Primary Cementing Operations

8/17/2019 Primary Cementing Operations

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Primary Cementing

Primary cementing is the cementing operation performed immediately after casinghas been run in the hole. This basic principle varies with the many materials used toperform the many cementing operations. More and deeper wells are being drilled thathave extreme temperatures, both hot and cold, in new and more hostile

environments. This presents a constant challenge to successful primary cementing.Meeting this challenge has led to an increasing number of ingenious and complexmaterials, tools, equipment, and techniques.

Primary cementing uses several basic techniques. The most widely used procedure isthe single-stage primary cement job using a two-plug method. ement is pumpeddown the casing between two rubber plugs. The plugs are equipped with wiping !nsto help prevent contamination of the cement by drilling mud, and to help clean theinterior wall of the casing.

The use of other common techniques depends on well depth and completionrequirements. Two-, three-, and four-stage cementing procedures decrease thehydrostatic pressure of the "uid column in the annulus, help protect wea# $onesagainst excessive high pressures, and help prevent circulation loss. %n addition to itseconomic advantages &i.e., it is not necessary to cement the entire string bac# to thesurface', multiple-stage primary cementing is also important in wells where two ormore $ones are separated by long intervals.

Terminology

(etting the casing at or near the bottom and perforating it for expected production iscommon practice in the industry. (ometimes the casing is suspended and cementedabove the producing formation and the well is produced from open hole) this is calledan openhole completion. The !nal casing string is called the fow string or oil string.

The !eld terms long string and production string are used in some areas. The term

casing string denotes the total footage of casing run in the well at one time.

The phrase *waiting on cement,* or +, has long been a misnomer in mostinstances the nonproductive and expensive time spent waiting has usually not beennecessary. %n most cases, the cement has !rmly set some time before operations areresumed. %n the early days of cementing, standards for curing concrete in theconstruction industry were adapted as appropriate + time for oil wells. The !rstwells were shut down for / days to allow the cement to set. Ten years later, oil !eldoperators were reducing the time to three wee#s, and some were resumingoperations after only two wee#s. Ten years after that, 01 days was consideredsu2cient. (ome 31 years after cement was !rst used, most operators accepted threedays as su2cient + time. 4ccelerators are currently added to cement, and ma#e itpossible to resume operations within a few hours.

%mportant considerations in the determination of + time are

• how much strength the cementing composition must develop before drilling

can continue


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• the strength development characteristics of the commonly used cementing

compositions, as well as those of available materials that are not being usedto their best advantage

• cement-curing temperatures that exist under wellbore conditions

5y using curing pressures that closely simulate those found in oil or gas wells, it ispossible to have a better understanding of additive performances and more realistic+ time for cementing compositions.

Downhole Temperatures

6ole conditions and curing environments for cement slurries vary in temperaturefrom below free$ing in permafrost $ones to 7118 9 &3708 ' in geothermal steamwells. The capabilities and versatility of most 4P% cements can be extended by usingadditives. 4 blend of additives usually produces an optimum range of cementqualities.

Temperature studies conducted along the :ulf oast of Texas and ;ouisiana in the

early 0 /18 9 ? @1.10= x depth infeetA. The cooling eBect of mud displacement lowers considerably the circulatingtemperature of the hole during casing cementing. Curing squee$e cementing, thereis less cooling because there is less well "uid preceding the slurry. Thus, a cementingcomposition can be pumpable longer during casing cementing than during squee$ecementing at the same depth.

The bottomhole cementing temperature may be determined from logs and 4P%temperature data. ;og temperatures ta#en approximately D hours after the lastcirculation ended may be considered static for use in 4P% (pec 01.

Slurry Volume

The amount of cement used in creating a slurry depends on the estimated total slurryvolume. %f a caliper log is available, an allowance of 01E above the volumecalculated from the caliper information is generally acceptable. %f volume from the bitsi$e of the drilled hole must be used, allow from =1E to more than 011E above thevolume calculated from the bit si$e. The excess-cement percentages of other wells inthe area of interest can be used as a guideline.

%n some areas, regulatory requirements dictate how far the top of the cement mustbe above the uppermost pay $one. Three hundred to !ve hundred feet of cementabove the top of the pay is typical for many locations. This volume is contingent uponcontact time — the time it ta#es for cement to "ow past a given point in the annulus.

The greater the contact time, the greater the chance of removing the drilling mudfrom the annulus. nly a speci!c part of the contact time should be considered incalculating slurry volume when the cement is pumped at high velocity past thehighest point in the annulus where good $onal isolation is needed. 4ny time spentdisplacing at a low rate just before bumping the top plug need not be included. ncewe have an estimate of total slurry volume,we can easily determine our bul# drycement requirements and water requirements along with the amounts of additivesneeded, by going to service company cementing tables.


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(uppose for example, that we have calculated a slurry volume of 0,111 ft.3 of class F:F neat cement for a surface string of 01 3GD* casing in a 0D 3GD*

hole. 9rom the cementing tables &6alliburton, 0


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Figure 1

The axial load which brea#s the cement bond was measured and the ability of thecement to support axial casing loads was found to be proportional to the area ofcontact between cement and the casing. Therefore, *support coe2cient,* *shearbond,* or *sliding resistance,* as described by various investigators, is the loadrequired to brea# the bond divided by the surface area between cement and pipe.

5ased on worst-case results, 5earden and ;ane provided a relationship fordetermining the support capability of a cement sheath, conservatively utili$ingresults for mud-wetted and nondisplaced conditions. Modifying their relationship toutili$e compressive strength &assumed to be 01 times tensile strength', gives theformula &in conventional oil!eld units',

9 > 1.


