Oilfield Review

Concrete Developments in Cementing Technology

Jean Marc BoisnaultDominique GuillotMontrouge, France

Abderrahim BourahlaTimothy TirliaAnadarko Algeria CompanyHassi Messaoud, Algeria

Trevor DahlPanCanadian Petroleum Ltd.Calgary, Alberta, Canada

Chris HolmesA.M. RaiturkarPetroleum Development OmanMuscat, Sultanate of Oman

Pierre MaroyClamart, France

Charles MoffettHunt Petroleum CorporationJena, Louisiana, USA

Genaro Pérez MejíaIgnacio Ramírez MartínezPetróleos MexicanosVillahermosa, Mexico

Philippe RevilHouston, Texas, USA

Robert RoemerAberdeen, Scotland

Perhaps the most difficult borehole fluid to handle, cement is critical to the

performance and life of a well. Optimal slurry properties for placement of

standard oilfield cements typically do not coincide with optimal mechanical

properties of set cement necessary for long-term zonal isolation. New

technology optimizes both slurry and set-cement properties simultaneously.

Since a flawlessly cemented wellbore protectsthe conduit that links reservoir fluids to the sur-face where they are used, high-quality oilfieldcement is an essential ingredient in any success-ful well. The quality and integrity of a cement jobcan determine how long a well remains stableand productive without requiring repair. In addi-tion to promoting ongoing operational safety andsuccess, today’s cements must also be designedwith cost savings and challenging operating envi-ronments in mind. Environmental protection is agreater concern than ever, especially protectionof shallow aquifers during and after drilling. Agood primary cement job is essential becauseremedial cementing (squeezing) is difficult toaccomplish and provides only temporary, localzonal isolation—it is preferable to do the job cor-rectly the first time. Overcoming the trade-offbetween cement slurry properties, includingrheology, fluid loss, pumpability and thickeningtime, and mechanical properties of set cement,such as compressive strength, porosity and per-meability, is a major challenge.

Traditional Cementing ApproachesThere are several fundamental purposes for plac-ing cement in oil and gas wells. Cement is usedto support the casing. In addition, it hydraulicallyisolates the various formations the well pene-trates, thereby protecting aquifers and prevent-ing fluid flow from high-pressure to low-pressureformations, which might result in a loss of hydro-carbon production or excessive water production.Cement guards against fluid broaching to the sur-face, which could lead to a catastrophic blowout.Cement also protects the casing from corrosionby chemically aggressive brines.

In the past, the least expensive material andtechnology—typically displacing drilling fluids by pumping Portland cement behind casing—were acceptable in all but the most difficultcases. Portland cement mixes easily with waterto produce a slurry that is readily pumpable andcan be placed anywhere within hydrostatic pres-sure constraints of a wellbore. Prepared at therecommended water-to-cement ratio, Portlandcement fulfills the most important objective,hydraulically isolating the formations. Further-more, Portland cement is readily available world-wide and is inexpensive.

For help in preparation of this article, thanks to AndrewAcock and Kevin England, Dowell, Houston, Texas, USA;Tyler Bittner, Walter Chmilowski and Mike Roy, Dowell,Calgary, Alberta, Canada; Leo Burdylo, Oilfield Services,Sugar Land, Texas; Erling Prado-Velarde, Dowell,Villahermosa, Mexico; Tarek Ramadan, Dowell, Muscat,Sultanate of Oman; and Eugene Toukam, Dowell, HassiMessaoud, Algeria.CemCADE, CemCRETE, DeepCRETE, DensCRETE, DESC(Design and Evaluation Services for Clients), FLAC (fluid-loss additives for cement), GASBLOK, LiteCRETE,SqueezeCRETE, USI (UltraSonic Imager) and VariableDensity are marks of Schlumberger. Ping-Pong is a mark of Parker Brothers, Inc.

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Spring 1999 17

The usual method for placing a slurry in a wellduring primary cementing operations consists ofpumping a series of fluids down the casing whilethe fluid already in the well—the drilling mud—flows out the casing-formation annulus to sur-face. The first fluid pumped is usually a preflushor spacer, or both, that separates the drilling fluidfrom the cement slurry. The spacer must be com-patible with both the drilling fluid and the slurry,yet keep those fluids apart to preclude contami-nation of the slurry by drilling fluid. Such con-tamination degrades the quality of the setcement. This is followed by as many as fourslurries. The preflush-spacer-cement series mustdisplace from the annulus all fluids ahead of it toprevent development of mud channels within thecement sheath.1 Such channels allow formationfluid migration. The presence of mud can alsonegatively affect set-cement properties, for

example by inducing shrinkage cracks, reducingcompressive strength or increasing permeability.A mechanical plug is then launched into the cas-ing and displaced to the bottom of the well byanother fluid, typically the drilling fluid needed todrill the next section of hole. At the end of theoperation, the cement occupies the annularspace between the casing and the penetratedformation from the bottom of the hole up to thedesired level.

