44605517 L15 Cementing Lecture 1

7/30/2019 44605517 L15 Cementing Lecture 1

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Cementing: Lecture 1

Fundamentals

of

Cementing Operations



Functions of Cement in Wells

• Cementing is the process by which cement slurry is placed inthe annulus, bonding the casing to the formation

• The conventional method of doing this is to pump cementdown the casing and displace it around the casing shoe intothe annulus

• A good cement job is essential to allow further drilling andproduction operations to proceed



Cementing Operation



Components of Cementing Operation

Float shoeA float shoe prevents cement from flowing back into the casing oncethe cement is displaced behind the casing. Shoes have either innerparts made of aluminum or cement; both being easily drillable, withthe advantage that cement is more resistant to impact.

Float Collar

A float collar is a one way valve placed at one or two joints above theshoe. The float collar provides the same functions as a float shoe bypreventing fluid back flow into the casing: mud backflow duringrunning in hole and cement slurry backflow after cementdisplacement. The distance between the shoe and float collar iscalled Shoe Track.

Wiper PlugsBoth top and bottom plugs are used during cementing operations.They are used to separate the various fluids from one another.



Bottom PlugThe red bottom plug has a shallow top, is made of rubber, and has ahollow core. It is used ahead of the cement slurry to preventcement/drilling fluid contamination and to clean the casing wall of filter cake. After the bottom plug comes into contact with the floatvalve, sufficient pressure (150 to 350 psi) causes the top diaphragmto rupture, allowing the cement slurry to flow through it.

Top PlugThe black top plug has a deep cup on its top and has a solid, moldedrubber core. It is dropped after the cement slurry has been pumped,to prevent contamination with the displacement fluid. The top plugalso signals the end of displacement by forming a seal on top of thebottom plug, causing a pressure increase.

The main functions of cement plugs are:

• Separate mud from cement• Wipe the casing from mud before cement is pumped and then wipecasing fromth cement film after the complete volume of cement is pumped.• Prevent over-displacement of cement• Give surface indication that cement placement is complete

• Allow the casing to be pressure tested



The bottom plug is first released and is followed by cement. Whenthe bottom plug lands on the float collar a pressure increase onsurface is indicated. A small increase in pressure will rupture thebottom plug and allow cement to flow through it, through floatcollar, shoe track, casing shoe and then around the casing.

The top plug is released from surface immediately after the total

volume of cement is pumped. The top plug is displaced by thedrilling fluid and it, in turn, pushes the cement slurry into theannulus.

When the top plug lands on the bottom plug a pressure increase isobserved at surface. This is called bumping the plug. Bumping

indicates that the total volume of cement is now displaced behindthe casing. Usually, at this time, the casing is pressure tested to aprecalculated design value to check its integrity. Pressure testingcasing while the cement is still wet is recommended as thisreduces the chances of breaking the set cement or creating micro-channels if the test is carried out a few hours later when the

cement sets.

Bumping the Plug



Washes & Spacers

Pre- FlushIn any successful primary cementing operation the cement slurrymust displace the fluid surrounding the casing. Mud and cement areoften incompatible and contact between them can lead to severechannelling or the formation of an un-pumpable viscous mass. Toavoid this problem an intermediate fluid is used as a pre-flush toclean the drilling mud from the annulus.

The simplest form of pre-flush is a 'wash' - usually water, with thepossible addition of a surfactant. Such a pre-flush is very effective inremoving mud from the annulus as turbulence can be achievedaround the complete annulus.

Spacers

Spacers are difficult fluids to design. They must be compatible withboth mud and cement and have the correct rheological properties tominimise mixing and channelling. Although weighted spacers cantheoretically achieve turbulence, care must be exercised in assumingthat turbulence will occur beneath eccentric casing, in the narrowsection of the annulus. If laminar flow exists beneath the casing and

turbulent flow above, channelling will result.



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 to

another through the annulus;• To protect the casing from corrosive fluids in the formations

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 cement

completely around the casing to prevent channelling



A secondary or squeeze cement job ..

may have to be done at a later stage to carryout some remedial work on the well (eg, sealingoff certain zones, repairing casing leaks). This

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



Planning the cement job

Each cement job must be carefully planned to ensure thatthe correct cement and additives are being used, and that asuitable placement technique is being employed for thatparticular application:

• The cement can be placed correctly using the equipmentavailable;

• The cement will achieve adequate compressive strengthsoon after it is placed;

• The cement will thereafter isolate zones and support thecasing throughout the life of the well



Classification of Cement

Compounds (a)

API Class C3S

%

C2S

%

C3A

%

C4AF

%

CaSO

%

FinenessSq cm/Gram

A 53 24 8 8 3.5 1600-1900

B 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

There are several classes of cementapproved by the API. The differences

between the cements lie in the distributionof the five basic compounds, which areused to make cement: C3S, C2S, C3A,C4AF, CaSO4.