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6 > height of cement column, ft

9or example, for one bonded foot of 7-in. casing, using =11-psi compressive strengthcement,

9 > 1. 33


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Type mud $ydrauli%

bond(psi)

Sura%e *nish +etting +ater ,as

Kew mill-varnished Kone 11-=1

Jarnish removed &chemical' Kone 311-D11

Jarnish removed &sandblast' Kone =11-711 0=1

Jarnish removed &sandblast' 9resh water 011 =1

Jarnish removed &sandblast' %nvert oilemulsion

011 =1

Jarnish removed &sandblast' il base 011 =1

Hesin-sand coat &new,sandblast'

Kone 0,111-,111

D=1


9resh water 011 ==


%nvert oilemulsion

011 D=


il base 011 D=

ement 4P% lass 4

+ater content =. galGs#

uring temperature /18 9


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uring time D hours

asing si$e in. inside D in.

Table ' - Hydraulic bond vs. casing surace and type o fuid wetting.

Sura%e %ondition Sura%e %oating $ydrauli% bond(psi)

Cry Mill varnish L 1

Mud !lm Mill varnish L 1

Cry Husty 3=1-D=1

Mud !lm Husty 1-=1

Cry 4cid-etched =1-D11

Mud !lm 4cid-etched D1-=1

Cry (andblasted =11-I11

Mud !lm (andblasted =1-I1

Cry poxy-coated, I-0mesh sand

711-


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Table .- Eect o mud lm on bond strength.

Annular De/i%es $elp

The pressure at which failure of the hydraulic bond occurred in the test can beincreased by

• preventing formation of the microannulus by controlling pressure diBerential

across the casing as the cement sets, andGor)

• attaching seal rings of deformable rubber &similar to those available for !eld

installation' to the exterior of the casing.

6owever, $one isolation is routinely obtained in the !eld at greater diBerentialpressures than those causing failure in these hydraulic-bond tests. Therefore, suchtests are probably not completely representative of downhole conditions.

0ud wetting and Ru1 Cote

9urther tests were conducted to more directly measure adhesion between cementand pipe. These tests showed an advantage to the resin-sand &HuB ote' exterior inthe mud-wetted condition, which was not apparent in the previously discussed test&see Table ., above'. +hen resin-sand coatings are used downhole, however, theireBectiveness should be increased by removing mud from the casing surface withpre"ushes ahead of the cement, and by cement scouring.

asing with HuB ote should be well centrali$ed to avoid the embedment of mudca#eor shale into its roughened surface. This may not be possible in irregular, doglegged,or high-angle holes, or where mud is poorly conditioned.

ne advantage of the resin-sand is that it inhibits formation of a microannulus undercertain pressure and temperature conditions. This appears to be veri!ed by cementbond logs.

Casing Support in the &orehole

The cement sheath can protect the casing against several types of downholedamage, including

• deformation by perforating guns)

• formation movement, salt "ows, etc.)

• bottom-joint loss on surfaceGintermediate strings during drilling.

6owever, added resistance to casing collapse for design purposes is questionable. %nfault-slippage $ones, doglegs, and certain sand-control failures, the cement sheathmay contribute to problems.

Perorating 2 3xpendable /ersus Carrier ,uns


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The cement sheath tends to minimi$e casing damage caused by expendableperforating charges. xpendable guns of nominal charge N for example, through-tubing guns N may be used in cemented pipe with little or no danger of seriouscasing damage. 6owever, expendable charges may split casing collars that areunsupported by cement, and expendable gun charges of over 1 g frequentlydamage partially supported or unsupported casing.

9igure &ement sheath aects casing deormation by perorating with e!pendableguns ' &top' shows lab tests on casing deformation with 1-g charges and three cases

representing no cement &top curve', 3GD-in.

Figure 2

&0.


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asing damage can be caused by lateral loads resulting from "ow of salt formations.(alt may "ow in various ways, and it may not be economically practical to designcasing for the most severe situations of nonuniform loading possible, such as the"attening eBect illustrated in 9igure 3 &"o cement or partial sheath results in

eccentric pipe loading '

Figure 3

and 9igure D ith ault slippage$ unconstrained pipe may minimi%e damage '.

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Figure 4

6owever, when the annulus is completely !lled with cement, casing is subject to anearly uniform loading approximately equal to the overburden pressure. 4lthough themodes of failure may diBer, casing designs for withstanding salt pressure can becomputed on the same basis as those for withstanding "uid pressure.

asing failure caused by formation movement along natural or induced fault planesN as opposed to salt "ow N is best handled by elimination of cement through theaBected interval and perhaps by opening the hole to enable fault slippage to occurwithout loading the casing in shear & 9igure 3 and 9igure D '.

ther downhole conditions, such as borehole doglegs and sand-control failures, alsomay cause casing damage similar to the types described above. Onowledge of thefailure mechanism is essential to the selection of the failure-prevention method-i. e..,cement sheath or no cement sheath.

4dequate cement strength and good cementing and operational practices may berequired to prevent parting or other failure in bottom joints of surface andintermediate casing strings.

Jisual inspection of a joint failure reveals whether the casing is unscrewed or bro#en.nscrewing occurs because of high-level torque impulses transmitted to the casingby the bit as it hangs up while drilling cement and cementing equipment out of thebottom joints. %t can also be caused later by tool joint torque action on the lowermost

joints as drilling proceeds ahead.



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The problem is usually prevented by welding or by using thread-loc#ing compoundson the connections of the lowermost two or three joints and controlling rotary speed.ther prudent cementing practices include the following

• 4pply standard good practices when cementing e.g., maximi$e casing

movement) use high-rate displacement, centrali$ation, and proper washes

and "ushes.

• 4round the shoe, use quality cement with early high-compressive strength.

• se two plugs to prevent mud !ll around the shoe joint, and do not

overdisplace even if a top plug is used.

• Helease pressure to avoid microannulus formation &if this is compatible with

landing methods'.

• Oeep drill pipe out of the hole until the cement has adequate initial set.

Minimum strength for drillout is =11 psi &possibly 0111 psi @I/


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• in deep, hot wells, to place faster-setting slurry above retarded compositions

in lower, hotter $ones

Jarious tools allow "exibility and variety of application.

(tage collars are most commonly used. The stage collar contains ports that are

initially isolated by a sliding sleeve. The sleeve can be moved downward to open theports N and moved up to close them N with a special bomb or tripping plug. 9igure 0

&application o stage tools - displaced type method '

Figure 1

and 9igure &application o stage tools& ree all plug method 'shows two types of

collar-opening methods displaced plugs and free-fall plugs.



