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CHAPTER-5

CEMENTING

Oil well cementing falls into three categories.

-primary cementing job on a casing string

-squeeze cementing

-plugs

Primary Cementing:

Casing strings are usually cemented :

-to isolate troublesome behind the casing from deeper formations to be drilled,

-to isolate high-pressure formations below the casing from the weaker shallow

zones behind the casing,

-to isolate producing zones from water bearing sands.

The cement is normally placed behind the casing in a single or multi-stage

technique. The single stage technique pumps cement down the casing and up to

annulus. The heavier cement in the annulus is prevented from U-tubing by back-

pressure valves in the bottom of the casing string. The initial stage of multi-

stage job is usually planned as if it were a single stage effort. Cement is pumped

down and up to annulus. The next stage is pumped through a special port collar at

the desired location up to annulus. The port is opened after the initial stage is

cemented.

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Squeeze Cementing:

A common method for repairing faulty primary casing jobs or performing

remedial operations on the hole is squeeze cementing. Major applications:

-supplement a faulty primary casing cement job,

-reduce water-oil, water-gas and gas-oil ratio

-repair casing leaks,

-stop lost circulation in an open hole while drilling

-bring a well under control.

Placement techniques and slurry design are important considerations

squeeze operations. Supplementing a faulty or ineffective primary casing cement

job is the most prominent application for squeeze cementing.

Cement Characteristics

The cement slurry pumped into oil and gas wells includes cement, special

additives and water. Portland cement is most commonly used. The additives are

used to control characteristics such as thickening time, density and compressive

strength. Water is an important agent in the cementing.

Portland Cement:

Portland cement is manufactured by calcining limestone, clay, shale, and

slag together at 2000-2600 oF in a rotary kiln. The resulting material, clinker, is

cooled and inters ground with small percentages of gypsum to form Portland

cement. In addition to the raw materials, other components such as sand, bauxite,

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and iron oxide may be added to adjust the chemical composition of the clinker for

the different types of Portland cement. The principal components of the finished

Portland cement are lime, silica, alumina, and iron. Each component affects the

slurry in a different manner. When water is added to cement, setting and

hardening reactions begin immediately. The chemical compounds in the cement

undergo hydration and re-crystallization, resulting in a set product. The API has

established a classification system for cements used in oil and gas operations.

Slurry Features

Variables involved in the design of the slurry include:

yield, density, mix water, thickening time, compressive strength, fluid loss

and downhole temperature.

-The yield of the cement in cubic ft per sack, is the volume of space that will

be occupied by the dry cement, water and additives when the slurry is mixed

according to design specifications. A major factor affecting the slurry yield is

the density, since water must be added in significant volumes to achieve low

weight cements that will not fracture shallow, weak zones.

-The density of cement is an important design criterion. It must be sufficient

to prevent kick and blow-outs yet it should not cause lost circulation.

-The mixing water requirements will vary, depending primarily on cement class

and slurry density. Most cement jobs use well site water. Quality of mixing water

is an important parameter in cement planning. The hydration and curing of the

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slurry will react differently with varying amounts of salt, calcium, or magnesium

the mix water.

-Thickening time is the amount of time that cement remains pumpable with

reasonable pressures. This is the perhaps the most critical property in the

displacement process. Factors affecting the thickening time include cement

composition and temperature. The compressive strength is measured in pounds

per square inch. A 500psi minimum compressive strength is generally

recommended before drilling operations resume, but higher strengths are

preferred.

-Temperature affects the compressive strength of the cement. Higher

temperatures reduce the time for the cement slurry to reach some compressive

levels. However, at temperatures above 230 oC, cement strength begins to

decrease.

-Fluid loss is the water lost from the slurry to the formation during slurry

placement operations. If a large volume of water is lost, the slurry becomes too

viscous or dense to pump. Neat cement, or cement with no special additives has

a fluid loss rate in excess of 1000 cc/30 min.

0-200 cc/30 min Good control

200-500 cc/30 min Moderate control

500-1000 cc/30 min Fair control

Over 1000 cc/30 min No control

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Cement Additives

Neat slurry is a mixture of water and cement only. Special chemicals are

often added to the slurry to achieve some desired purposes. These additives are:

Accelarators, retarders, density adjuster, dispersants, fluid loss additives

Accelarators: Most operators wait for cement to reach a minimum of 500 psi

compressive strength before resuming operations. At temperatures below 100 oF

common cement may require a day or two to develop 500 psi strengths.

