011607 0.14881. ))f s f fV g r r r

V= critical transport velocity(cm/sec) =dynamic viscosity of fluid (mPs) g=gravity (980 cm/sec2)

s =density of sphere (gm/cm3)

f =density of fluid (gm/cm3)

r=sphere radius (cm)

CRITICAL TRANSPORT VELOCITY FOR GAS WELLS In gas producing wells, the appropriate equation to predict critical velocity of formation sand that can be carried up the production string is:

f

fdV

)36.165(052.2

Where, the particle diameter (d) is in inches. These equations help predict whether formation sands will be lifted through the production string or the sands will be accumulated at the bottom of the borehole.

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7.2 Characteristics of Formation Sand In order to control the sand production successfully one need to look at the background of the sand production problem so that potential solutions can be found. Formation sand characteristics which impact sand production include depositional environment, physical and chemical properties. 7.2.1 Physical and chemical properties of formation sands CEMENTATION Sand can be defined in the geological sense as a granular material with particle size ranging from 2 to 0.625 mm in diameter. The primary composition of sand is silicon dioxide (SiO2) although other material is present. The strength of sandstone is given by a process called oregenesis where after deposition sand grains undergo compaction by the overburden pressure of subsequent layers. This process also includes cementation between grains which leads to natural consolidation. The most common cementing material is calcite calcium (carbonate), dolomite (magnesium carbonate) and clays. Some sandstone formations are formed in environments where cementing materials are not available which cause the sand to remain unconsolidated. Most sandstone formations contain some clays and detrital fines. They can be attached to the sand grains or interbedded within the formation or placed interstitially. If sandstones are consolidated by clays as the cementing agent this will results in a very weak formation. POROSITY AND PERMEABILITY Porosity is a measure of void space while permeability is the fluid’s ability to flow in the rock. Porosity and permeability of rocks generally decreases with poorer sorting, tighter packing, more cementing and smaller grains. There are two types of packing: tight packing and loose packing as described in Fig.7.1.

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Fig. 7.1: Idealised comparison of tight pack vs. loose pack.

RELATIVE PERMEABILITY AND SATURATION Relative permeability is a measure of the effective permeability of one phase (oil) in the presence of another phase (water) and a function of relative saturation of individual phases. Most sandstone formations are naturally water wet and the individual grains are surrounded by a thin film of water resulting in about 10% to 30% water saturation. In water drive reservoirs water saturation increases as the production continues. In water drive reservoirs, when water saturation increases sand production also increases.

7.2.2 Sand categories There are three general categories of sands which are related to sands strength:

quick sand, partially unconsolidated sand and friable sand.

Quick Sands:

Quick sands usually refer to completely unconsolidated formations. Sands of this type have no effective cementing capabilities and are only held together by small cohesive force and compaction. Quick sands occur all over the world, including California, Libya, Venezuela and Nigeria where tons of sands are produced from wells each year. This type of formation usually produces sands of constant concentration. Special gravel packs with screen liners could be used to control sand production (to be discussed later).

Partially Consolidated Sands:

Sand grains are partially cemented making the formation weakly consolidated. When production is initiated the formation crumbles behind the casing to form small cavities which cave in to fill the rat hole and form bridges in the tubing (rat hole meaning a hole that extends below the perforation level). Because of partial consolidation, these formations show inconsistent sand production. If production is continues without sand

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control measures, shale beds in close proximity may collapse to form a mixture of clay and sand which is harder to control and thus reduces near well bore permeability (as discussed in chapter 1).

Friable Sands:

Friable sands or fragile sands are semi-competent and susceptible to movement. Core analysis from this type of formation appears to be consolidated enough not to produce sand; however, when fluids are produced the face of the formation will produce sand.

7.2.3 Formation Sand Analysis Usually sand types are identified by core sampling. Types of sand also can be identified by monitoring the concentration of sand produced and logs which measure the relative strength of rocks. The following guide can be used to identify sand types:

Quick sand: usually indicated by a constant sand production. Partially unconsolidated sands: indicated by fluctuating sand concentration. Friable sands: indicated by a high initial sand concentration, this eventually tapers

off. Figure 7.2 describes the relation between sand production and stress.

Fig. 7.2: Regimes of sand production.

Types of information essential for characterising sand include:

regional geology, statigraphy and hydrodynamics.

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Other sources with respect to formation evaluation include:

wire line logs, porosity analysis, completion histories, temperature and pressure, permeability, porosity and saturation and production histories.

The most essential design parameter for sand control, however, is the determination of formation sand size and their distribution. This information is used to decide which sand control measure is most suitable. Formation sand size and the respective distribution can be best obtained by:

collecting a formation sample, conducting sieve analysis plotting data on a cumulative weight and frequency diagram.

FORMATION SAND SAMPLING

To acquire a representative formation sand sample entire interval through the field must be considered as the formations are heterogenous in nature. Samples are usually obtained through:

coring with rubber sleeve core barrel, side wall cores and bailings.

Rubber Sleeve Core Barrel:

The most representative of actual size can be obtained from rubber sleeve core samples which are undisturbed and provide accurate information about the lithology. This form of sampling is expensive. Sidewall cores: Sidewall coring can be carried out where there are no full size cores available. Due to their small sample size these samples are less representative than full cores.