During the cementing operation, the criticalgoal is to maintain the pressure in the annulusbetween the pore and fracture pressures of thepenetrated formations at all times and all depthsthroughout the openhole interval.2 If the annularpressure becomes lower than the formation porepressure, fluids can flow into the annulus andlead to a potentially catastrophic situation, ablowout. At the other extreme, if the annularpressure becomes higher than the formation frac-ture pressure, then annular fluids can split thesurrounding rock, damaging the borehole andescaping into the formation.

The first factor affecting annular pressureduring drilling or cementing operations is thedensity of the fluids, which exert hydrostaticpressure on the exposed formations. The secondfactor, fluid rheology, governs the frictional pres-sures during placement. Though density is aparameter that can be controlled easily duringthe design and operation phases, the actualrheology of a fluid is more difficult to control ormodify. Once these properties have beendesigned properly for a given operation, such aswith CemCADE cementing design and evaluationsoftware or other simulators, it is important thatthey be maintained within reasonable toler-ances during the entire placement operation.The cement slurry must be stable—solid parti-cles that are denser than the water in which theyare suspended must not separate from the liquidduring either static or dynamic conditions. The

1. Bonett A and Pafitis D: “Getting to the Root of GasMigration,” Oilfield Review 8, no. 1 (Spring 1996): 36-49.

2. Aldred W, Cook J, Bern P, Carpenter B, Hutchinson M,Lovell J, Rezmer-Cooper I and Leder P: “Using DownholeAnnular Pressure Measurements to Improve DrillingPerformance,” Oilfield Review 10, no. 4 (Winter 1998): 40-55.

slurry must not lose excessive interstitial waterto the formation when the pressure in the annu-lus is higher than in the formation. Excessivefluid loss from a slurry can increase the viscosity,which might result in incomplete placement ofthe slurry and bridging of the annulus, and canalso lead to volume reduction in the cement, pro-ducing channels or other defects.3 Finally, theslurry should not thicken or set prematurelyduring placement.

Performance of conventional cement slurriesultimately is a function of many variables, includ-ing the amount and types of solids, water, chem-ical additives, temperature and pressure.Weighting agents increase density; extendersdecrease it. Dispersants control rheology bybreaking larger particles into smaller ones, whichcan reduce viscosity. Stability is either intrinsic tothe design, or improved by using free water con-trol or solid-suspending agents (antisettlingagents). Fluid-loss control is achieved by addingFLAC fluid-loss additives for cement. Retarders oraccelerators control thickening time. Clearly,chemical additives define the performance ofPortland cement slurries.

Once in place, the cement slurry should setquickly and develop adequate strength to mini-mize the time spent waiting on cement (WOC) sothat the operator can proceed with the nextphase of well construction as soon as possible.

Limitations of Conventional Cementing TechnologyGood slurry and set-cement properties are mutu-ally exclusive in many conventional cementingsituations. For example, standard high-densitycements, while necessary for well control inhigh-pressure drilling, are difficult to pump andprone to sedimentation as weighting agents set-tle out of suspension. Low-density slurries withproportionately higher liquid volumes developcompressive strength slowly and attain low finalcompressive strengths, limiting their value whencementing production casing. Although chemicaladditives are crucial to successful cementingoperations, the ultimate performance of conven-tional cement systems is dominated by thewater-to-cement ratio.

The optimal water-to-cement ratio is about44% by weight for a low-viscosity, stable slurryof API (American Petroleum Institute) Class Gcement, one of the most commonly used Portlandcements in the oil field. This gives a density ofaround 15.8 lbm/gal [1900 kg/m3]. Higher densi-ties can be reached by either decreasing thewater-to-solid ratio or increasing the density ofthe solid blend at a given water-to-solid ratio.When the water-to-cement ratio is close to theoptimal value, the better choice is to reduce theamount of water; but this quickly leads tounpumpable or unmixable slurries. At that point,the only option is to add weighting materials tothe cement, normally high-density minerals suchas barite, hematite (the most common weightingagent) or ilmenite. The densities of these miner-als are 35 to 43 lbm/gal [4200 to 5200 kg/m3],whereas the density of Portland cement is about27 lbm/gal [3200 kg/m3].