Accelerators are useful at reducing the amount of waiting-on-cement (WOC)

time. Low concentration of cement accelerators, usually 2-4 % by weight of

cement, shorten the setting time of cement and promote rapid strength

development. Calcium chloride is perhaps the most commonly used chemical for

this purpose.

Retarders: High formation temperatures associated with increased well depths

necessitate the use of chemicals that retard the setting time of the cement; i.e.

increase the pumping time. The most common retarder may be calcium

lignosulfonate. Its effectiveness is limited in temperatures above 200 oF. Other

retarders such as carboxymethyl-hydoxyethylcellulose, can be used to about 240

oF.

Density Adjusters: High formation pressures for neat slurry densities require

additions in cement density. Formations with low fracture gradients require

reductions in cement weight. Dispersants as an additive can increase slurry

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densities to 17.5 ppg due to their effect on viscosity. Adding more water to the

slurry and adding materials to prevent solid separation achieve density

reductions.

Dispersants: Dispersants provide several beneficial features for the slurry.

-reduce slurry viscosity

-allow slurry turbulence at lower pump rates

-assist in providing fluid loss control for densified slurries

Fluid Loss Additives: Fluid loss agents are used in cement slurries for the

following reasons:

-minimize cement dehydration in the annulus

-reduce gas migration

-improve bonding

-minimize formation damage.

Slurry Design

A well plan is not complete until the cement slurry has been designed.

Major aspects of the design are as follows:

-Volumetric requirements for the casing and annulus

-cement

-mixing water

-density selection

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Calculation of slurry density or “weight” usually expressed in pounds per gallon, is

based on the following equation.

Slurry weight = (lb cement + lb water + lb addit.) / (gal cement + gal water

+ gal addit.)

Cement has a bulk density of 94 lb/cu ft, an absolute density of 94/0.48 = 195.8

lb/cu ft and an specific gravity of 195.8/62.4 = 3.14.

The absolute volume of all solid constituents must be calculated in gallons, where:

Absolute Volume, gal = (lb of material) / (8.34 lb/gal x spec. grav. of

material)

The volume of slurry to be realized from 1 sack of cement when mixed with a

specified amount of water and possibly other additives is called the yield. The

yield in cubic feet per sack of cement is :

Yield = (gal cement + gal water + gal additive) / 7.48 gal/cu ft

Example 4-1

Calculate the weight, percent mix and yield or set volume of a slurry given?

Water-cement ratio = 5.5 gal/sx

Spec. Grav. of cement = 3.14

1 sx = 1 cu ft = 94 lb

Density of water = 8.34 ppg

Solution:

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Slurry weight = (lb cement + lb water + lb additive) / (gal cement + gal water +

gal additive)

Slurry weight = [(94 lb/sx + (5.5 gal/sx x 8.33 lb/gal)] / [(94 lb/sx / 8.33 lb/gal x

3.14) + 5.5 gal/sx]

Slurry weight = 15.4 lb/gal

Yield = (gal cement + gal water + gal additive) / 7.48 gal/cu ft

Yield = [(94 lb/sx / 8.33 lb/gal x 3.14) + 5.5 gal/sx] / 7.48 gal /cu ft

Yield = 1.215 cu ft

Absolute Volume, gal = (lb of material) / (8.34 lb/gal x spec. grav. of material)

Absolute Volume = 94 lb/sx / (8.34 lb/gal x 3.14)

Absolute Volume = 3.6 gal/sx

Percent Mix = (5.5 gal/sx x 8.34 lb/gal x 100) / 94 lb/sx

Percent Mix = 48.8 % by weight of cement

Example 4-2

Calculate the number of sacks of cement and bentonite required to obtain

cement returns on surface casing.

Volume of 9 5/8 inch 40 lb/ft casing = 0.4256 cuft / lin ft

Class-A cement with 4 % gel

Water-cement ratio = 7.73 gal/sx

Slurry weight = 14.10 lb/sx

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Casing to be landed at 1400 ft

Excess cement required = 35 %

Solution:

Cement left in casing = 30 ft x 0.4256 cu ft / ft

Cement left in casing = 12.77 cu ft

Cement required to fill annulus = 1400 ft x 0.3469 cu ft / ft x 1.35

Cement required to fill annulus = 655.64 cu ft

Total Cement required = 12.77 cu ft + 655.64 cu ft = 668.41 cu ft

Sacks of cement required = 668.41 / 1.536 = 435 sx

Pounds of cement = 435 sx x 94 lb/sx = 40890 lb

Bentonite required = 40890 x 4/100 = 1636 lb or 163.6 sx

Cement planning involves evaluating and selecting equipment to be used with the

cementing process. The down-hole equipment includes shoes and collars that are run as

integral sections of the casing string. In addition, many cementing aids attached to the

exterior of the pipe may be used, i.e., centralisers, scratchers and cement baskets.