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Figure 2

Port collars can be opened or closed by mechanical action of an inner string, such asdrillpipe. Two types of port collars are shown schematically in 9igure 3 &two types o

port collars opened and closed by pipemovement ') these are actuated by inner-string rotation or vertical manipulation.

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Figure 3

The tools shown in 9igure 3 are applicable to speciali$ed jobs such as the placementof "uids behind pipe for corrosion protection or sand control, or cementing withtreating pac#ers.

'etal-(as)et 'ac)er

Mechanical devices are frequently used below stage tools to prevent upper-stageslurry from dropping through the mud. They may also be used below conventionalshoes where casing is landed above the open hole.

The most common support device is the metal petal bas#et attached around thecasing exterior. The bas#et allows vertical "uid movement but opens against the

borehole to prevent downward movement. (trength and sealing ability limit its use toshallow depths.

4 more rugged support device is the solid rubber or in"atable e!ternal casing pac)er. Typically, this is placed below the cement-outlet port and in"ated with mud orcement prior to the opening of the cementing port& 9igure D , E!ternal pac)er supports cement column over a wea) %one at shoe or orstage cementing'.



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Figure 4

Types o Casing

Condu%tor

The conductor pipe & 9igure 0 , conductor pipe' is the !rst string set in the well. %tmay be set by the rotary rig drilling the hole, or by a smaller rig &or rathole machine'

before the larger rotary rig is moved in.



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Figure 1

This casing string serves as a conduit to raise circulating "uid high enough to return itto the pit. onductor casing also supports part of the well load where ground supportis inadequate or where the well is going to be drilled to a great depth.

The purposes of conductor casing are

• to prevent washing out under a rig

• to provide elevation for the "owline

• to provide support for part of the wellhead

4 blowout preventer &5P' is not usually attached to the conductor casing.

(ome characteristics of conductor casing and its placement are as follows

• asing is usually large 1 to 31 in. in diameter.

• The hole for the casing may be severely eroded.


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• asing can be easily pumped out, and is usually tied down.

• (etting depth can vary from as little as 1 ft to as much as a few hundred

feet.

• The most common pipe and hole si$es are 0I-in. pipe in a 1-in. hole, and

1-in. pipe in a I-in. hole.

Hecommended cements for use with conductor casing are • accelerated neat

• ready-mix concrete

• thixotropic cement

• ;M additives

Typical slurries for conductor-casing application include 4P% lass 4, , :, or 6 withE calcium chloride as the accelerator. ;ost-circulation additives such as sand,gilsonite, and cellophane may be added without signi!cant eBect on the slurry-thic#ening time or compressive strength. +here lost circulation is severe, athixotropic cement can be used.

The following is a brief summary of conductor-casing cementing practices

• The amount of cement used should be su2cient to provide returns to surface

• The casing is often cemented through drillpipe with a sealing sleeve.

•

+hen cementing down casing, plugs may not be used) cement is simplyplaced.

• ;arge-diameter &31-, I-, and 1-in.' casing plugs are wooden body plugs. %f

bumped on baRe or "oat, care must be ta#en in pressuring up to preventbypassing the plug with displacement "uid.

• The amount of excess cement is usually determined by experience in the

area of interest.

The following factors must be considered in the selection of the correct slurrycomposition for a conductor casing

•

The set cement must have a compressive strength high enough to supportan appreciable wellhead load) therefore, high-compressive-strengthcompletion cements are best.

• (ince the temperature at shallow depth is usually only /1 or


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• +hen the cement cannot be circulated bac# to surface, a top-out slurry is

usually pumped into the annulus through small tubing on top of the primarycement.

Sura%e Casing

(urface casing & 9igure 0 , surace casing' is usually the second string of pipe set inthe well. 6owever, when a conductor casing is not set because the well is on !rmground or will not be drilled to great depth, the surface pipe is the !rst string set.

Figure 1

The purpose of surface casing is to

• protect freshwater sands, particularly underground sources of drin#ing water

• case unconsolidated formations

• provide primary pressure control &5P is usually nippled up on surface

casing'



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• support future casings

• case-oB potential lost-circulation $ones

The casing head and other !ttings used for completing the well are attached later.

(ome characteristics of surface casing and its application are as follows

• asing si$es normally range from 7 =G/ in. on shallow wells to 1 in. on deep,

multistring wells.

• The hole in which it is set may be severely eroded.

• (hallow strings can be easily pumped out.

• Crilling muds are often viscous, with little water-loss control.

• asing may stic# easily in unconsolidated formations.

• ;oss of circulation may be a problem.

• Most areas require that cement be circulated.

• 4 guide shoe &or "oat shoe', "oat collar, scratchers, and centrali$ers are

commonly used.

• asing may be set from a few hundred feet to several thousand feet) the

depth depends on the proposed total well depth, the competency of shallowformations encountered, and state regulations regarding protection of

freshwater $ones.

*ecommended ements

(hallow surface casing is cemented in the same manner as conductor casing.ompletion cements with accelerated thic#ening times and compressive strengthsare used. Top-out slurries are used on surface jobs when cement is not circulatedbac# to surface.

9or deeper strings of surface casing, a lightweight lead cement is used, followed byheavier-weight completion cement. (ometimes, when $ones are penetrated by longsurface casings, a lightweight lead cement may help #eep these formations from

brea#ing down under the hydrostatic pressure exerted by a long column of cement. The bottom of the surface casing around the shoe is cemented with the high-strengthcompletion cement.

This creates a strong seal with the pipe and formation for solid support of the casing.

There is a cost advantage to using a high-yield completion slurry to cement the entirestring. +hen the well is shallow and a signi!cant load is to be placed on the wellhead,


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a densi!ed !ller slurry can be used to cement the entire casing. mission of the tail-in slurry is not economically advantageous.

Hecommended cement types include

• The amount of cement used should be su2cient to provide returns to

surface. This is a regulatory requirement in many locations.