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Bailings:

Where there are no cores (full cores or sidewall cores) available, produced sands or bailings can be used in order to characterise sand size and size distribution. Because of intermixing and tendency for larger grains to settle at the bottom (including the rat hole) these samples cannot be related to any statiography or depth. Sometimes it is better to have some form of information than no data. SIEVE ANALYSIS Sieve analysis involves sorting of sand grains of similar sizes using a series of sieves. Prior to conducting sieve analysis, it is important to clean and dry the sample (core or bailings). It is also important that the samples do not break during preparation. Then the weighted sample is placed on the top sieve of a series of sieves (see Fig. 7.3) which are shaken either mechanically or sonically. The screens are arranged progressively finer mash as the sample moves downward by combination of gravity and shaking. Materials (grains) retained in each sieve are weighed and plotted in a cumulative weight vs. diameter graph.

Fig 7.3: A series of sieve showing sieve analysis procedure.

Common measuring scales for mesh are the opening sizes of the screens as presented in Table 7.1 whose typical ranges for sands, silts and clays are given in inches and millimetres. For example, mesh 8 corresponds to 8 openings for every linear inch as shown in Fig. 7.4 and has the opening diameter of 0.094 inch (2.380 mm).

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Fig. 7.4: Sieve opening and its relation with US mesh.

Interpretation of grain size and grain size distribution: A typical grain size distribution is presented in Fig 7.5. Where d10 represents sand in the 10 percentile on the distribution scale and is described as the point on the distribution scale where 10% of the sand size (by weight) is of a larger grain size. Where d90 represents sand in the 90 percentile on the distribution scale and is described as the point where 90% of the sand (by weight) is of a larger grain size and 10% of smaller grain size.

Fig. 7.5: Graph for grain diameter vrs. cumulative weight percentage.

Sand size distribution varies greatly from formation location to formation location. An example is shown in Fig. 7.6 where the left-hand graphs show a wider grain size distribution (non-uniform grain size distribution) whereas right hand curves show grain size distribution in a narrow band (uniform grain size distribution).

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Fig. 7.6: Cumulative sand size distribution for different sand sorting.

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Table 7.1: Sand sieve sizes

US Mesh D[mm] D[in] 4 4.670 0.187 5 4.000 0.157 6 3.360 0.132 7 2.830 0.111 8 2.380 0.094

10 2.000 0.079 12 1.680 0.066 14 1.410 0.056 16 1.190 0.047 18 1.000 0.039 20 0.841 0.033 25 0.707 0.028 30 0.595 0.023 35 0.500 0.020 40 0.420 0.0170 45 0.354 0.0140 50 0.297 0.0120 60 0.250 0.098 70 0.210 0.0083 80 0.177 0.0070 100 0.149 0.0059 120 0.125 0.0049 140 0.105 0.0041 170 0.088 0.0035 200 0.074 0.0029 230 0.063 0.0025 270 0.053 0.0021 325 0.044 0.0017 400 0.037 0.0015

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7.3 Sand Control Methods Production of sand can be controlled by one or combination of the following methods:

1. reducing drag force, 2. bridging (mechanical) and 3. increasing formation strength (chemical).

7.3.1 Reducing Drag Force Reducing drag force on sand grains by production fluid makes sand grains less prone to movement and hence is a more effective method of avoiding sand production problems. Drag force can be reduced by:

increasing flow area and restricting production rate.

INCREASED FLOW AREA When flow area increases, the velocity of the produced fluid decreases. This can be achieved through a number of ways:

large perforation interval, large number of perforations by increasing perforation density, large diameter, long and clean perforations and a long conductive path some distance into the reservoir by hydraulic fracturing.

Good Engineering practises use perforation charges that produce large diameter long holes and less debris which is an effective method to controlling sand production.

RESTRICTING PRODUCTION RATE

Reducing production rate is, will in essence, reduce the drag force on the sand grain by the producing fluid velocity. According to Allen and Roberts, 1982 it is possible to control sand production by carefully observing sand production rate with fluid production rate which is known as the “Bean-up” technique.

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7.3.2 Mechanical Method of Control Mechanical sand control provides a physical barrier to sand movement while allowing fluid to flow across passages. In the rock, physical barrier is provided either by

a screen, a combination of a screen and gravel pack

The flow passages through screen or gravel or gravel pack and screen must be small enough to stop the formation sand but large enough to achieve adequate well productivity. The flow passages are reduced with time due to plugging by clays, asphaltenes, wax and scales. Choice of sand control method depends on specific well characteristics which include:

grain size distribution, interval length, bottom hole temperature and pressure, clay content, well bore deviation, mechanical deviation, mechanical configuration, anticipated production rate and economics.

SCREENS

Screens are effective in controlling sand production from formations which are composed of clean large grained sands with very narrow grain size distribution. These are primarily water wells. Oil and gas wells are much deeper and formation sands are smaller grained, poorly sorted and often contain clay sized particles. This type of sand plugs the screens. Use of screens as a sand control technique has a number of inherent problems. They include:

slots or openings are eroded before sand control is achieved, well productivity is reduced due to sand plugging, because screens have always annular gap (between screen and formation)

formation collapses and fills the annular gap which leads to sand movement and causes intermixing of sands and

intermixing of sands results in reduction in near wellbore permeability(please refer to chapter 2).

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DESIGN CONSIDERATION OF SCREEN/LINER

The screen/liner must be designed to effectively trap formation sand while retaining maximum productivity. This is achieved by selecting appropriate size (slot width), geometry and density of slots to trap the larger grains which inturn stop smaller grains. Smaller grains are trapped in the interstices of larger grains. Hence, important design considerations for screens are as follows:

slot opening and slot geometry, ratio of screen outside diameter to well bore inside diameter and slot spacing, orientation and density.