To achieve lower densities, the methods arereversed: either increase the water-to-solid ratioor add lightweight aggregates. Another possibleoption is foaming the slurry with gas—usuallynitrogen or air. When the water-to-cement ratioapproaches the optimal value, the simplestapproach is to add more water to the slurry, butthis jeopardizes its stability, reduces the strengthof the set cement and increases porosity and per-meability. To rectify stability problems, intersti-tial water can be viscosified using colloidal clays(bentonite or attapulgite), sodium silicates orhydrosoluble polymers. However, these cementsystems exhibit higher porosity and permeabilityonce set, which often precludes their use in crit-ical casing strings. Another technique consists ofblending Portland cement with lighter solid mate-rials such as diatomaceous earth, perlite, fly ash,fumed silica, blast furnace slag or hollow micro-spheres. This method works only in relativelynarrow density ranges where the water-to-solidratio is maintained above a given threshold forthe slurry to be mixable and pumpable.

A further problem with standard cementsystems is that remediation of unsatisfactory pri-mary cement jobs is difficult. Squeeze cementing,even when performed satisfactorily, merely pro-vides a temporary patch. Conventional cementsare difficult to place in small defects, such aspartially plugged perforations and damaged cas-ing, because of their relatively large particlesizes and poor injectability.

18 Oilfield Review

> Particle-size optimization. A slurry made from particles of a single size (left) contains larger water-filled spaces than a slurry made from an optimized blendof several particle sizes (right). The smallest particles fill the spaces between larger particles and function much like lubricating ball bearings.

3. Bonett A and Pafitis D, reference 1: 38.4. For more on primary cementing: Fraser L, Stanger B,

Griffin T, Jabri M, Sones G, Steelman M and Valkó P:“Seamless Fluids Programs: A Key to Better Well Con-struction,” Oilfield Review 8, no. 1 (Spring 1996): 42-56.

Spring 1999 19

Operators seek cementing materials that notonly are easier to place the first time, but alsooffer the best long-term performance. Cementsthat achieve compressive strength earlierreduce waiting time and increase efficiency.Because drilling a well is typically the culmina-tion of months or even years of intensive effort,including the acquisition and interpretation ofseismic data and planning well construction, itis critical to achieve 100% cementing successat the outset.4

Concrete ImprovementTypically, cements are weighted without consid-eration for the particle sizes of the ingredients(primarily cement and weighting agents). As therequired density increases, conventional addi-tives alone quickly lead to either an unpumpableor unmixable slurry if the solid-to-liquid ratio istoo high, or to a system that does not containenough cement to develop a reasonable strength.A new system, CemCRETE technology, is con-crete-based slurry technology to optimize slurryperformance during placement while ensuring ahigh set-cement quality. By adjusting the parti-cle-size distribution (PSD) of the different solids,this technique uses more solid particles in agiven slurry volume while keeping slurry rheologyreasonably low. This allows slurries with densi-ties as high as 24 lbm/gal [2900 kg/m3] to be

used to cement critical casing strings in wellswith high pressure gradients.

Because many traditional cement slurrieshave single-size particles, they can be visual-ized as a box full of Ping-Pong balls (previouspage). Between each ball, there are large air-filled voids. In a real slurry, the void spacewould be filled with water rather than air. In ahigh-performance slurry with engineered PSDoptimization, particles of three or more differentsizes are carefully selected. A box of Ping-Pongballs with green peas and grains of sand fillingthe voids is crudely analogous to a trimodal PSDCemCRETE system.

By adjusting the PSD of the solids in theblend, CemCRETE technology increases thesolids per unit volume of slurry above that of Portland cement slurries. This increasescompressive strength and reduces porosity andpermeability by achieving a higher packingvolume fraction (PVF) independent of slurrydensity. Packing volume fraction is defined as the ratio of the sum of the absolute volumesof all particles in the dry blend divided by the bulk volume of the dry-blend components. HigherPVF values generally indicate better set-cementproperties. For example, hexagonal packing ofidentical spheres results in a PVF of 0.74, butrandom packing of the same spheres achieves a PVF of 0.64. The packing volume fraction of

an optimized dry blend is increased by using a trimodal PSD, which in turn decreases set-cement permeability (below left).

Because the remaining fluid content is usedmore efficiently, CemCRETE technology usuallyrequires lower concentrations of most chemicaladditives compared to traditional approaches.Gas-migration technology is more easily appliedbecause of the lower water-to-solid ratio andbecause of the lower permeability and porosityof the cement slurry during the transition fromliquid to solid as the cement sets. The 35 to 45%porosity, or water content, of the new high-performance slurries is significantly lower thanthe average 55 to 75% porosity for standardslurries (below right).