• !ller cements &with a high water content' followed by neat or high-strength

tail-in

• accelerated cements

• ;M additives

• high-strength cements, which are often used on deep-well surface casing to

support future strings

The following is a brief summary of surface-casing cementing practices

• ;arge-diameter strings are often cemented through drillpipe with a sealing

sleeve.

• 5oth bottom and top plugs should be used to prevent mud contamination.

• The bottom joints and thread loc# should be centrali$ed to prevent bac#ing

oB during drilling.

• Hegulator rules usually require + of / hours, or =11 psi minimum

compressive strength.

Curing displacement with mud, a "oat collar placed two joints above the guide shoehelps prevent mud from contaminating the cement around the shoe joint. (cratchersand centrali$ers are the !nal consideration of casing equipment. (cratchers aresometimes used to help clean the mud from the formation face. entrali$ers centerthe casing in the hole to help place the cement completely around the pipe.

4 top and a bottom plug should be used to wipe the pipe clean ahead of the cementwhen mud is the drilling "uid. %f only a top plug is used, the mud wiped oB the casingbuilds up behind the cement and contaminates the cement around the shoe.

"ntermediate Casing

The intermediate casing & 9igure 0 .,intermediate casing' is the !rst string of pipe set

after the surface casing.



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Figure 1

%t is sometimes called the protection casing. %ntermediate casing strings extend from

the surface to a formation able to hold the mud weights expected at greater depth. This depth can vary by several thousand feet in a single-stage job. +hen a secondintermediate string is set, the casing is run to just below the wea# $one to acompetent formation and is cemented at that point.

The purpose of intermediate casing is to

• separate the hole into wor#able increments for drilling

• case-oB lost-circulation $ones, water "ows, etc.

• isolate salt sections

• protect the open hole from increases in mud weight

• prevent "ow from high-pressure $ones if mud weight must be reduced

• control pressure) the 5P is always installed

• support subsequent casings


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• (ome characteristics of intermediate casing and its application are the

following

• Pipe and hole si$es are determined largely by the number of casing strings to

be run below the intermediate string.

• asing si$es range from I =G/ in. to 1 in. Most common are < =G/ in. 01 3GDin., and 03 3G/ in.

• (ome sections N particularly salt sections N may erode severely.

• (trings may be very heavy and set on bottom.

• 5oth extremely wea# $ones and high-pressure $ones are covered by

intermediate strings.

• ement volume is dictated by wellbore condition.

• 4 guide shoe &or "oat shoe' and "oat collar are commonly used.

• ement volumes are usually largest in the well.

• %ntermediate casing is often cemented in stages.

• Prolonged drilling may be done through this casing, and damage is common.

• ompletion may be made in intermediate casing.

*ecommended ements

5ecause of the large volume of cement required, and the types of formation to becovered, both !ller and composition cements are used to cement most intermediatecasing. (ometimes, as many as three diBerent slurries are needed. 9ormation-fracture gradients, lost-circulation $ones, formation temperatures, possible futureproducing $ones, and well depth determine the number and types of slurries to use.

The slurry requirements for a single-stage cementing job are similar to those for along surface job. The !ller slurry needs to be light enough not to brea# down thewea#er formations. The completion slurry needs to have enough strength to hold thepipe and provide a good seal between the pipe and the formation.

The bottom of the pipe is cemented &usually at 011 to 3111 ft' in a single-stageintermediate job because of cost considerations, or because the uncementedsections of casing may be reclaimed from the well later and reused. %n the lattercase, only a high-strength completion slurry with a retarder is needed. Hetardersensure su2cient pumping time to get the slurries in place, and also impart somefriction-reducing properties to the slurry.

nli#e the conductor and surface casings, additives such as friction reducers, "uid-loss additives, and retarders are required for intermediate slurries. +here the annulus


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is small, friction reducers lower pump pressures and reduce the chance of losing"uids in a lost-circulation $one. 9luid-loss additives prevent slurry loss into lost-circulation $ones and dehydration in the annulus caused by permeable $ones, andalso give better bonding results.

The following is a brief summary of intermediate-casing cementing practices

• 5oth bottom and top plugs should be used to minimi$e contamination of the

cement.

• (tage tools are used occasionally in cementing long strings of pipe where

there is ris# of brea#ing down a wea# formation.

• The number of slurries required may be determined by possible production,

wea# $ones, and wellbore temperatures.

• (cratchers, centrali$ers, and "ushes can be important in the successful

completion of an intermediate-casing cementing job.

Produ%tion Casing

The production casing & 9igure 0 , production casing' is the last full string of pipe set

in the well, and extends to the surface.



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Figure 1

Production tubing, downhole pumps, and other equipment needed for the productionof oil and gas are housed in this casing. The production-casing cement must give a

pressure-tight seal between the formations and the production casing. %t is essentialto isolate the reservoir from "uids both within the producing $one itself and fromother $ones penetrated by the wellbore. These "uids &e.g., oil, water, gas' can createemulsions, scale deposits, para2n deposits, severe corrosion, and a decline inproduction. 5esides primary producing operations, remedial wor#over jobs such assquee$es or chemical treatments are also run through the production string.

The purpose of production casing is to

• complete the well for production

• eBect $onal isolation

• protect pay $ones from unwanted "uids

• provide pressure control

• cover worn or damaged intermediate casing

(ince the production casing may extend from total depth to surface, the settingdepth can vary from a few thousand feet to as much as 0D,111 ft &D71 m'. 5elow0D,111 ft, liners may be set to reduce cost and because less pipe weight is needed.

The si$e of the casing depends on the number of strings of production tubing to berun into the well and the si$e of production equipment used.

The following are some characteristics of production casing and its application

• ommon casing si$es are D 0G in., = 0G in., and 7 in.

• Crilling mud is usually in good condition.

• The cement job is usually not circulated, but cemented bac# to intermediatecasing depth.

• 4 good cement job is vital to a successful completion.

To achieve a pressure-tight seal and protect the reservoir, special consideration mustbe given to the production-casing cement properties. 4s with intermediate casing andlong surface pipe, both !ller and completion cement are usually employed. The !llercement needs good "uid-loss control. %t must have enough compressive strength toprotect upper, potentially productive $ones which might be completed in the future.