Casing Shoe: A casing shoe is a short, heavy walled pipe run on the bottom of the

casing string. It has a rounded “nose” to guide the casing into the hole. The shoe is

screwed on the casing and generally is “glued” with a thread-locking compound. Casing

shoes are generally available in three types.

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i) guide shoe; ii) float shoe and iii) differential fill shoe.

Figure 4-1 Guide shoe (Courtesy World Oil’s Cementing Book)

Figure 4-2 Float Collar (Courtesy World Oil’s Cementing Handbook)

A guide shoe contains an orifice through the centre that allows mud to pass

freely. A float shoe contains a back pressure valve that prevents mud from flowing into

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the casing from the bottom yet allows fluid to be pumped through the shoe. Float valve

prevents surface casing pressure resulting from cement U-tubing. The driller must fill,

or partially fill, the casing with mud periodically to prevent casing collapse as the

annulus hydrostatic pressure increases with depth. Differential fill shoe are similar in

concept to float shoes.

Figure 4-3 Centralizers (Courtesy World Oil’s Cementing Handbook)

Collars: A cementing collar is typically run as an integral part of the string and is

placed at the top of the first or second casing joint. The collar serves as a stop for the

cement wiper plug so that all the cement is not inadvertently pumped completely out of

the casing and into the annulus. Multi-stage cementing requires special collars with

sliding sleeves and ports. The sleeves are usually closed during the primary stage of

cementing. The sleeves are activated with either the free-fall or displacement

methods.

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Centralizers: Centralizers are placed on the exterior of the casing string to provide

stand-off distance between the well bore and the pipe in an effort to assist in

attaining cement encirclement of the pipe. Numerous types of centralizers are

available. The bow spring type is most common.

Scratchers: To achieve an effective cement job, the slurry must bond to the

formation. Scratchers assist by scraping and scratching the mud cake on the formation

to promote bonding to the virgin formation.

Cement Baskets: Cement baskets provide support for the column of cement while it

cures, or hardens. The baskets are often placed above lost circulation zones that

cannot support a full column of cement.

Plugs: The cement slurry is normally separated from the mud column by plugs that

minimise interface contamination. The bottom plug has a diaphragm that is ruptured

with pump pressure after it seats on the collar or shoe. The top plug has a solid

aluminium insert. The plugs are mounted in a cementing head at the top of the casing.

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Displacement Process

Pumping the cement into the annulus is an important to the successful cementing

program as the slurry design. The displacement rate affects the flow regime in the

annulus. High flow rates convert the flow regime from laminar to turbulent. Although

annular turbulent flow is not desirable in most drilling operations, it is desirable in

cementing operations because it erodes the mud cake on the formation. Contamination

of the interface between the mud and cement is a problem that can reduce the

effectiveness of the cement job. This problem can be controlled by separating the mud

and cement with a spacer fluid (Figure 4-4, 4-5).

Primary Cementing Technique

Primary cementing operations are usually conducted in single or multiple stages.

The single stage method has been used traditionally for conductor, surface,

intermediate and production casing strings. Procedure:

1.Drill hole to desired depth.

2.Pull drill string and run intermediate casing.

3.Circulate hole with rig pump.

4.Attach cementing head with plugs to casing.

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Figure 4-4 Diagram of a casing cementing job (Courtesy World Oil’s Cementing

Handbook)

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Figure 4-5 Equipment typically used to install and cement a drilling liner

(Courtesy BJ-Hughes Services)

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Figure 4-6 Setting and cementing casing (Courtesy Oil & Gas Journal)

5.Connect lines to pump truck and cementing head.

6.Start circulation with pump track.

7.Release bottom plug.

8.Pump spacer to remove mud.

9.Mix cement and displace until all cement is mixed and in casing.

10.Release plug.

a) Release top plug for a single-step job.

b) Release bottom shut-off plug for second-stage job (Figure 4-7).

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11.Pump until sharp pressure increase is noted on pump truck gauge, indicating top plug

has bumped.

Step 12-16 is for stage cementing.

12.Drop bomb, open ports.

13.Circulate any excess cement around the stage tool.

14.Wait at least 6 hr. for cement to gain initial strength.