Slot Opening and Slot Geometry:

Once the formation has collapsed behind the screen, the largest sand grains tend to form a trap fro th smaller grains entering the screen. These large grained sands must be stopped by the screen slot opening. Common slot openings used by the industry are:

Parallel face opening and V-shape opening

Parallel Face Vrs. V-Shaped Opening:

In Figures 7.7 and 7.8 describe the bridging mechanism of sand grains at the slot opening. In the case of parallel slot openings sand grains are likely to bridge across the opening, thus forming a plug, whereas in a V-shape opening sand grains are stopped at the entry and form bridges at the face of the slot opening. Because of the V-shape, sand grains entering the slot can easily pass through the slot and prevent formation of bridges across the opening. This V-shape is achieved in wire-wrapped screens by using trapezoidal cross section wire (see Fig. 7.9).

Fig 7.7: Parallel face screen slot opening and sand bridging across the slot opening.

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Fig 7.8: V-shaped screen slot opening and sand bridging at the face of the slot opening.

Fig. 7.9: Wire Rap Screen with continuous trapezoidal wire.

Slot Size (width):

Common industry practice of selecting slot size is based on correlations. These correlations are derived from laboratory experiments and field experience. Most widely used correlations are the work of Coberly,1930 and Wilson, 1938. Coberly’s correlation is based on average sorting of formation sand where slot width, w is determined from the correlation: W=2 10d ( 10d is the 10 percentile of formation sand)

The above correlation is based on the understanding that the sand grains form stable bridges on slots that are twice the size of 10 percentile of formation sand. Following

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Coberly’s work Wilson derived a correlation for the Gulf Coast where the sand grains tends to be more uniform as: W=2d10 This means that the width of the screen should be selected based on 10 percentile of the formation sand. Gill, 1937 suggested a more conservative correlation to select slot width as: W= 10d

A general rule was provided by DePriester,1972 in relation to selecting slot size for formation for which very little information is available. To avoid plugging the minimum slot width should be 0.05 inch. If the 20 percentile of sand is less than 0.05 inch then an alternative approach should be adopted.

2005.0 dwinch

Slot Spacing and Slot Orientation:

The two basic types of screens which are widely used by the industry are slotted pipe and wire wrapped. On tubing slots of various patterns are milled to produce screen. These patterns include taggered, multiple vertical and horizontal patterns (see Fig.7.10)

Fig. 7.10: Horizontal, staggered and multiple vertical patterns (clockwise).

Comentario [A1]: Why are these the same?

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Wire wrapped screens are manufactured in many forms which include ripped welded, grooved and wrapped on pipe. They are made as continuous slot on outside of the pipe that has already milled or machined holes or slots. The wrapping wire is usually made of 403 stainless steel and the core pipe is usually grade S or K. The typical screen sizes used by the industry are presented in Table 7.2.

Comentario [A2]: One for H2S? explain

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Table 7.2: Typical dimensions of slots for slotted and wire wrapped screens (dimensions vary with different manufacturer)

Pipe size

Slotted pipe Wire Wrapped Ribbed(Channel)

All-welded ribbed(Channels)

OD 0.010 0.020 0.030 0.010 0.020 0.030 0.010 0.020 0.030 1-1/2 0.4 0.8 1.3 4.9 9.3 13.0 9.0 16.4 22.0 3-3/8 0.5 1.1 1.6 5.7 11.4 17.0 10.8 19.7 27.1 2-7/8 0.7 1.3 2.0 6.9 13.8 20.7 12.7 23.1 31.8 3-1/2 0.9 1.8 2.7 8.4 16.8 25.3 15.1 27.4 37.7 4 1 2 3.1 9.4 18.8 28.2 17 30.8 42.4 4-1/2 1.1 2.3 3.4 10.6 21.2 30.6 18.8 34.2 47.1 5 1.3 2.5 3.8 11.8 23.6 35.6 20.7 37.6 51.8 5-1/2 1.4 2.8 4.1 13.0 26.1 37.8 22.6 41.1 56.5 Special applications of screen liners include:

highly deviated and horizontal wells where gravel placemtn become cumbersome and

wells completed in semi-competent (not fully consolidated) reservoir formation where moderate sand production is expected.

GRAVEL PACKS

Gravel Pack refers to uniform graded commercial sand placed between the wellbore and slotted screen to retain formation sands from movement. Figure 7.11 describes a typical gravel pack. The main advantages and disadvantages of a gravel pack are described in Table 7.3.

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FORMATION SAND GRAVEL

Oil

flow

up

Fig. 7.11: Schematic of gravel packing.

Table 7.3: Advantages and disadvantages of gravel pack Advantages Disadvantages

1. Effective control of formation sand as sands are stopped in pores formed by gravel

2. High production is achieved because the near wellbore permeability remained mostly undamaged due to grain intermixing

3. Screen is subject to less erosion 4. No chemical reaction involved 5. Regulating acid wash is feasible

1. Effective flow diameter of the wellbore is reduced

2. Zone isolation is not feasible 3. Screens are susceptible to

erosion

Key to the successful sand control using gravel pack includes:

selection of gravel size, selection of screen type and slot operating, selection of gravel pack interval, gravel placement and selection of gravel pack to minimise formation damage.