The completion slurry needs to have good "uid control and su2cient compressivestrength to hold the weight of the pipe and to bond the formation to the pipe. Thesetting times of both slurries should be minimi$ed to help prevent cementcontamination from formation "uids and formation contamination by cement !ltrate.


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Hecommended types of cement are

• !ller cements with high-strength tail-in

• low-water-ratio cements &for all potential pay $ones'

• densi!ed cements &for high competency and pressure control'

• "uid-loss control additives

+ummary o 'roduction asing

5atch mixing or continuous batch mixing is recommended for all large or critical jobs.(ince the production string aBects the success of the well more than any othercasing, a good job may mean the diBerence between success and failure of the well.

2cient removal of the mud is essential. (pacers or "ushes may be used to removemud and to water-wet the pipe and formation face for good cement bonding. suallycasing reciprocation or rotation is used. 4 "oat shoe and "oat collar, centrali$ers,scratchers, and pipe movement should be used.

Liner Cementing

4 liner is a string of casing that does not extend up to the wellhead. %t is used tocase-oB the open hole below an existing casing string.

(everal types of liners may be categori$ed by their function

• Crilling liners permit deeper drilling operations by isolating lost circulation or

highly pressured intervals and controlling sloughing or plastic formation. %nlieu of full-length casing, the drilling liner improves drilling hydraulics, i.e., thegreater cross section above the liner top enables the use of larger drillpipeandGor reduces annular pressure drop.

• Production liners provide isolation and support functions when casing has

been landed above the producing interval.

• 4 tie-bac# stub liner extends from the top of a liner to a point uphole, inside

another string of casing or liner. The stub liner is used to cover damaged orworn casing above an existing liner, and to provide added protection againstcorrosion andGor pressure.

• Tie-bac# casing extends a liner to the wellhead. %t is used primarily for thesame purposes as the tie-bac# liner. Hunning such a string at the end of adrilling operation ensures that the completion will be run in unworn casing.

9igure 0 &E!ample o deep-well tubulars$ liner,tiebac) application ' shows the tubular

program of a modern deep well using two liners and tie-bac# casing.



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Figure 1

Cementing Pro%edures

ementing procedure is illustrated in 9igure ,



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Figure 2

9igure 3 ,



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Figure 3

9igure D , and 9igure = &ypical liner cement procedure.

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Figure 4

+ith the liner hung in the casing but still attached to the drillpipe, slurry is pumped

into the drillpipe without a bottom wiper plug.


31/46

Figure 5

The cement is followed by a drillpipe wiper plug that latches into a liner wiper plug,positioned below the liner hanger. The combination plug then wipes the liner cleanand !nally latches into a landing collar to complete slurry placement.

The following are important details in cementing procedure

• +ith the liner in position, mud is circulated to ensure that the liner and the

"oat equipment are free of any foreign material, and to condition the mud. 4clean mud system is important so that materials will not fall out on top of theliner-running assembly during the cement job.

• The cement can be batch-mixed, circulated through a holding tan# or ribbon

blender, andGor double-pumped to obtain the desired cement-slurryproperties.

• ement slurry should be pumped in turbulent "ow, or as fast as possible,

refer to the heading titled *9luid 9low Properties and Mud Cisplacement*. (uch"ow minimi$es excess cement-volume requirements. Most operators prefer tolimit excess cement volume, which, of course, is pumped into thedrillpipecasing annulus. %t is usually desirable to pump some type of spacer"uid &buBer' ahead of the cement.

• %f no bottom plug is used, the drillpipe and liner plugs wipe mud !lm oB the

%C of the drillpipe and liner. This mud collects below the plugs and cancontaminate cement in the bottom of the liner. (pacing between landing collar


32/46

and "oat shoe should be adequate to #eep contaminated cement out of theliner-openhole annulus.

• +ith cement in place, it is standard procedure to pull the liner-setting

assembly out of the liner hanger. +ith the tailpipe of the liner-settingassembly above the liner top, excess cement can be reversed out. 6owever,

reverse circulation places an extra pressure on the annulus that must becontrolled to prevent formation brea#down. 4 liner pac#er #eeps reverse-circulation pressures oB the formation.

• ne method is to pull the drillpipe all the way out of the hole and leave

cement inside the casing to be drilled out. + time depends on cementcomposition and hole conditions.

Liner2Cementing 3quipment

4 liner is usually run on drillpipe that extends from the liner-setting tool to surface.(pecial tools perform various running, setting, and cementing operations & 9igure 0 ,

ypical e/uipment used to install and cement liners'.

Figure 1



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+hoes,ollars

4 "oat shoe placed at the bottom of the liner contains a chec# valve designed to

prevent bac#"ow of the cement. 4 "oat collar can be run above the shoe to provide abac#-up chec# valve. 4utomatic !ll-up-type "oat equipment may be selected, but it israrely run on liners.

4 landing collar is usually run one joint above the "oat collar or two or more jointsabove the "oat shoe to provide space for mud-contaminated cement inside the liner.

The collarFs function is to latch and seal the liner wiper plug. %t prevents the linerwiper plug from moving uphole if a chec# valve fails, and prevents it from rotating,which aids drillout.

Liner wiper plugs can be attached to the end of the tailpipe or slic# joint with a shear-pin arrangement. The selection of the proper shear rating is very important in theprevention of premature shearing and release of the liner wiper plug.

The liner wiper plug can also be latched to the tailpipe to prevent prematureshearing. Helease of this type can only be eBected by engagement of the drillpipewater plug.

Hangers and +etting ools

The liner hanger is installed at the top of the liner. 6angers are usually classi!ed bythe method used to wedge slips against the casing wall) two such classi!cations aremechanical and hydraulic.

The presence of slips between liner and casing reduces the bypass area for

circulating. This reduced area can create high-pressure loss during circulation andcementing. 6angers are available with multiple split slips that increase bypass areaand provide increased slip-contact area.