15.Mix second stage cement and displace until all cement is mixed and in casing.

16.Release top closing plug and displace until a sharp increase is noted on the pump

truck gauge, indicating the plug has bumped.

17.Release pressure to determine if single stage or stage tool is holding.

Liner and Squeeze Cementing

The liner is run on the bottom of the drill pipe with a hanger and setting tool.

Hangers are usually set mechanically or with a hydraulic action. A typical liner assembly

is given. Plugs sweep cement from the interior of the liner to the float collar. If the

primary cement job is nor successful, squeeze cementing will not be required. However,

potential problems must be considered to overcome poor primary jobs. Application for

squeeze cementing in drilling and producing operations include:

-casing shoe; -liner top

-perforation

-plug a producing zone or sections of the zone

-seal lost circulation problems.

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Example 4-3

Assume that a 10000-ft well is in an area where the geothermal gradient is 1.8

oF/100 ft. Determine the bottom hole temperature (BHT) if the ambient temperature

is 70 oF.

Solution:

BHT = (D / 100 x G) + TA

BHT = (10000 / 100 x 1.8) + 70

BHT = 250 oF

Figure 4-7 Cementing plugs: (a) top and (b) bottom plugs (Courtesy World Oil’s

Cementing Handbook)

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Example 4-4

A cementing engineer was preparing to runcompressive strength tests on a

cement slurry prior to upcoming casing job. The following data wre available from the

logging engineer. The well had been circulated for 6 hr prior to logging.

Run No Time Diff. Btw Runs, hr Temp. oF

1 7 220

2 4.5 225

3 8 228

Estimate the bottom hole static temperatuıre (BHST) ?

Solution:

TD = time after circulation / (time of circulation + time after circulation)

Run 1, TD = 7 / (6 + 7) = 0.538

Run 2, TD = (7 + 4.5) / (6 + 7 + 4.5) = 0.657

Run 3, TD = (7 + 4.5 + 8) / (6 + 7 + 4.5 + 8) = 0.765

Plot the data and extrapolate to a BHST of 235 oF.

Figure 4-8 Estimation of BHST

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Example 4-5

A 7 5/8 inch 39 lb/ft production casing string will be run inside 51 lb/ft, 10 ¾

surface casing set at 2000 ft. The bottom of the 9 inch hole is at 9100 ft (casing

seat). Compute the volume of casing and annulus. A 6 ½ inch x 18 inch duplex pump will

be used to pump the cement plıg against the float shoe. If the pump operates at 90 %

efficiency, how many strokes will be required? After the job was completed, the

drilling engineer at the well site observed that 1990 strokes were required to bump the

plug. What is the actual pump efficiency?

Solution:

7 5/8 inch pipe capacity = 0.2394 cu ft / lin ft; 0.0426 bbl / lin ft

7 5/8 in. x 9 inch hole annulus = 0.1247 cu ft / lin ft

7 5/8 in. x 9.85 inch annulus = 0.2148 cu ft / lin ft ; 0.0382 bbl / lin ft

Compute the pipe and annulus capacities:

7 5/8 inch pipe capacity = 910 ft x 0.0426 bbl / lin ft = 387.6 bbl

7 5/8 in. x 9 inch hole annulus = (9100 – 2000) ft x 0.0222 bbl/lin ft = 157.6 bbl

7 5/8 in. x 9.85 inch annulus = 2000 ft x 0.0382 bbl / lin ft = 76.4 bbl

The output of the 6 ½ inch x 18 inch duplex pump is obtained as:

0.2280 bbl/stroke = 100 % efficiency

0.2052 bbl/stroke = 90 % efficiency

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Determine the pump stroke requirements to bump the plug.

387.6 bbl / (0.2052 bbl / stroke) = 1888 stroke

If the pump required 1990 strokes, determine the output.

387.6 bbl / 1990 strokes = 0.1948 bbl / stroke

Determine the actual efficiency.

(0.1948 bbl / stroke) / (0.2280 bbl / stroke) x 100 = 85.4 %

Example 4-6

A 3000 ft 13 3/8 inch surface casing is to be cemented in a 17.5 inch hole. The

1000 ft tail slurry is 14.2 lb/gal Class-A cement with 4 % gel.The remaining lead slurry

is 12.2 lb/gal Class-A cement with 16 % gel. Use 100 % volumetric wash out. Compute

the cement, water and gel requirements.