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Selection of Gravel Size:

Selection of gravel is important as the gravels must form interstices which can effectively trap formation sands and prevent sand movement. Similar to selection of screen slots gravel size is determined based on well established correlations. These correlations are summarised in Table 7.4. Table 7.4: Summary of gravel to formation sand relationships (correlation) developed by the industry: Authors Pack sand Formation sand Rule Coberly and Wagner 1937

Narrow Broad 1010dD

Gumpertz.1940 Narrow Broad 1010dD

Hill,1941 Narrow Broad 108dD

Department of US Agriculture,1952

Narrow Broad 505050 8.35.6 dDd

Depriester,1957 Broad Broad

5050 8dD

9090 12dD

9010 3dD

Stien,1969 Broad Broad 1585 4dD

Soucier,1974 ------ ------ 5050 dD

Note: D is the diameter of gravel and d is the diameter of formation sand. From the table, it is apparent that the diameter of the gravel sands is selected by matching the diameter of certain percentile formation sand. Rules for selection of gravel sands are based on the correlation present in Table 7.4 which varies significantly and that the distribution of sand size is described by a particular percentile on the distribution curve. To overcome the above anomaly of the selection process, Schwartz,1969 came up with a unique uniformly coefficient, C. The uniformly coefficient is determined by comparing 40 percentile formation sand (d40) with 90 percentile formation sand (d90) as:

90

40

d

dC

C<3, sand is considered to be uniform C>5, sand is non-uniform C>10, Sand is very non-uniform. For the above uniformity coefficient Schwartz gave the following correlations:

for uniform sand(C<5), 1050 6dD

fro a non-uniform sand (C>5), 4040 6dD

The flow velocity for all the range of C should not exceed a critical value and be calculated as: Flow velocity = production rate (ft3/sec)/ 50% of the open area of slots, ft2.

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Gravel Thickness:

Gravel thickness is also an important factor which affects productivity of the well. Through laboratory experiments, Allen and Roberts, 1989 have shown that the gravel pack thickness of 3 to 4 grain diameter should be sufficient in order to stop sand movement. In practice, however, a thicker gravel pack is needed for effective sand control. Sage and Lacey, 1942, based on their work, provided a rule for gravel thickness. Based on their work it is clear that 3 inch or greater gravel pack thickness is required to effectively control sand production.

Mixing Gravel with Sands:

The work of Sperlin, 1972 has shown that the mixing of gravel sand with formation sand reduces pack permeability significantly. With 100% of gravel sand (no mixing of gravel sand with formation sand) the pack sand has shown to have the highest permeability whereas with 50-50 mixing of gravel and formation sand a 20 fold reduction in permeability can be observed. Based on this experiment it is recommended that well sorted gravel sand should be used as pack sand.

Physical Properties of Gravel Sand:

In addition to gravel diameter, suitability of gravels for a sand control job depends on a number of physical properties. They include:

roundness or sphericity, grain strength, acid solubility, uniformity, presence of clay and clay size materials and wetness.

Gravels should have a uniform geometry with roundness or sphericity 0.6 in Krumbein scale or better. Flat or angular geometry reduces the porosity and hence reduces trap capacity of gravels. Gravels should have strength greater than 2000 psi so that they do not breakdown on formation stress and produce clay size particles. Presence of clay size materials reduces gravel pack permeability. Usually turbidity is used to determine the presence of clay and the turbidity should be less than 1. Gravel should have a Uniformity Coefficient, C=1.5. Materials finer than 1.5 (C=1.5) would have little effect on the control of sand movement. Gravel should also be resistant to acid as often acid treatments are employed to clean up of sand pack to remove pore blockage (formation damage).

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Finally, gravel should be water wet to increase the effective permeability. It has been shown that when water wetness increases relative permeability to oil increases greatly (Williams et al, 1972).

PLACEMENT OF GRAVEL PACK

Gravel pack can broadly be classified as:

1. inside pack between casing and screen 2. outside pack between formation and casing 3. combination-inside and outside casing packing 4. open hole gravel

Inside Gravel Pack:

A slotted or wire wrapped screen/liner is placed inside the casing as shown in Fig.7.12. Various methods are used to place the gravels between the casing-screen annular gap.

Fig. 7.12: Inside pack.

Outside Casing Gravel Pack:

Perforations are cleaned and washed prior to the gravel placement. Then slurry (mixture of gravel and viscous fluid) is circulated under pressure to squeeze the gravel behind the casing as shown in Fig.7.13. Under pressure the gravel transport fluid squeezed into the

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formation leaving the gravel dehydrated. The gravel transport fluid must be carefully selected in order to avoid formation damage due to fluid invasion.

SCREEN

CASING

Fig.7.13: Outside pack.

Combination of Inside and Outside Casing Pack: Combination packs involves 3 steps: 1. washing behind the casing, 2. placing the gravel behind casing, 3. finally, placing gravel between the casing and screen as shown in Fig.7.14. The combination pack provides an effective sand control and is widely used in the industry.

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SCREEN

CASING

PERFORATIONS

Fig. 7.14: Combination- washout technique.

Open Hole Gravel Pack:

Open hole gravel pack involves underreaming to enlarge the hole diameter and then placement of gravel between the open hole section and screen/liner (see Fig.7.15). This technique provides a thick gravel pack with large unrestricted flow area and is very effective in controlling sand production and increasing oil/gas production. Since it is not possible to produce from multiple zones simultaneously, open hole completions are often used to control sand production fro each completion.

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SCREEN

GRAVEL

Fig. 7.15: Inside pack-open hole completion-under reamed.

Under Reaming of Open Hole Section:

A hole opener is used to under-ream the pay section as shown Fig.7.16. About 4 to 6 inch on the diameter should be under-reamed to provide sufficient annular gap for the gravel pack.

Fig.7.16 Under reaming for open hole gravel pack.