The liner-setting tool$ a rental item furnished by the liner service company, providesthe connection between drillpipe and liner. (wab cups attached to tailpipe, or apac#oB bushing and slic# joint, are inserted into the liner to provide a seal betweensetting tool and liner.

nce the liner is hung, the setting tool can be released and pic#ed up a shortdistance to con!rm, by indicator weight loss, that the setting tool has separated. 4new retrievable pac#oB bushing eliminates bushing drillout.

Liner pac)ers can be installed at the top of liners to seal between liner and casing,after cement placement. (eal elements may be rubber or lead, or a combination ofthe two. They may be run as an integral part of the liner hanger and set bymanipulation of the liner-running tool. 6owever, this type of pac#er should beconsidered only if clearance is such that the hole can be circulated at desired rateswithout increasing bac#-pressure excessively.


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(pecial pac#ers can be set in conjunction with a tie-bac# sleeve after cementing andcleanout operations have been completed. These pac#ers seal both in the tie-bac#sleeve and against the suspending casing.

xternal casing pac#ers have been used on liners to isolate between $ones in openhole. They are in"ated following cement displacement before the cements set up to

provide more eBective $one isolation.

+etting on (ottom

xcept in unusual cases where buc#ling is not expected or can be prevented throughcentrali$ation, liners to be cemented should be suspended from slips set in existingcasing, or the drilling liner. 6owever, equipment is available for the specialapplication in which liners are cemented and set on bottom.

4 special "oat shoe can be run on the bottom of the liner with an extra internal left-hand thread. 9irst the liner is run into the well and hung from surface slips. Then thecementing string is run and engaged into the thread at the shoe. The liner is run tobottom on the cementing string, and the cement job is completed. The inner string is

disconnected from the shoe by rotating to the right.

Typi%al Problems in Deep5 $ot +ells

;iner cementing is a major problem in deep wells for the following reasons

• Tight 6oles. 4s seen in Table ' &below', liner-to-borehole clearances can be

very small. This is a highly undesirable situation that frequently results frompoor planning, misguided economics in selecting well tubularGbit programs, orunforeseen downhole conditions. (mall clearances require "ush joint liners that approach drill-collar si$e. Oeyseats and collar-worn grooves cause diBerential stic#ing and prevent eBective

mud removal by cement

Liners5 in6 $ole5 in6 Casing5 in6 Cementsheaththi%7ness5in6

< =G/ 01 =G/ 00 3GD 0G

7 3GD < 0G 01 3GD 7G/

7 =G/ < 0G 01 3GD 0=G0I

7 3GD / 0G < =G/ 3G/

7 =G/ / 0G < =G/ 03G0I

= 0G I 0G 7 =G/ 0G

= I 0G 7 =G/ 3GD

= I 0G/ 7


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3 0G D 3GD = =G/

Table '- Typical liner/casing and hole size combination. • ;ong %ntervals, Mud a#e. %n +est Texas, liner lengths may be =11 to

00,111 ft &7I to 33=1 m') they average /111 ft &DD1 m'. 6igh temperatureand prolonged exposure causes mud to gel excessively. (hale instability in long geopressured sections causes hole irregularities. Mudis di2cult to remove from enlarged sections.

• Temperature CiBerential Top to 5ottom. 4s shown in 9igure 0 &Long #est

e!as drilling liner has high temperature dierential over its length', staticgeothermal temperature may vary as much as 011 9 over the length of a

long liner.

Figure 1

ement composition and setup time must be compatible with this gradient. Set, circulating temperatures may be radically diBerent, actually causingmaximum temperatures to occur perhaps 111 ft &I01 m' uphole for nearly anhour after circulating.



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• :as utting, ;iner Top ;ea#age. Premature cement setup uphole by high

temperature or !ltrate lea#oB to long permeable $ones can reduce hydrostaticpressure and allow gas to permeate the partially cured column. The resultingchannels are too small to eBectively squee$e cement, but they continue tochannel gas to liner tops. Two compositions that minimi$e problems are &0'"uid-loss additive spotted across upper $ones and &' retarder that #eeps

slurry "uid, then sets up rapidly, rather than thic#ening gradually. (pecialcompressible cement that maintains its volume during setting to prevent gaslea#age and thixotropic cementing compositions are also available.

Spe%ial Primary Cementing 0ethods

"nner2String Cementing

ementing large-diameter casing requires some special considerations. (uch casingis subject to being pumped out of the hole. This occurs when pump or hydrostaticpressure acting on the cementing-head area and on the bottom of the hole throughthe shoe opening provides an upward force exceeding the buoyed weight of the

casing. Pressure increase on bumping a plug is, of course, oBset, and does notcontribute to the problem.

;arge casing can also be "oated out of the hole if the weight of the casing and themud in the pipe do not exceed the buoyancy provided by the annular column ofcement. The possibility of casing collapse must also be considered. 6eavy mud maybe required to prevent these occurrences.

%nner-string or stab-in cementing is now a fairly common practice for large-diametercasing. The string is cemented through drillpipe stuc# into a special sealing sleeve in

the shoe & 9igure 0 , stab-in cementing techni/ue or large-diameter casing'.



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Figure 1

(mall-diameter plugs can be used. The drillpipe can be raised from the seal and theexcess cement reversed bac#.

%n geothermal wells where no voids can be tolerated outside the casing because oflater heating and boiling problems, slurry can be continuously pumped until goodcirculation to surface is established. onventional methods, conversely, pump acalculated volume that is di2cult to determine in surface holes without the use ofcaliper logs.

Jarious adaptations are possible using cup pac#ers, etc. with stab-in methods. Portcollars can be opened or closed, and external pac#ers can be in"ated to permit stagecementing of long, large-diameter pipe.

3xternal Cement24illed Pa%7er

xtra-long &1- or D1-ft' elastomer-sheath covered in"atable pac#ers can be run aspart of the casing string. ne pac#er &or more' is landed across the productive $oneto be perforated and the primary job is completed conventionally.