In contrast to conventional Portland cement,state-of-the-art cements contain a specific blendof particles engineered for each specific slurrydensity. The PVF of the optimized blends com-monly exceeds 0.80. The high solids contentresults in stable systems that disconnect theslurry density from rheology, require few addi-tives and are easy to mix and place in operationsthat are as simple as ordinary jobs yet require nospecialized equipment. These systems exhibitlow porosity and permeability once set, even for slurry densities as low as 10 lbm/gal [1200 kg/m3]. More simply stated, physics suc-ceeds where chemistry often fails.

Extendedlightweight

cement

15.8-lbm/galClass Gcement

CemCRETEcement

0

0.05

0.10

0.15

0.20

Perm

eabi

lity,

mD

> Set-cement permeability. Permeabilities to gasof set conventional and extended lightweightcements can be as high as 0.20 mD. (“Extended”lightweight cements have high porosities, typi-cally 75%, because the slurry density is loweredby increasing the water-to-cement ratio.) Thegranulometric optimization of CemCRETE blendsresults in set-cement permeability below 0.05 mD.

12.5-lbm/galExtended

lightweightcement

15.8-lbm/galClass Gcement

10- to 24-lbm/gal

CemCRETEcement

0

12

10

8

6

4

2

14

77%porosity

59%porosity

40%porosity

Mix

wat

er n

eede

d, g

al/s

ack

> Slurry porosity. High water content, or porosity,of a cement slurry improves its pumpability, butcan lead to sedimentation in the slurry andhigher permeability and lower compressivestrength once the cement sets. Conventionalslurry porosities range from 55 to 75% or more,whereas CemCRETE slurry porosities are typi-cally 35 to 45%. Sized particles in the optimizedblend ensure high strength in the set cement andgood slurry rheology despite low water content.

20 Oilfield Review

DensCRETEcement

Conventionalcement

DensCRETEcement

Conventionalcement

DensCRETEcement

Topsectiondensity

Middlesectiondensity

Bottomsectiondensity

17.8lbm/gal

18.1lbm/gal

17.5lbm/gal

18.7lbm/gal

19.3lbm/gal

19.6lbm/gal

18.7lbm/gal

20.7lbm/gal

20.9lbm/gal

18lbm/gal

18lbm/gal

19.5lbm/gal

19.5lbm/gal

21lbm/gal

21.2lbm/gal

> Sedimentation and segregation. In the BP settling test, a column of set cement cured under controlled pressure andtemperature is cut into sections and the density of each cylindrical section is measured. High-density conventionalcements tend to show greater vertical density variation because the weighting agent tends to settle out of suspensionas the cement sets. DensCRETE cements, or high-density CemCRETE cements, show little variation in density from topto bottom because the network of particles and associated reduced water content inhibit sedimentation or segregationof the heaviest particles. Each column represents a different cement type and density, with density variation measuredin the top, middle (where the designed density is most likely to be found) and bottom sections of the column.

0

50

100

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30 40 50 60Solid volume fraction, %

Conventional slurryCemCRETE slurry

Conventional slurry (20% fluid loss)CemCRETE slurry (20% fluid loss)

Plas

tic v

isco

sity

, cp

> Fluid-loss effects. As slurries lose fluids to permeable formations, plasticviscosity tends to increase. Compared with optimized slurries, conventionalPortland cement slurries tend to suffer greater increases in plastic viscosityper unit of fluid loss. The bottom two curves show the difference in viscositybetween an optimized blend and a standard blend. The top two curves showthe increase in viscosity after both slurries have lost 20% of their fluid. Optimized blends suffer less viscosification per unit of fluid loss.

030 40 50 60

250

200

150

100

50

Solid volume fraction, %

Monomodal silica suspension in 0.15 M NaCl, PVF 0.5

Trimodal silica suspension in 0.15 M NaCl, PVF 0.8

Plas

tic v

isco

sity

, cp

> Plastic viscosity of silica suspensions. A dry blend consisting of amonomodal particle-size distribution produces a high-viscosity slurryeven at a relatively low solids content. The blend with the trimodalparticle-size distribution, typical of CemCRETE technology, achievesbetter slurry properties and contains more solids per unit volume.

Spring 1999 21

The rheology of CemCRETE slurry is decou-pled from its density (previous page, top left).These water-reduced slurries have constant vis-cosities even at high densities, low gel strengthsand are easy to place. Low water content dimin-ishes sedimentation (previous page, bottom), orseparation of liquid and solids during cementing,yielding higher compressive strength and lowerpermeability (previous page, top right). The spe-cially engineered particle sizes allow easy mixingand pumping because the smallest particles actlike ball bearings to provide lubricity for thelarger solids in the slurry. The compressivestrength of set CemCRETE slurries, whether ofhigh or low density, develops faster and reacheshigher levels than conventional cements (right)because of the low water content.