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Washing Behind the Casing:

In order to provide thick gravel pack, perforations are required to be washed. A typical washing technique (after Tausch and Corley, 1958) or a cup type (after Allen and Roberts, 1989) can be employed to wash behind casing and perforation tunnel (see Fig.7.17a)

Fig. 7.17a.: Washing of perforations.

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Fig.7.17b: Cup type perforations.

According to Tausch and Corley, 1958 a wash pipe is run with a packer which is set half way through the perforation interval. Brine is pumped through the annulus into the perforation and back to the surface through the wash pipe as shown in Fig.7.17a. In a cup type washing technique, a ball is dropped before circulation commences. A selective circulating sleeve is used to direct brine to the perforation tunnel via wash pipe. Brine and sand mixtures are recovered back to the surface via the annular gap (casing and wash pipe, see Fig.7.17b). Telltale: Telltale is a device to indicate the position that can not be easily seen, in this case a short section of the screen/liner located below the screen. A seal sub is installed between the telltale and the screen to seal the wash pipe therefore ensuring that the return is from the telltale only. The objective is to direct the gravel to the bottom of the screen to achieve a tight pack.

Crossover Tool:

Crossover tool is used to divert down flowing slurry (mixture of sands and fluids) to the outside of the liner and flowing fluid into the annulus to return to surface. The slurry is pumped down the tubing, as shown in Fig.7.18, through the crossover tool into the casing-screen annulus. This prevents the gravel to travel back to surface.

Eliminado: at the

Eliminado: Selective

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Fig. 7.18:Crossover tool.

Gravel Blending Unit:

The surface equipment needed to run a gravel pack job is a truck mounted fluid tank, a mechanical mixture/blender, a slurry tank (gravel and fluid mixture) with a positive mechanical injection device shown in Fig.7.19. Fluid (viscous brine) is injected from the tank into a jet which sucks the gravel from the mixing hopper and mixes the gravel with the viscous brine (slurry). The slurry is then directed to the well head. Usual pumping ranges from 2 to 6 barrels per minute.

Fig. 7.19: Gravel blending unit (Courtesy of Solum Oil Tool Corporation).

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SLURRY CIRCULATION TECHNIQUE

Gravels are placed down hole by a number of techniques which include:

gravity circulation, normal circulation reversed circulation and squeeze gravel pack

Each technique is unique for certain bottomhole conditions and has associated advantages and disadvantages. Selection of appropriate circulation technique is important for effective sand control.

Gravity Circulation:

In gravity circulation the slurry is dumped down the casing and allowed to settle (see Fig.7.20). To allow the gravel to settle, low viscous brine is used as a carrier fluid. This technique results in a poor gravel pack as the different gravel size travel down the well at different velocities leading to segregation of gravel. This results in poor compaction of the gravel and hence is primarily used in shallow water wells. Gravity circulation would be used as a cost effective solution for water wells.

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Fig 7.20: Gravitate placement technique.

Normal Circulation:

In normal circulation, the slurry is pumped down the working string via a crossover tool and the carrier fluid is returned through the annulus as shown Fig.7.21.

Fig. 7.21: Normal Circulation Technique.

It is primarily used for the inside casing gravel pack. Due to the injection pressure, the same gravel may flow through the perforation tunnel. In order to achieve a gravel pack a two step procedure is used: Outside pack and inside pack. The circulation is described in the following steps:

1. Gravel is pumped down the tubing and forced into the perforation as shown in Fig.7.22. The carrier fluid passes through the liner and up the wash pipe leaving the gravel behind.

2. The tubing and wash pipe are then pulled up to some distance and step1 is repeated again allowing more gravel to settle (see Fig.7.23).

Eliminado: a

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3. Figure 7.24 shows the final outcome of repeating stage one. Gravel is tightly packed behind the casing and inside the casing.

4. The second stage is a wash down procedure. It consists of pumping fluid down the wash pipe to displace the gravel, thus allowing the screen to be placed down hole. This procedure is demonstrated in Fig.7.25.

Fig.7.22: Gravel placement behind casing.

Fig. 7.23: Gravel placement inside casing.

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Fig. 7.24: Gravel packed inside and out casing.

Fig. 7.25: The Wash pipe is pushed down with the screen to remove gravel from inside.

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Wash Down Procedure: The wash down procedure, described in Fig.7.25, is also used inside the gravel pack. In the wash down method, the gravel is injected into the perforations before the screen is placed. Then the screen is run into the hole. The assembly is then “washed down” into its final position by circulating brine through the wash pipe and shoe. When the shoe reaches the bottom,circulation is stopped and the gravel is allowed to settle around the screen and liner (see Fig.7.26).

Fig. 7.26: Wash down technique.

Reverse Circulation:

In reverse circulation a conventional water/gravel mixture is circulated down the casing-tubing annulus allowing the fluid to return up the tubing (see Fig.7.27). The slurry flows down the annulus and the gravel is retained on the outside of the screen. The carrier fluid flows through the screen and up to the surface through the tubing. Assemblies for reverse circulation usually involves running in the hole, down hole functional check, packing of gravel to a selected point in the casing, pack off after packing and disengagement. A production packer with an overshot assembly is then run over the polished bore nipple. A prepack should be used as discussed earlier in case of outside casing gravel pack.

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WATER/GRAVEL DOWN ANNULUS

Fig 7.27: Reverse Circulation Technique.

