Cements Cementing Slides v1

8/10/2019 Cements Cementing Slides v1

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Introduction to Cement& the Fundamentals ofCementing Operations

Robert Gordon University 1


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This topic covers

Functions of oilwell cement

API classification and properties of drycement and neat slurry and the effect of

additives on these properties Primary and stage cementing procedures

Equipment used e.g. float collars, stagecollars, surface equipment, centralisers

and scratchers Inner string and liner cementing are

covered



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Cementing is the processby which cement slurry isplaced in the annulus,bonding the casing to theformation.

The conventional methodof doing this is to pumpcement down the casingand displace it around thecasing shoe into theannulus.

A good cement job isessential to allow furtherdrilling and productionoperations to proceed.

Functions of cement in wells



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The most important functions of theinitial or primary cement job are:

To support the casing string;

To prevent the movement of fluids from one formation toanother through the annulus;

To protect the casing from corrosive fluids in theformations.

The cement slurry is able to meet these requirements byproviding adequate compressive strength and lowpermeability when the cement hardens. The critical factor inobtaining a satisfactory cement job is to place the cementcompletely around the casing to prevent channelling.



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A secondary or squeeze cement job...

May have to be done at a later stage to carry outsome remedial work on the well (e.g. sealing offcertain zones, repairing casing leaks). This

involves forcing cement through holes orperforations in the casing into the annulus andformation.Like this



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Planning the cement job

Each cement job must be carefully planned toensure that the correct cement and additives arebeing used, and that a suitable placementtechnique is being employed for that particular

application:

The cement can be placed correctly using theequipment available;

The cement will achieve adequate compressivestrength soon after it is placed;

The cement will thereafter isolate zones andsupport the casing throughout the life of the well.



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There are several classes of cement approvedby the API. The differences between thecements lie in the distribution of the five basiccompounds, which are used to make cement:C3S, C2S, C3A, C4AF, CaSO4.

Compounds (a)

API Class C3S

%

C2S

%

C3A

%

C4AF

%

CaSO

%

FinenessSq cm/Gram

A 53 24 8 8 3.5 1600-1900B 44 32 5 12 2.9 1500-1900

C 53 16 8 8 4.1 2000-2400

D & E 50 26 5 13 3.0 1200-1500

G 52 27 3 12 3.2 1400-1600

H 52 25 5 12 3.3 1400-1600

Classification of Cement



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Classes A and B:These cements are generally cheaperthan other classes of cement and can only be used at

shallow depths where there are no special requirements.Note: Class B has a higher resistance to sulphate thanClass A.

Class C:This cement has a high C3S content and soproduces a high early strength.

Classes D, E and F:These are known as retarded cements

due to a coarser grind, or the inclusion of organic retarders(lignosulphonates). Their increased cost must be justifiedby their ability to work satisfactorily in deep wells at highertemperatures and pressures.

Class G and H:These are general purpose cements whichare compatible with most additives and can be used over awide range of temperature and pressure. Class G is themost common type of cement used in most areas. Class Hhas a coarser grind than Class G and gives better retardingproperties in deeper wells.



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Other types of cement not covered by the API specificationinclude:

Pozmix cement- formed by mixing Portland cement withpozzolan (ground volcanic ash) and 2% bentonite. Very

durable & less expensive than most other types. Gypsum cement- formed by mixing Portland cement with

gypsum, giving a high early strength and can be used forremedial work. It expands on setting and deteriorates inthe presence of water.

Diesel oil cement- a mixture of one of the basic cementclasses (A, B, G, H) with diesel oil or kerosene with asurfactant. It has an unlimited setting time and will only setin the presence of water. Consequently it is often used toseal off water producing zones, where it absorbs and setsto form a dense, hard cement.



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Following tabulated figures are based on:

The need to have a slurry that is easily pumped; A minimum amount of free water

Effects of reducing the amount of mixwater:

Slurry density, compressive strength, and viscositywill all increase;

Pumpability will decrease; Less volume of slurry will be obtained from each

sack of cement

API Cement Classification

API Class MixingWater

Gals/Sk

SlurryWt.

Lbs/Gal

Well Depth(a)

Ft

StaticTemp

deg F

A (Portland) 5.2 15.6 0-6000 80-170

B (Portland) 5.2 15.6 0-6000 80-170

C (High Early) 6.3 14.8 0-6000 80-170

D (Retarded) 4.3 16.4 6-10000 170-230

E (Retarded) 4.3 16.4 6-14000 170-290

F (Retarded) 4.3 16.4 10-16000 230-320

G (Basic Calif) 5.0 15.8 0-8000 80-200

H (Basic GulfCoast)

4.3 16.4 0-8000 80-200

Mixwater Requirements



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Properties Compressive Strength

To support the casing string a compressive strength of 500 psi isgenerally thought to be adequate (includes a generous safety factor).The casing shoe should not be drilled out until this strength has beenattained - referred to as waiting on cement (or WOC).