CemCRETE technology benefits not only pri-mary cementing applications, but also remedia-tion. Particle-size optimization inhibits prematuredehydration of the slurry and the associated fric-tion-pressure increase that commonly preventsany remedial slurry from achieving deep penetra-tion. Water-reduced primary cements have alower incidence of costly remediation thanPortland cements.

Additional benefits are that CemCRETE tech-nology does not require specialized equipment orpersonnel, and while never desirable, mixingerrors are better tolerated in the new slurriesthan in Portland cement. Optimized dry blendsmay be mixed with fresh water, seawater or saltwater. Optimized slurries can include conven-

tional defoamers, accelerators, dispersants,retarders, fluid-loss control additives, right-angleset (RAS) additives and GASBLOK gas migrationcontrol cement technology. In fact, the combina-tion of specialized gas-migration control addi-tives, low bulk shrinkage and rapid strengthdevelopment of optimized cements is breakingnew ground in gas-migration control. Clearly, asexemplified in the case histories that follow,advanced cementing technology can be tailoredto specific needs by changing components of the dry blend.

Specialized ApplicationsThere are four broad applications of CemCRETEtechnology, encompassing low-density, high-density, remedial and deep-water cementingsituations. LiteCRETE slurry systems have lowdensities and are ideal for cementing weak for-mations or eliminating a casing string or a riskymultiple-stage operation (below). LiteCRETE slur-ries of 9.7 to 13 lbm/gal [1166 to 1563 kg/m3]perform comparably to ordinary 15.8-lbm/gal[1900 kg/m3] slurries. Optimized lightweightcement develops compressive strength earlier

6000

5000

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Time, hr

8 16 24

18-lbm/gal DensCRETE slurry 12-lbm/gal LiteCRETE slurry15.8-lbm/gal conventional slurry

Com

pres

sive

stre

ngth

, psi

> Compressive-strength development. CemCRETE slurries, both low-density LiteCRETEcement and high-density DensCRETE cement, develop compressive strength earlier and reach higher levels than conventional cement slurries. Rapid compressive-strengthdevelopment reduces waiting-on-cement time and speeds well construction.

Conventional cement LiteCRETE cement

Weak zone

LiteCRETE Stage-Operation Replacement

Conventional cement LiteCRETE cement

Zone 2

Zone 3

Zone 1

LiteCRETE Cementing Production Liner

Tailslurry1

2

LiteCRETE Cement Plugs

Conventional cement LiteCRETE cement

Fillerslurry

> New approaches to common problems. LiteCRETE cement (left) can replace stage-cementing operations, saving rig time and avoiding a complex, moreexpensive operation. Here, the two-stage cementing operation on the left has a weak zone that is eliminated in the single-stage LiteCRETE operation on theright. For cementing production liners (center) or casing across a weak or depleted zone, high-quality cement is placed across the primary pay zone as atail slurry at the bottom of the well. Shallower formations, isolated with lower-quality filler slurry, cannot be completed without additional cementing work.LiteCRETE cement can be placed throughout the entire annulus so that any zone may be completed without additional cementing work, such as blocksqueezes. Placing a higher density cement plug in a lightweight fluid (right) can lead to instability as the fluids intermix. Cement placement is improved bymatching low fluid densities with LiteCRETE slurries, which prevents fluid contamination and degradation of set-cement properties.

than conventional cement, reducing WOC time.In addition, this type of slurry is more stable thanlow-density Portland cement slurries because ofits low water content. It is strong enough to beperforated cleanly and withstands fracturing andstimulation treatments (above left).5

DensCRETE technology offers better rheol-ogy at high density, adjustable density at thewellsite and improved well control duringcementing (above right). High-density, water-reduced cement is useful for whipstock plugsand high-pressure cementing operations, for sit-uations where the fracture and pore pressuremargin is narrow, and for grouting (injection of cement to consolidate seabed sediments orinjection of high-strength cement betweenpipes such as the legs of offshore platforms). A

high-performance, high-density slurry of 17 to24 lbm/gal [2040 to 2900 kg/m3] has a lowerequivalent circulating density than that of a con-ventional high-density cement slurry, allowingplacement even when the window between porepressure and fracture pressure is tight and con-ventional high-density slurries are inadequate.Slurry density can be adjusted by as much as 1 lbm/gal at the last minute on location withoutperturbing other slurry properties. DensCRETEslurries usually develop compressive strengthswell in excess of 5000 psi [34.5 MPa] and canreach 20,000 psi [138 MPa] in especiallydemanding applications.