Solution:

The annulus volume is computed as:

0.6946 cu ft / lin ft x 2000 ft = 1389.2 cu ft

0.6946 cu ft / lin ft x 1000 ft = 694.6 cu ft

Accounting for 100 % wash outs:

(2000 ft lead slurry) : 1389.2 x 2 = 2778.4 cu ft

(1000 ft lead slurry) : 694.6 x 2 = 1389.2 cu ft

Lead slurry calculations are as follows:

Cement: 2778.4 cu ft / (2.55 cu ft / sx) = 1089.5 sx of cement

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Gel: (1089.5 sx) (16 % gel) (94 lb/sx) = 16386 lb gel = 163.86 sx of gel

Water : 14.7 gal/sx x 1089.5 sx = 16015 gal = 381 bbl

Tail slurry calculations are as follows:

Cement: 1389.2 cu ft / (1.52 cu ft / sx) = 913.9 sx of cement

Gel: (913.9 sx) (4 % gel) (94 lb/sx) = 3436 lb gel = 34.4 sx of gel

Water : (913.9 sx) x 7.57 gal/sx = 6918 gal = 164.7 bbl

Applications of API Cements:

Class-A

-Used at a depth range of 0 – 6000 ft.

-Used at a temperature of up to 170 oF.

-Intended for use when special properties are not required; well conditions permit.

-Economical compared with premium cements.

Class-B

-Used at a depth range of 0 – 6000 ft.

-Used at a temperature of up to 170 oF.

-Intended for use when moderate to high sulfate resistance is required; well conditions

permit.

-Economical compared with premium cements.

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Class-C

-Used at a depth range of 0 – 6000 ft.

-Used at a temperature of up to 170 oF.

-Intended for use when early strength is required; its special properties are required.

-High in tricalcium silicate.

Class-D & E

-Class-D is used at a depth range of 6000 – 10000 ft.

-Class-E is used at a depth range of 10000 – 14000 ft.

-Class-D is used at a temperature of 170 oF to 260 oF.

-Class-E is used at a temperature of 170 oF to 290 oF.

-Intended for use when moderately high temperature and high pressure are

encountered; its special properties are not required.

-Available in types that exhibit regular and high resistance to sulfate.

-Retarded with an organic compound, chemical composition and grind.

-More expensive than Portland cement.

Class-F

-Used at a depth range of 10000 – 16000 ft.

-Used at a temperature of 230 oF to 320 oF.

-Intended for use when extremely high temperature and high pressure are

encountered; its special properties are not required.

-Available in types that exhibit moderate and high resistance to sulfate.

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-Retarded with an organic compound, chemical composition and grind.

Class-G & H

-Used at a depth range of 0 – 8000 ft.

-Used at a temperature up to 200 oF without modifiers. A basic cement compatible with

accelerators or retarders.

-Useable over the complete range of classes A to E with additives.

Class-J

-Used at a depth range of 12000 – 16000 ft.

-Used at a temperature of 170 oF to 320 oF without modifiers.

-Useable with accelerators and retarders.

-Will not set at temperature less than 150 oF if used as a neat slurry.

API Cement Composition

API

Class

C3S C2S C3A C4AF Fineness

Sq cm/g

Water /

Cement

A 53 24 8 8 1500-1900 0.46

B 47 32 3 12 1500-1900 0.46

C 70 10 3 13 2000-

2400 0.56

D 26 54 2 12 1100-1500 0.38

G 52 32 8 12 1400-1600 0.44

H 52 32 8 12 1200-1400 0.38

J 53.8 - 8.8 - 1240-

2480 0.44

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API Cement Properties

Cement Class Mix Water

Gal /s x

Slurdy density

Ppg

Slurry yield

Cuft / sx

Thicken.

Time

113 oF, hr

Comp.

Streng.

110 oF, psi

A 5.2 15.6 1.18 2 ½ 4000

C 6.3 14.8 1.32 1 ¾ 2700

G 5.0 15.8 1.15 1 ¾ 3000

H 4.3 16.5 1.05 2 3700

Effect of Temperature on Compressive Strength

Curing Time 80 oF 100 oF 120 oF 140 oF 160 oF

8 203 1100 2320 2235 2900

12 750 1710 2600 3420 4150

24 1570 2720 3740 4580 5190

Effect of Gel Additions on Class H Slurries

% Gel Mixing Water

gal/sx

Slurry Density

lb/gal

Slurry Volume

cu ft /sx

0 5.18 15.7 1.17

4 7.57 14.2 1.52

8 9.96 13.3 1.86

12 12.4 12.6 2.21

16 14.7 12.2 2.55