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+hen initial slurry displacement is complete and the top plug is bumped, increasedpressure opens shear-pin controlled valves in the pac#ers and additional cement ispumped into the pac#er elements to expand them tightly against the borehole wall.%n"ation cement is pumped down the casing between the !rst top wiper plug and asecond top wiper plug.

4fter curing, the cement-!lled pac#er and the casing joint mandrel on which it is runare perforated by conventional methods. %n"ation cement transport and thecompleted production system are illustrated in 9igure 0 &0nfatable e!ternal cement-

lled pac)ers during running and ater perorating '.

Figure 1

Two cement-!lled pac#ers can be used eBectively to straddle a productive interval for$one isolation.

xternal cement-!lled pac#ers oBer several advantages in primary cementing

• omplete mud channel removal is ensured by the application of high internal

pressure to the end-reinforced element, to squee$e mud from the rubber-formation interface.



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• 4 quality pipeGcement bond is ensured by the use of uncontaminated

cement, mud-free pipe, and pressure setting. %nter$onal "ow and the need forsquee$ing may be eliminated.

• Producing $ones are supported by the pressure-set cement and will not

dilate or "ow when "uid in"ow causes a pressure diBerential.

• +ithout conventional slurry circulation !rst, cement may be placed solely in

the pac#ers, and thus not contact water-sensitive $ones.

(ystem limitations include the following • ;ong elastomer-covered pac#ers are durable in properly conditioned holes

but are inherently sensitive to restricted-clearance holes and problem holeswith casing burrs, bro#en centrali$er pieces, nontapered liner shoulders, etc.

• Heliable supply and pumping methods are required to prevent long delays or

job interruptions that may complicate in"ation-procedure control afterprotective #noc#-oB plugs are removed by the bottom wiper plug.

• C completions may prove less "exible for long-term production

adjustments than conventionally cemented wells.

4a%tors That A1e%t Primary Cementing

$ole Conditions

+loughing

%n many cases, this is the reason for setting an intermediate casing. (loughing cancreate several cementing problems bridging the annulus, stic#ing the casing, andincreasing the annular hydrostatic pressure.

1rill pipe 1rag

The cause and location of the drag could be very signi!cant. Crag may indicate theneed for centrali$ed casing or "uid-loss control cements.

Low-'ressure 2one

ne of the most persistent problems is an incompetent formation that will notsupport eBective columns of cements. This most commonly occurs in intervalscovered by surface and intermediate casing.

3ud ondition

4 well-conditioned mud greatly increases the mud-removal capability of "ushes andcement slurries.

4luid 3ovement


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one isolation fails any time "uid movement is allowed to occur in a cement slurrybefore it is completely set. %f the cement moves during the hardening process, it willnot set properly.

4ormation 3ovement

The most common formation movement occurs with salt intrusions.

0ud2Contamination 31e%ts

The possibility of mixing cement and mud always exists during pumping anddisplacement. (uch contamination can result in

• accelerated or retarded thic#ening times

• reduced compressive strength

• reduced bond strength

• increased !ltrate loss &higher than in either mud or cement'

• severe thic#ening &with oil-base mud'

Table ' &below' shows typical mud additives and their eBects on cement.

%norganic chemicals have an erratic eBect on oilwell cements, but generally tend toaccelerate thic#ening times. rganic chemicals generally retard, and may completelyinhibit thic#ening in some instances.

(evere thic#ening occurs with oil muds in cement mixing because these muds arethic#ened by water-wet solids that are readily available in the high-solids-contentcement. The problem is most serious when mud and cement slurry densities are high.4lso, oil-emulsion muds often contain calcium chloride in the water phase, which canaccelerate setting.

Additi/e Purpose 31e%t on %ement

5arium sulfate &5s(oD' +eighing agent %ncreases densityHeduces strength

austic &Ka6, Kao3, etc. p6 adjustment 4cceleration

alcium compounds onditioning 4cceleration

a, a&6', al,a(D and 6

p6 control

6ydrocarbons &diesel oil, 9luid-loss control, Cecreases density

lease crude oil' lubrication


41/46

(ealants &scrap, cellulose,rubber, etc.'

(eal against lea#age toformation

Hetardation

Thinners, &tannins,lignosulfonates,quebracho,lignins, etc

Cisperse mud solids Hetardation

mulsi!ers lingnosulfonates,al#yl ethylene oxide adductshydrocarbon sulfonates'

9orm oil-in-water orwater-in-oil muds

Hetardation

5actericides &substitutedphenols, formaldehyde, etc.'

Protect organic additivesagainst bacterialdecomposition

Hetardation

9luid -loss control additives,M,starch, guar,Ployacrylamides;ignosulfonate

Heduce "uid loss frommud to formation

Hetardation

Table ' Eects o mud additives on cement.

0ud2Contamination Pre/ention

To prevent mudGslurry problems, it is best to minimi$e contact. The bottom wiper plugprevents contamination in the casing, and a spacer "uid reduces cementGmudcontact in the annulus.

Two bottom plugs may be required N one preceding, and one behind the spacer "uidN to prevent mudGcement contact if contamination is li#ely to create seriousproblems, and the spacer "uid does not by itself strip the mud !lm from the casing

bore.

4 single bottom plug, ahead of the cement, removes the !lm and accumulated mudahead of the plug and behind the spacer "uid. This accumulated mud can then

contaminate the cement & 9igure 0.

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Figure 1.

, he lac) o bottom wiper cases mud accumulation below top plug'.

4 variety of spacer or pre"ush "uids are available, including water, brine, solutions of acid phosphates, diesel oil &weighted or unweighted', oil-base "uids, and emulsions&oil in water, water in oil'.

ompatibility of both spacer and mud, and spacer and cement should be veri!ed onevery cement job. (election of the amount and type of spacer depends on the type of mud and on potential reaction problems between the cement and the mud.

4 water "ush, normally in turbulent "ow, may aid mud-displacement e2ciency. (altwater has less tendency to cause shales to swell or slough. 6owever, fresh water, saltwater, or "uids containing dispersing surfactant should not immediately precede ahigh-density cement slurry) this can cause thinning and weight material settling

Casing

The following are general rules for casing preparation and application

• Tally all pipe) count, number, and rabbit &gauge' all casing joints on the pipe

rac#.