• 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



Other types of cement not covered by the API

specification include:• Pozmix cement - formed by mixing Portland cement withpozzolan (ground volcanic ash) and 2% bentonite. Verydurable & less expensive than most other types;

• Gypsum cement - formed by mixing Portland cementwith gypsum, giving a high early strength and can be

used for remedial work. They expand on setting anddeteriorate in the 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. They have unlimited setting times and willonly set in the presence of water. Consequently they are

often used to seal off water producing zones where theyabsorb and set to form a dense, hard cement



Mixwater Requirements

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 – Gulf Coast)

4.3 16.4 0-8000 80-200

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 viscosity will all

increase;• Pumpability will decrease;

• Less volume of slurry will be obtained from each sack of cement



Properties – Compressive Strength

To support the casing string a compressive strength of 500 psi isgenerally thought to be adequate (includes a generous safetyfactor). The casing shoe should not be drilled out until thisstrength has been attained - 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



Compressive Strength

Temperaturedeg F

Pressure

(psi)

Typical compressive strength (psi) at 24 hours

Class A & B

Portland

Highearly

streng

thclass C

APIclass G

APIclass 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



Thickening Times (‘pumpability’)

Thickening Times

Depth(ft)

StaticTempdeg F

Circu-latingTemp

deg 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+



Properties – Slurry Density

Standard slurry densities (shown in an earliertable) may have to be altered to meet specificrequirements (eg, a low strength formation may

not be able to support the hydrostatic pressure of a cement whose density is around 15 ppg). Thedensity can be altered by changing the amount of mixwater or by using certain additives. Mostslurry densities vary between 11-18.5 ppg.



Properties – Water Loss

The setting process is the result of a dehydration reaction.If water is lost from the cement slurry before it reaches itsintended position its ‘pumpability’ will decrease and watersensitive formations may be adversely affected. Theamount of water loss that can be tolerated depends on thetype of cement 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 slurryshould 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 of 50-200 cc. For a primary cement job 250-400 cc isadequate



Properties – Corrosion Resistance

Formation water contains certain corrosiveelements, which may cause deterioration of thecement. Two commonly found compounds aresodium sulphate and magnesium sulphate. These

will react with lime and C3S to form large crystalsof calcium sulphoaluminate. These crystalsexpand and cause cracks to develop in thecement structure. Lowering the C3A content of the cement increases the sulphate resistance.For high sulphate resistant cement the C3Acontent should be 0-3%



Properties - Permeability

After the cement has hardened thepermeability is very low (<0.1 millidarcy).This is much lower than most producing

formations. However if the cement isdisturbed during setting (e.g.. gasintrusion) higher permeability may occur(5-10 darcies).



Cement Additives

Most cement slurries will contain some additives to modifythe properties of the slurry to produce a better cement jobto suit particular requirements. Most additives are knownby certain trade names used by various cement servicecompanies. 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



MajorCementAdditives



Accelerators

These are added to shorten the time taken for the cementto set. WOC time is therefore reduced and less rig time iswasted. Accelerators are especially important in shallowwells where temperatures are low. In deeper wells thehigher temperatures promote the setting process, andaccelerators may not be necessary. The WOC time is

usually based on the time taken for the cement to attain acompressive strength of 500 psi.Common types of accelerator used include:

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

• Seawater



Retarders

In deep wells the higher temperatures will reduce thethickening time of the cement slurry and the cementbecomes less pumpable. Retarders are used to prolong thethickening time and avoid the risk of the cement setting inthe casing prematurely. The bottom hole temperature is the

critical factor for the use of retarders. Above a statictemperature of 260 - 275°F the effect of retarders shouldbe measured in pilot tests.Common types of retarders used include:

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

• Saturated Salt Solutions (eg, sea water)



Lightweight Additives (extenders)

These are used to reduce slurry density for jobs where thehydrostatic head of the cement may exceed the fracture strengthof certain formations. In reducing the slurry density thecompressive strength is also reduced and the thickening timeincreases. The use of these additives allows more mixwater to beadded, and hence increases the yield of the slurry. Such additives

are therefore sometimes 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, andincrease sulphate resistance;

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



Heavy Additives

These are used when cementing through over-pressured zones

Common types of additive used include:

• Barite (barium sulphate) – This can be used to attain slurrydensities of up to 18 ppg. It also causes a reduction instrength and pumpability;

• Hematite (Fe203) - The high specific gravity of hematitecan be used to raise slurry densities to 22 ppg. (Frictionreducing additives may be required);

• Sand – graded sand (40-60 mesh) gives a 2 ppg increase inslurry density



Fluid Loss Additives

Used to prevent dehydration of thecement slurry and premature setting

Common additives used include:• Organic polymers (cellulose) 0.5% -

1.5%;

• Carboxymethyl hydroxyethyl cellulose

(CMHEC) 0.3% - 1.0% (CMHEC will alsoact as a retarder)



Friction Reducing Additives(dispersants)

These are added to improve the flow propertiesof the slurry. In particular they will lower theviscosity so that turbulence will occur at a lower

circulating pressure, thereby reducing the risk of breaking down formationsCommonly 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



Mud Contaminants

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 the mudmay react with the cement to give undesirable side effects. Someof 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



Cement Excess

44605517 L15 Cementing Lecture 1

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