For remediation of faulty cement jobs and for water control, SqueezeCRETE technologyoffers a new solution for wellbore repairs, such

as casing leaks, liner top leaks, old partiallyplugged perforations, channels behind casing,leaking stage tools, fractures or even squeezinga gravel pack (below). A SqueezeCRETE slurrysystem applies the new technology at themicroscale for injection into very small gaps orfractures in primary cements and casing.Optimized slurries with specially engineeredparticle-size distributions penetrate deeply notonly because of the small particle sizes of theblend, but also because their improved resis-tance to dehydration reduces viscosification dur-ing placement. The improved injectability thatresults from fine-sized particles is key to successin remediation. In addition to high injectability,SqueezeCRETE cement has high compressivestrength and low permeability. Strength makes

22 Oilfield Review

Squeeze throughgravel pack

Microannulussqueeze

Casing leakrepair

SqueezeCRETE Applications

Old perforationsqueeze

Top of linersqueeze

Repair of channelbehind casing

> Remediation success. Perhaps the most versatile application of CemCRETE technology, SqueezeCRETE slurries penetrate more effectively than othercement slurries. SqueezeCRETE slurries repair small microannuli and leaks in casing, channels in cement and liner tops. They can also isolate old, partiallyplugged perforations and even be placed through gravel packs.

> Clean perforating. While conventional cements can shatterduring perforating, CemCRETE cement remains intact afterperforating. The perforation diameter is 0.4 in.

DensCRETE Applications< Cementing high-pressure formations. In high-pressurewells with narrow pore-fracturepressure windows, the frictionpressure increase in a tight annulus during cementing canfracture the formation (left), leading to improper zonalisolation. DensCRETE slurrieshave lower viscosity, allowingslurry placement throughout the annulus. In deviated holes, standard high-density slurries are prone to sedimentation ashematite particles settle on the low side of the wellbore and do not contribute to the totalhydrostatic pressure (right). This instability can lead to serious well control problems.

Spring 1999 23

SqueezeCRETE cement an appropriate materialto plug wells upon abandonment, although it ismore commonly applied to remediate wellboreproblems that cannot be repaired with typicalcementing materials.

SqueezeCRETE technology succeeds wherestandard gels used for water-control applica-tions might fail, including remediation of cross-flow behind casing and as a tail behindconventional gel treatments. When water cross-flow behind the casing is diagnosed, the paththrough the primary cement sheath might not yet be large enough to place ordinary squeezeslurries. On the other hand, the path may alreadybe so large that a standard gel used for water-control applications cannot perform correctly or withstand the differential pressure once thewell returns to production. The advanced slurryexperiences a lower viscosity increase for thesame volume of fluid loss than conventionalsqueeze cements. Its enhanced fluid-loss controlproperties, commonly better than those ofdrilling fluids, greatly improve slurry penetrationproperties: it can penetrate 120 micron slotsmore than 10 times farther than well-dispersedsqueeze slurries (top right).

Engineered slurry for squeeze applications isplaced after deep penetration through the chan-nel and set like ordinary primary cement. In thismanner, SqueezeCRETE technology restores theintegrity of the cement sheath and provides com-petent zonal isolation.

An alternative to foamed cement, DeepCRETEtechnology, has been developed for deepwaterwells. Foamed cement—cement plus nitrogen or air—requires specialized equipment and acementing team trained in its use (as well asavailability of nitrogen when air is not used),which might be logistically challenging andcostly on some offshore rigs and platforms.DeepCRETE cement develops strength faster,even at temperatures as low as 39°F [4°C], soWOC time is reduced when rig costs are calcu-lated by the minute, such as in deepwater areas.No specialized equipment clutters up limitedfloor space. LiteCRETE slurry systems can alsosubstitute for foamed cement.

Traditionally, cement jobs were planned byidentifying the application of the cement and thetotal hydrostatic limitations on the placedcement column. The liquid slurry density wasinferred from the physical properties necessaryfor the set cement. A major change precipitatedby new cementing technology is that the initialplanning step is to decide the slurry density first

and then the slurry porosity. From that, the spe-cific gravity of the dry blend is calculated and ablend designed according to the job parameters.6

CemCRETE technology results in cementproperties that ensure long-lasting zonal isola-tion. Its strong resistance to corrosion from acidstimulations and formation fluids is enhanced byits low permeability (above left). Its mechanicalintegrity is high, even in workover, perforatingand other specialized applications (above right).

Oilfield cement must withstand corrosionand CemCRETE cements provide good sulfateresistance when designed for that purpose.