Development of compressive strength is a function of severalvariables:

Temperature Pressure Amount of mixwater Elapsed time since mixing

With proper accelerators added - the WOC time may be reduced to3-6 hours. Following table shows some typical compressivestrengths for different cements under varying conditions:



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Compressive Strength

Temperaturedeg F

Pressure

(psi)

Typical compressive strength (psi) at 24 hours

Class A & B

Portland

Highearly

strength

class C

APIclas

s G

APIclas

s H

Retarded

class

D,E,F

60 0 615 780 440 325 -

80 0 1,470 1,870 1,185 1,065

95 800 2,085 2,015 2,540 2,110

110 1,600 2,925 2,705 2,915 2,525

140 3,000 5,050 3,560 4,200 3,160 3,045

170 3,000 5,920 3,710 4,830 4,485 4,150

200 3,000 - - 5,110 4,575 4,775

Compressive Strength

Temperaturedeg F

Pressure

(psi)

Typical compressive strength (psi) at 72 hours

60 0 2,870 2,535 - - -

80 0 4,130 3,935 - - -

95 800 4,670 4,105 - - -

110 1,600 5,840 4,780 - - -

140 3,000 6,550 4.960 - 7,125 4,000

170 3,000 6,210 4,460 5,685 7,310 5,425

200 3,000 - - 7,360 9,900 5,920



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Properties of Dry Cement & Neat SlurriesProperties of Dry Cement

Class Aand B

(Portland)

High earlystrength

class C

BasicAPI class

G

BasicAPI class

H

Retardedclass

D,E,F

Specific gravity(average)

3.14 3.14 3.15 3.15 3.16

Surface area(range), (sq cm /

gm)

1,500-1,900

2,000-2.800

1,400-1,700

1,400-1,700

1,200-1,600

Weight per sack(lb)

94 94 94 94 94

Bulk volume (cuft/sack)

1 1 1 1 1

Absolute volume(gal / sk)

3.6 3.6 3.58 3.58 3.57

Properties of Neat Slurries

Water (gal/sack)(API)

5.19 6.32 4.97 4.29 4.29

Slurry weight(lb/gal)

15.6 14.8 15.8 16.5 16.5

Slurry volume(cu ft/sk)

1.18 1.33 1.14 1.05 1.05



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Thickening Times (pumpability)

Thickening Times

Depth(ft)

StaticTempdeg F

Circu-latingTempdeg F

High Pressure Thickening Time (hours:min)

Portland Highearly

strength

APIclass

G

APIclass

H

Retard

class

D,E,F

2000 110 91 4::00+ 4:00+ 3:00+ 3:57

4000 140 103 3:36 3:10 2:30 3:20 4:00+

6000 170 113 2:25 2:06 2:10 1:57 4:00+

8000 200 125 1:40* 1:37 1:44 1:40 4:00+



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Properties Slurry density

Standard slurry densities (shown in anearlier table) may have to be altered to meetspecific requirements (e.g. a low strength

formation may not be able to support thehydrostatic pressure of a cement whosedensity is around 15 ppg). The density canbe altered by changing the amount of

mixwater or by using certain additives. Mostslurry densities vary between 11-18.5 ppg.



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Properties Water loss

The setting process is the result of a dehydration reaction. Ifwater is lost from the cement slurry before it reaches itsintended position its pumpability will decrease and watersensitive formations may be adversely affected. The amountof water loss that can be tolerated depends on the type ofcement job, for example:

Squeeze cementing requires a low water loss since thecement must be squeezed before the filter cake builds upand blocks the perforations.

Primary cementing is not so critically dependent on fluidloss. The amount of fluid loss from a particular slurry

should be determined from a pilot test. Under standardlaboratory conditions (1000 psi filter pressure, with 325mesh) a slurry for a squeeze job should give a fluid loss of50-200 cc. For a primary cement job 250-400 cc isadequate.



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Properties - Permeability

After the cement has hardened thepermeability is very low (


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Cement Additives

Most cement slurries will contain some additives tomodify the properties of the slurry to produce abetter cement job to suit particular requirements.Most additives are known by certain trade namesused by various cement service companies.

Additives used to:

Vary the slurry density; Change the compressive strength;

Accelerate or retard the setting time; Control filtration and fluid loss; Reduce slurry viscosity



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Major Cement Additives



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Accelerators

These are added to shorten the time taken for the cement toset. WOC time is therefore reduced and less rig time iswasted. Accelerators are especially important in shallow wellswhere temperatures are low. In deeper wells the highertemperatures promote the setting process, and acceleratorsmay not be necessary. The WOC time is usually based on the

time taken for the cement to attain a compressive strength of500 psi.

Common types of accelerator used include:

Calcium chloride (CaCI2) 1.5 - 2.0%; Sodium chloride (NaCl) 2.0 - 2.5%; Seawater



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Retarders

In deep wells the higher temperatures will reduce thethickening time of the cement slurry and the cement becomesless pumpable. Retarders are used to prolong the thickeningtime and avoid the risk of the cement setting in the casingprematurely. The bottom hole temperature is the critical factorfor the use of retarders. Above a static temperature of 260 -

275F the effect of retarders should be measured in pilottests.