• hec# all casing threads for cleanliness and damage. 4dditionally, chec# the

threads on all crossover equipment for proper thread type and cleanliness.


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• %dentify all joints by weight and thread type, and place them in proper order

for running into the hole.

• ;anding joints should be spaced out so the cementing head can be installed

from the stabbing board or rig "oor after the casing is landed.

4loating 3quipment

The "oating equipment &"oat collars and "oat or guide shoes' must be on locationand in good wor#ing condition. hec# operational features if diBerential "oatingequipment is used. Measure and prepare stage and "oating cementing equipmentseparately. se thread-loc#ing compound or tac# weld &if necessary' on all !eldma#eup connections between the "oating equipment, plus one or two joints above.

Running Casing

The following are general rules for running casing

• se of a movable stabbing board can minimi$e downtime.

• 4 safety valve is advised for long production casing strings or suitable

pressure rating.

• ontrol running speed of casing to prevent fracturing and lost circulation.

Cir%ulating Time

The importance of circulation before cementing is recogni$ed by all operators, butthere are considerable diBerences of opinion regarding optimum circulating time.

Many believe that because the number of variables aBecting the success or failure ofa cement job is so great, it does not seem possible to correlate the degree of successwith the amount of time spent in precementing circulation.

The following are circulation guidelines

• ondition the drilling mud with good rates up to anticipated cementing rates.

• 6igh circulation rates remove gel led mud that develops during static periods

because of temperature and "uid loss.

• 5egin pipe movement and mud conditioning immediately after the casing is

on bottom.

• 4pply scratching technique when wall ca#e and cuttings in the mud returns

have either virtually stopped or declined rapidly in volume.

• The casing moves at the start of circulation and continues throughout the

circulation period. +ith reciprocating scratchers, the pipe is commonly movedthrough a 1-ft stro#e, with a -min interval for the cycle. %f rotating scratchers


44/46

are used, the pipe is rotated as slowly as possible, usually between 01 and 1rpm.

Cementing Composition5 Volume5 and Slurry +eight

%n primary cementing, the cement slurry should have a viscosity that will give the

most e2cient mud displacement and still permit a good bond between the formationand the pipe. The following are some cementing guidelines.

• Cetermine the maximum allowable downhole density to prevent fracturing.

The density of cement should be at least 0 lbGgal &preferably or 3 lbGgal'heavier than the drilling mud.

• Cesign "uid loss using diBerential pressure of 0111 psi. To prevent gas

channeling, design on 1 ccG31 min or less.

• Cesign cement slurry to be displaced in turbulent "ow for a minimum of 01

to 1 min contact time at the top of the pay $one, if possible.

• 9or slurries to be placed across salt formations, use saturated sodium

chloride.

• se 3=E silica at static temperatures above 318 9 &0018 '.

• ontrol free water to 0E or less for normal slurries. To prevent gas

channeling, control free water to $ero.

• Cetermine cement-slurry thic#ening time at bottomhole cementing

temperature and pressure. Minimum thic#ening time should be job time plusone hour of thic#ening time to a consistency of =1 5c. &5earden nits of slurry

consistency are dimensionless units formerly called *poises.*' Minimumthic#ening time is the time required to mix the slurry, and pump it down thehole and up the annulus behind the pipe.

"mportant Tips

se top and bottom wiper plugs, and inspect the plugs before loading. The bottom&hollow' plug is loaded !rst, then the top &solid' plug. Co not slit the diaphragm of thebottom plug with a #nife before loading.

se a two-plug cementing head

• Cisplace the top plug out of the cementing head without shutting downoperations. Co not open the cementing head to drop the top plug or a vacuumwill be created and the well will ta#e in air.

• Pump pre"ush or spacer ahead of the bottom plug. %f you use two bottom

plugs, put the !rst bottom plug in !rst, then the pre"ush or spacer, and thenplace the second bottom plug just before the cement.


45/46

• 5efore mixing, chec# calibration of all density devices with fresh water for

proper calibration.

• 6oo# up bul# tan#s to the cement mixer. The rate of delivery of cement to

mixer should be su2cient to maintain pump rate in the annulus at the designrate.

• 5atch mix all cement slurries by using a ribbon or batch blender. This

operation is extremely important for good control of slurry properties.

4 bottom plug is not recommended for use with slurry containing large amounts oflost-circulation material or with badly rusted or scaled casing. (uch material maycollect on the ruptured diaphragm, bridge the casing, and thus prevent totaldisplacement.

Personnel

The supervisor and the person on the pump throttle should understand the

importance of the pumping rate to the success of a job. They both need to #now

• why variations in cement density should be held to close limits

• control parameters, so they will not be easily satis!ed with less control

• how to change over from pumping to displacement in 0= seconds, rather

than minutes

Maintain a log of operations that includes time, density measurements, mixing rateand displacement rate, wellhead pressure, operation in progress, volume of "uidpumped, etc. Hecord pump speed &stro#es per minute' and total stro#es. %nsist on a

properly operating pressure-recording chart from the operator.

Ma#e sure that all service personnel involved are given ample notice ofcommencement of casing operations, so they can be available and rigged up beforethis time.

9or the following items, prepare and record data on the appropriate company casingcementing report as required

• Cetermine the elapsed time and volume or stro#es required for the cement

slurry to leave the casing shoe after the start of displacement) to reach thepressure equali$ation point after the start of displacement) and to displace the

top plug to bump "oat. Kote that when a stage-cementing collar is used, theabove calculations should be made for both phases of the cementingoperation.

• Cetermine the theoretical weight of the casing in 0111-ft intervals. +hen

diBerential !llup equipment is used, use all available literature regardingpercent !llup and record the weight at 0111-ft &31=-m' intervals during casingdescent.


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• Cetermine the number of barrels or pump stro#es needed to displace the

pipe after the casing is landed and to circulate one full hole volume, and thenumber of barrels of mud required to displace the cement.

• stimate the rate of cement mixing and displacement, plus annular

velocities.

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Primary Cementing Operations

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