Squeeze Gravel Pack:

In a squeeze gravel pack, the gravel pack assembly is positioned opposite to the completed interval, the packer is set and the crossover tool is opened. The gravel is pumped with a viscous carrier fluid down the tubing via crossover tool into the casing-screen annulus and the perforations under pressure (see Fig.7.28). The viscous fluid is squeezed into the formation leaving the gravel in the annulus as a dehydrated gravel pack. Pumping is continued until screen out occurs. After the circulation of completed excess gravel above the screen is circulated back as the wash pipe is pulled out. This technique is designed for only a short interval of 30 feet or less. The major disadvantage fro the squeeze gravel pack is that the carrier fluid is “squeezed” into the formation causing formation damage.

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SLURRY DOWN WORSTRING

PACKER AND CROSSOVER

CARRIER FLUID LOST TO

FORMATION

DE

HY

DR

AT

ED

GR

AV

EL

SCREEN

Fig 7.28: Squeeze Technique.

GRAVEL TRANSPORTING FLUID CHARACTERISTICS

In order to obtain a successful gravel pack it is important to select fluids with the appropriate properties for effective transportation. Essential characteristics of gravel transport include:

Viscosity, fluid leak off control and density.

Viscosity:

Brine has been the common fluid used for transportation of gravel as it readily leaks off into the formation, thus providing a tighter gravel pack. It must be a clean and contain minimum amounts of any clay-like solids (clear fluid) and its water wettability should not be impaired. Low viscosity has limited transport capability to 0.5-1 lbs gravel/ gallons of fluid with a pump rate of 5 bbls/min. High viscous fluid is needed to increase the carrying capacity characteristic of the fluid. This is achieved by adding gelling agent such as the hydroxyethyl cellulose (HEC) or x-tham gum polymers. For 8 to 9 lbs of HEC in 100 gallons of brine can yield a viscosity of 100-200 mPaS at 100 S-1 and can transport upto 15 lbs of gravel /gallon of brine. By

37

adding beaker fluids this viscosity can be easily broken and recovered by producing fluids (oil and gas) without much formation damage.

Fluid Leak off Control:

Formation damage by transport fluid leak off must be considered. Most viscosity building agents provide good fluid leak off control, however additional fluid leak off control material can be added. Common material used to control fluid leak off include: ground calcium carbonate and oil soluble particles. Ground calcium carbonate provides good fluid leak off by plugging pores of formation sands. Calcium carbonate is also acid degradable such that with acid (HCl) treatment carbonate particles can be removed. Finely graded oil soluble particles are used to control leak off. Fluid Density: Densities of up to 10 lbs/gallon can be achieved in common brines which are adequate to control formation pressure in shallow and low pressure reservoirs. In deeper and high pressure wells brine densities are often required to increase densities up to 19 lbs/gallon. Using calcium chloride, brine density can be increased to 11.4 lb/gal. Calcium and zinc bromide can provide densities between 12 and 19.2 lbs/gal. These brines are expensive and corrosive. In most cases, ground calcium carbonate is added to the common brine in order to increase density between 12 and 14 lbs/gallon and polymers to suspend both gravel and carbonate particles, leading to a cost effective solution,

GRAVEL PACK FOR HIGH ANGLE WELL COMPLETION

When the inclination angle exceeds 60 degrees from the vertical, significant changes must take place in the dynamics of gravel transportation. This can be illustrated by calculating the fall of gravel. For example, 15 lbs/gallon slurry has a fall velocity of approximately 500 ft/min in a vertical wellbore. The slurry fall rate in a horizontal well will become zero. And any angle between 90 degrees and 0 will depends on the cosine of the angle. For a 60 degree inclination angle, in a high angle well slurry flows along the low side of the well forming dunes of gravel as shown in Fig.7.29.

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>60o

Fig.7.29: Dune formation in lateral holes.

Laboratory studies by Maby et al, 1974 using 1/10th scale model show that the packing efficiency decreases rapidly as the hole angle reaches 60 degrees and more (see Fig.7.30). Similar results were also observed by Gruesbeck et al., 1977.

Hole Angle[Degrees from vertical]

100

Unipack tool

Conventional tools

30 60 90 120

20

40

60

80

Fig 7.30: Effect hole angle on gravel packing efficiency(After Maly et al).

Various laboratory and field experience have shown that the transport efficiency of grave can be significantly improved by:

Eliminado: ed

39

Use of flexible baffles in the wash pipe/screen (Unocal). Selection of outer diameter of the casing 0.8 (or more) times greater than the inner

diameter of the screen (Chevron, Halliburton, Dowell and Marathon). Vibration of the liner during gravel packing operation (Solum oil Tool

Corporation). Selection of transport fluid with high viscosity and low fluid leak off. Circulating the slurry using reverse flow.

These measures have been successful in improving transport efficiency however they cannot solve every problem. For example, high viscous fluids can improve suspending gravel, but do not allow the slurry to fully dehydrate when the gravel reaches the perforation interval. Also, the use of reverse flow has the potential to mix formation sand with gravel sand resulting in a looser gravel pack.

7.3.3 Chemical Method of Sand Control In this method, sand grains are consolidated by plastic resin. In general, with the use of plastic resin, strengths between 1000psi and 3500psi of the consolidated sand can be achieved. Measured limits plastic consolidations are:

It is more expensive (per foot) than conventional gravel pack. The formation permeability is reduced due to plastic consolidation. Limited length of interval can be treated (15-30ft). Storage and handling of plastic is hazardous. Plastic resins are subject to chemical degradation (temperature related).