SqueezeCRETE slurry Standard microcement slurry

Injection point Injection point

> Improved penetration of remedial cement. Squeeze cementing materials were injected through the valve on the left side of the 120-micron slots shown in the photographs. As indicated by the blackarrows below the slots, the SqueezeCRETE slurry (left) achieved deeper penetration into the narrowslot than the conventional microcement slurry (right), which lost more water earlier, viscosified andplugged the left side of the slot. Improved penetration reflects lower fluid loss and reduced viscosifi-cation, allowing SqueezeCRETE slurry to better repair tiny wellbore defects.

0

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40

Extendedlightweight

cement

15.8-lbm/galClass Gcement

CemCRETEcement

Cem

ent s

olub

ility

in m

ud a

cid

afte

r 4 h

r, w

t %

> Resistance to acid attack. Better zonal isolationis inherent in all CemCRETE systems because oftheir improved resistance to aggressive, corro-sive fluids, as demonstrated in laboratory testson cement solubility by acid or brine. This prop-erty makes LiteCRETE systems particularly valu-able for geothermal applications or when acidstimulation is planned, since low density andresistance to corrosive fluids are of paramountimportance in those situations.

1.00

1.05

1.10

1.15

1.20

1.25

1.30

12-lbm/galCemCRETE

cement

15.8-lbm/galClass Gcement

18-lbm/galCemCRETE

cementT/

E

> Cement integrity. The mechanical integrity ofcement, or its ability to withstand stresses fromperforating, hydraulic fracturing and other opera-tions, is critical for long-term zonal isolation. Theratio of the tensile strength (T) and Young’s modu-lus (E) is one indicator of the relative performanceof different cements. The higher T/E of CemCRETEcements reflects their superior integrity.

5. For more on high-performance, lightweight cement slurries: Moulin E, Revil P and Jain B: “Using ConcreteTechnology to Improve the Performance of LightweightCements,” paper SPE/IADC 39276, presented at theSPE/IADC Middle East Drilling Technology Conference,Bahrain, November 23-25, 1997.Revil P and Jain B: “A New Approach to Designing High-Performance Lightweight Cement Slurries for ImprovedZonal Isolation in Challenging Situations,” paperIADC/SPE 47830, presented at the IADC/SPE Asia PacificDrilling Technology Conference, Jakarta, Indonesia,September 7-9, 1998.Sumartha I and Martinez R. JA: “Application of a New Technique for Lightweight Cement Formulation toImprove Cement Placement in Campeche Bay Area,”paper SPE 39889, presented at the SPE InternationalPetroleum Conference and Exhibition, Villahermosa,Mexico, March 3-5, 1998.

6. Moulin E et al, reference 5.

Also, their low permeability inhibits water per-colation into the cement, slowing corrosion (seebottom left figure, page 19 ). Destructive events,such as repeated freeze-thaw cycling, tectonicactivity, production-induced subsidence andthermal expansion during production and testsprior to abandonment of wells, can impactcement integrity.

Protection of shallow aquifers is an ongoingconcern, so regulatory requirements for cementperformance, such as in well abandonments, arebecoming stricter in many areas. Recently,prudent operators have recognized that surfacecasing should be cemented as carefully asproduction liners. New high-performance oilfieldcements have greater reliability than traditionalcements, even in extreme conditions, so using thebest technology available might help operatorsmeet stricter environmental protection standards.

During 1998, more than 250 CemCRETE jobswere carried out in 20 countries (left). LiteCRETE,DensCRETE and SqueezeCRETE technologieshave been used in most cases, althoughDeepCRETE technology, introduced at the end of1998, is also gaining popularity.

Elimination of Stage-Cementing OperationsIn the Hassi Berkine field in the Ghadames basinof Algeria, Anadarko Algeria Company usesLiteCRETE cement to avoid stage-cementingoperations and better protect the supply of freshwater coming from the overpressured Albiansandstone. The Albian aquifer overlies oil-producing Cambrian sandstones and underliessalty Senonian carbonate and evaporite rocks.Additional geologic complications include theweakness of certain formations below the Albianthat are prone to lost circulation during drillingand the potential for flowing salt. The previousapproach had been to set a stage tool below the Albian, cement the lower zones, and thenisolate the Albian in the second stage of cement-ing operations.

Stage cementing resulted in higher costs thana single-stage operation and suboptimal zonalisolation that often required remedial cementing.After careful consideration of the risks andrewards of different approaches, Anadarko chosea solution proposed by Dowell engineers—single-stage cementing using a LiteCRETE slurry.Key factors that make this preferable to conven-tional cementing include rapid setting time, highcompressive strength, low set-cement porosityand permeability that result in better zonal isola-tion and superior resistance to corrosive forma-tion fluids (left).