Common types of retarders used include:

Calcium lignosulphanate (sometimes with organic acids)0.1 - 1.5%;

Saturated Salt Solutions (e.g. seawater)



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Lightweight Additives (extenders)

These are used to reduce slurry density for jobs where thehydrostatic head of the cement may exceed the fracture strength ofcertain formations. In reducing the slurry density the compressivestrength is also reduced and the thickening time increases. The useof these additives allows more mixwater to be added, and henceincreases the yield of the slurry. Such additives are thereforesometimes called extenders.

Common types of lightweight additives used include:

Bentonite (2% -16%) This is by far the commonest type of additive usedto lower slurry density. Bentonite absorbs water, and therefore allowsmore mixwater to be added. It will also however reduce compressivestrength and sulphate resistance. The increased yield due to the bentoniteadded may be seen in cement tables.

Pozzolan This may be used in a 50% / 50% mix with the Portlandcements. The result is a slight decrease in compressive strength, andincreased sulphate resistance.

Diatomaceous earth (10% - 40%) - The large surface area allows morewater absorption, and produces low density slurries (down to 11 ppg).



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Heavy Additives

These are used when cementing through over-pressuredzones.

Common types of additive used include:

Barite (barium sulphate) This can be used to attain slurry

densities of up to 18 ppg. It also causes a reduction instrength and pumpability.

Hematite (Fe2O3) - The high specific gravity of hematite canbe used to raise slurry densities to 22 ppg (friction reducingadditives may be required).

Sand Graded sand (40-60 mesh) gives a 2 ppg increasein slurry density.



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Fluid Loss Additives

Used to prevent dehydration of the cement slurry andpremature setting.

Common additives used include:

Organic polymers (cellulose) 0.5% - 1.5%;

Carboxymethyl hydroxyethyl cellulose (CMHEC) 0.3% -1.0% (CMHEC will also act as a retarder)



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Friction Reducing Additives (dispersants)

These are added to improve the flow properties of the slurry.In particular they will lower the viscosity so that turbulencewill occur at a lower circulating pressure, thereby reducing therisk of breaking down formations.

Commonly used additives include:

Polymers 0.3-0.5 lb/sx of cement;

Salt 1-16 lb/sx;

Calcium lignosulphanate 0.5-1.5 lb/sx



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As well as the compounds deliberately added to the slurry onsurface to improve the slurry properties, there will also be theeffect of the mud downhole which comes into contact with thecement in the casing or in the annulus. The chemicals in themud may react with the cement to give undesirable side

effects. Some of these are listed below:

Mud additive Effect on cement

Barite increases densityreduces compressive strength

caustic calcium compounds acts as an accelerator

diesel oil decreases density

Thinners act as retarders

Mud Contaminants



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Primary Cementing

For a primary cement job the object is to place the cementslurry behind the casing in the annulus. In some cases thiscan be done in a single operation by pumping cement downthe casing, displacing it around the casing shoe and up intothe annulus.

For conductor and surface casing the whole annulus iscemented back to surface.

In longer casing strings (e.g. production casing) the cementjob may be carried out in two stages:

The first stage is completed in the conventional manner asdescribed above, with the exception that the cement slurry does

not fill the entire annulus, but reaches only a pre-determinedheight above the shoe. The second stage involves opening a special tool in the casing

string which allows cement to be displaced from the casingdirectly into the annulus. This is known as stage cementing andwill be discussed in more detail later.



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Downhole Cementing Equipment

Guide shoe:This is run on thebottom of the first joint. It has a rounded

nose to guide the casing past any ledges

or other irregularities in the hole

Float collar:This is positionedone or two joints above the guide

shoe. It acts as a seat for the cementplugs used in the pumping and

displacement of the cement slurry. At

the end of the cement job there will be

some cement left in the casing

between the float collar and the guide

shoe which must be drilled out



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Centralizers

These are hinged metal ribs,which are installed on the casingstring as it is run. Their functionis to keep the casing away fromthe borehole so that there is

some annular clearance on allsides. The proper use ofcentralisers will help to:

Improve displacementefficiency (i.e. placecement all the way around

the casing) Prevent differential sticking Keep casing out of keyseats



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Surface Cementing Equipment

Mixing and Pumping Facilities

On most rigs cement materials are handled in bulk, whichmakes blending and mixing much easier. For large volume

cement jobs several bulk storage bins may be required onthe rig. On offshore rigs the cement is transferredpneumatically from supply boats to the storage bins.



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Cementing Heads

The cement head provides the connectionbetween the discharge line from thecement unit and the top of the casing -designed to hold the cement plugs usedin the conventional primary cement job.The cement head makes it possible to

release the bottom plug, mix and pumpdown the cement slurry, release the topplug and displace the cement withoutmaking or breaking a connection. Forease of operation the cement headshould be installed as close to rig floorlevel as possible. Cement jobs are oftenunsuccessful because the cement plugs

are installed incorrectly or not releasedproperly.

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Cements Cementing Slides v1