Consolidation by plastic resin involves injection of liquid plastic resin into the formation following a preflush. Then an after-flush is carried out to displace the resin from the pore space and distribute it around the grain surface. Certain resin treatments have an inherent self-activation and others are activated during the afterflush. Thus the procedure for chemical consolidation is as follows:

1. Preflush 2. Resin injection 3. Afterflush

CONSOLIDATION MECHANISM USING RAISIN

Consolidation of formation sand by plastic resin is dependent on a) the type of resin used b) the activation mechanism and c) formation temperature.

40

Type of Plastic Resin:

Commonly resins used in industry are furan, phenolic and epoxy. Furan and phenolic deteriorate over time due to their inherent imperfection (chemical makeup). This can result in stress concentrations leading to premature failure when subject to production related stress (hydrodynamic stress). Expoxy resins on the other hand, deposit around sand grains and spread out to coat the surface and create strong bonding between grains. After consolidation it produces little shrinkage, no harmful polymerisation as a by product and also crosslinks to produce a strong inert plastic.

Activation Mechanism:

Activation is the reaction process by which liquid resin turns into a solid resin, which provides a physical strength to bond individual grains. A catalyst is used to activate the reaction that causes the resin to cure with time and temperature. The two types of activation mechanisms are:

internal and external

In an interval activation system the resin contains a catalyst which activates the curing with time or temperature. Once the resin reaches a certain time or temperature the curing begins. The drawback of this system is that it can put a constraint to the operation. The resin could set prior to proper placement of around the sand grain and therefore can plug the entire interval. In an externally activated system catalyst is pumped with the afterflush. This system has no time or temperature constraints. It is, however, possible that the activator may not contact the resin leaving behind an uncured resin.

DESIGN CONSIDERATIONS FOR PLASTIC CONSOLIDATION

Important considerations in the design and implementation of plastic consolidation include:

resin characteristics, pay thickness, formation characteristics, operational conditions and timing of the treatment.

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Resin Characteristics:

A number of chemical and physical properties of resin must be considered in the sign. They include:

viscosity, adhesion, strength, storage, chemical resistance, formation compatibility and safety.

Resin must remain pumpable during the execution of the job. To achieve this, the viscosity of the resin must not be high. Also high viscosity prevents the resin to be placed deeper into the formation and remain trapped in the formation pores. It should be wet to the sand grain so that it can easily coat the grain to provide a strong bond. It is important that the resin, after forming a coating around the grain, develop a strong tensile strength which can resist formation forces. During the life of the well the resin must be resistant to any chemical deterioration and remain compatible with formation fluid. Its operational safety must meet the HSE guide lines and have a reasonable storage life.

Pay Thickness:

Pay thickness is an important consideration as the resin consolidation is limited to a short interval. If possible, long intervals should be treated at different stages for quality assurance.

Formation Characteristics:

Operational characteristics that are essential include:

heterogeneity, clay content, formation fluid compatibility and temperature

There is a potential for the plastic to bypass sections with low porosity and permeability when intervals with high heterogeneity (porosity and permeability varing vertically or horizontally or both) exist. To ensure good zone coverage it is important to select small section (preferably 30 feet or less) with uniform porosity and permeability for the treatment. Formations with high clay content in the pore space are susceptible for fines mobilisations, resulting in formation damage. To avoid this damage, the formation must

Eliminado: and

Eliminado: varies

42

be treated (acidised) before the consolidation treatment commences. The general rule of them is to avoid consolidation treatment if the formation contains clay in excess of 10%. Formation fluids (water, oil, gas) may contain chemicals that may reduce the effectiveness of the resin. To overcome this problem a preflush must be designed to separate the formation fluid coming into contact with the resin. This will keep the resin free from contamination by formation fluids. Bottom hole or formation temperature influences the activation of catalyst. Selection of plastic system is very much dependent on the formation temperature.

Operational Requirements:

Operational requirements that must be taken into considerations include: storage and handling facilities at the rig, surface facilities such as pumps and rig time.

Rig should have sufficient quality storage facilities so that the resin does not degrade overtime. Appropriate handling facilities are required in order to prevent physical contact with handling crews. Surface facilities should have excess pumping capacity as viscous fluids require high injection pressure in order to be injected into low to average formation permeability. It is also important to consider mixing the resin offsite to reduce rig time which can significantly reduce job cost and prevent contamination at the rig site.

Depth of Treatment:

Most plastic treatments use 200-300 gallons per foot to allow sufficient penetration depth better distribution of the resin and allow well compacted sand matrix with a high strength bonding of grains.

Timing of the Treatment:

Plastic consolidation should be considered at the time of well completion to achieve a good result. This is because sands and clays remain undisturbed and have less chance to intermix due to production of hydrocarbon. The strength of the consolidated sand become high as the sand remain tight rather than loose due to production. IMPLEMENTATION OF CHEMICAL CONSOLIDATION As discussed earlier, sand consolidation by using plastic involves: preflush, injection of resin and an after flush. This sequence is important for a successful execution of the treatment. The pore level displacement process involved during the implementation of chemical consolidation is described in Fig. 7.31.

Eliminado: ,

43

Fig 7.31: Displacement process at pore level.

Preflush:

Preflush is designed to separate resin from the formation fluids and clean sand grains for good bondage. For phenolic and furan resins, diesel oil is used to drive the water and prevent water dilution or acids that may accelerate the settings process. Similarly, for an epoxy resin, all free water must be removed and a large volume of oil and alcohol is used. A typical preflush is presented in the Fig.7.32.

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Fig. 7.32: Plastic consolidation - preflush

Resin Solution Injection:

Once the well and formation grains are cleaned and formation fluid is displaced by the pre-flush, the resin solution of a given volume is injected (see Fig.7.33). The resin solution usually contains solvent, coupling if internally activated - a curing agent and a catalyst or activator.

45

Fig. 7.33: Plastic consolidation – resin injection

Afterflush:

The final step of this process is to distribute the resin around the grain forming a coating deep into the formation. This is achieved by an afterflush of a non-contaminant and an immiscible fluid usually hydrocarbon (see Fig.7.34). The viscosity and density is about the same as the resin to have a uniform displacement. For external afterflush an activator is mixed with afterflush. The amount of afterflush determines the thickness of the resin coating and hence the porosity and strength of the bonding.

46

Fig. 7.34: Plastic consolidation – activator injection

Fig. 7.35: Plastic consolidation - overflush

47

7.4 Exercises Example 7.1 For a formation with the following sand distribution, plot a relationship to determine liner slot size:

Cumulative Weight (%) Diameter (inch) 0 0.04 5 0.03 10 0.02 15 0.015 20 0.012 25 0.0093 30 0.008 35 0.0075 40 0.007 45 0.0065 50 0.006 55 0.0055 60 0.005 65 0.0048 70 0.0045 75 0.0042 80 0.004 85 0.0037 90 0.0035 95 0.0027

100 0.002 Solution

1. Plot sand distribution (see Fig. 7.35). 2. Calculate the value of the uniformity coefficient, C as:

C = d40/d90 = 0.007/.0035 = 2

3. Determine the liner slot width for fifty percentile (D50) and ten percentile (d10)

rule as:

inchdD 12.002.066 1050

4. Plot liner slot distribution line on same curve (see Fig. 7.36).

48

Fig.7.36: Sand size distribution plot as function of diameter.

Fig.7.37: Selection of slot width based on sand size distribution. Selection of a screen or slotted liner, once the slot size has been determined, depends on well conditions. Sawcut slots are cheaper. Wire-wrapped screen permits use of harder,

49

more corrosion-resistant metal. Screens set inside casing usually reduce productivity since fine sand moving through the perforations fills the annulus between the screen and casing. Use of largest diameter screen possible is good practice.

REVIEW QUESTIONS 1. What are the two major factors controlling sand production? 2. What are the three basic sand-controlling mechanisms? 3. How large should the screen slot size (i.e. slot width) be? What is the minimum thickness of a

gravel pack? 4. In planning/designing a gravel-packing job, what are the two key aspects that you should

normally consider? 5. Describe the major parameters which are normally considered in selecting a gravel pack

fluid? 6. What are the advantages and disadvantages of sand control by plastic resin?

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References 1. Allen T.O and Roberts AP, Production Operations - well completion, Workover and

stimulation, 3rd Edition, 1989, Oil and Gas Consultants International Inc., pp 37-59. 2. DePriester, C L, Sand Control - Production Operations Course1- Well Completion,

1972, Soceity of Petroleum Engineering’s Publication, pp 354-401.

3. Coberly, CJ, Selection of Screen Openings for Unconsolidated Sands, API Drilling and Production Practices, 1937, pp 189-201.

4.Coberly,CJ, and Wagner, EM, Some Consideration in the Selection and Installation

of Gravel Packs for Oil Wells, Journal of Petroleum Technology, Aug. 1938 pp 1-20.

5.Wilson, HD, Discussion of Some Consideration in the Selection and Installation of Gravel Pack for Oil Wells, Journal of Petroleum Technology,Vol 20, August1938.

6.Gill S, Discussion of Selection of Screen openings for Unconsolidated Sands API

Drilling and Production Practices, 1937.

7.Gumpertz.B, Screening Effect of Gravel on Unconsolidated Sands, Transactions, AMIE, 1941, pp 76-85.

8.Hill, KE, Factors Affecting the use of gravel in Oil and Gas Wells, Drilling and

Production Practices,1941, pp 134-143.

9.Stein and Hilchie, DW, Estimation of Maximum Production Rate Possible from Friable Sandstone Without Using Sand Control, Journal of petroleum Technology, Sept. 1972, pp 1157-1160.

10. Smith, HF, Gravel Packing Water wells, Water Well Journal, Jan-Feb 1954.

11. Saucier, RJ, Consideration in Gravel Pack Design, Journal of petroleum

Technology, Feb 1974, pp 205-212.

12. Schwartz, RH, Successful Sand Control Design for High Rate Oil and Gas Wells, Journal of Petroleum Technology, Sept 1969, pp 1193-1198.

13. Sage BH and Lacy, WN, Effectiveness of Gravel Screens, Transactions, AMIE

1941, pp 89-107.

14. Sparlin DD and Copeland CT, Pressure Packing with Concentrated Gravel Slurry, SPE 2649, 1972.

15. Williams BB, Elliott, LS and Weaver R.H, Productivity of Inside Casing Gravel

Pack Completion, Journal of Petroleum Technology, April,1972, pp 419-425.

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16. Tausch, GH and Corley CB, Sand Exclusion in Oil and Gas Wells, Drilling and

Production Practices, 1958, pp 66-81.

17. Maly, GP, Robinson, JP And Laurie, AM, New Gravel Pack Tool for Improving Pack Placement, Journal of Petroleum Technology, Jan 1974 pp 19.

18. Gruesbeck, C, Salathiel WM and Echols, DE, Design of Gravel Packs in Deviated

Wells, 10 October 1977, SPE 6805.

19. Thompson, GD, Effect of Formation Compressive Strength on Perforator Performance, Drilling and Production Practices, 1959, pp 249-260.