24 Oilfield Review

257 CemCRETE jobs worldwide in 1998

> Locations of CemCRETE operations. The size of each circle is proportional to the number of jobs inthe area. During 1998, more than 250 cementing operations using CemCRETE technology demonstratedthe versatility of optimized cement blends in a variety of critical casing operations. Stage-operationreplacement has been the most significant application to date.

Fresh water

Typical Casing Program LiteCRETE Casing Program

Low fracturegradient

Fresh water

Low fracturegradient

The stage tool created aweakness in the 9 5/8-in.casing, requiring 7-in.casing to surface

9 5/8-in. intermediate casingcemented in two stages to cover freshwater zone withlow-permeability cement

9 5/8-in. casingcemented in onestage with LiteCRETE slurry

7-in. full production string

7-in. production linerreplaces the full stringdue to the eliminationof the stage tool

> Elimination of stage-cementing operations. In the Hassi Berkine field, Alge-ria, LiteCRETE technology meets multiple operational challenges: protectionof freshwater supplies, high strength with low density and reduced cost andrisk. Senonian carbonate and evaporite rocks must be isolated from underly-ing Albian sandstone, a freshwater aquifer. Oil production comes from deeperCambrian sandstones. By eliminating stage-cementing operations, a 7-in. pro-duction string to the surface can be replaced by a 7-in. production liner.

Spring 1999 25

The cost savings associated with the single-stage operation and decreased need for remedialcementing were also compelling. A typical sin-gle-stage operation in this area can save almosta full day of rig time and decrease costs of fluidcontamination that might occur during the firststage of cementing. Additional savings stemfrom the low incidence of remedial work, whichtypically requires two days of rig time as well asadditional cementing costs. The elimination ofthe stage tool removes a known weak point fromthe 95⁄8-in. casing string, making it possible toreplace a full 7-in. production casing to surfacewith a 7-in. production liner, saving on tubularand cementing costs as well as rig time (above).7

In the United Arab Emirates, Abu DhabiCompany for Onshore Oil Operations (ADCO) has performed similar successful single-stageLiteCRETE cementing operations.8

Ongoing collaboration between engineersfrom Dowell and Schlumberger Wireline &Testing has improved interpretation of bond logsof lightweight cementing systems. In the past,acoustic properties were incorrectly related tocompressive strengths of cement, resulting in afalse expectation of similar log responsesbetween 15.8-lbm/gal Portland and LiteCRETEsystems. The new systems have compressivestrengths as high as 15.8-lbm/gal Portland

cements, but their acoustic impedances arebetween 15.8-lbm/gal cements and ordinarylightweight cements. LiteCRETE systems displaya lower acoustic impedance contrast withdrilling fluids, producing a different logresponse, so log interpretation for these systemsis not as straightforward.

Fluid compensated CBL amplitude (CBLF)

0 MV 50Transit time (TT)

400 µsec 200Transit time (Sliding Gate) (TTSL)

0 MV 50

CCL (CCLU)

-35 5

Predicted Amplitude for 100% BI fromDowell cement data (DCD PA 100 BI)

0 MV 50

Predicted Amplitude for 80% BI fromDowell cement data (DCD PA 80 BI)

0 MV 50

10 20in.Bit size (BS)

10 20in.

Caliper 1(DCD CALI1)

0 100API

Gamma Ray(GR)

Gas fromLHF2 to USGI

Liquid fromUSGI to USBI

Bonded fromUSBI to LHF2

0.00000.30001.90002.09092.28182.47272.68362.85453.04543.23643.42733.61823.80914.0000

Cement map withimpedance

classification

Min Amplitude

Sonic_VDL_Curve (VDL)

Max

200 µsec 1200

< Evaluation of LiteCRETE cement using bondlogs. The USI UltraSonic Imager (USI) log,cement bond log (CBL) and Variable Density log(VDL) from a well in Algeria give informationrelated to the presence of a 10.85-lbm/gal [1.33-kg/l] LiteCRETE cement behind 95⁄8-in. casing. In the first track (from left to right), thegreen gamma ray curve shows minor lithologyvariation with depth; the black curve indicatesbit size and the red curve hole size (as uploadedfrom the CemCADE software). The bond index isdenoted from 100% to 0% in track 2, with yellowindicating cement behind the casing. Thecement map in the third track is a circumferen-tial representation of the material presentbehind the casing. The cement map was generated by rescaling USI UltraSonic Imagerdata from the default (0 to 8 MRayl) to a scaleof 0 to 4 MRayl to better fit the lower acousticimpedance of LiteCRETE cement, which averages 3 to 4 MRayl. Dark areas, equivalent to 4 MRayl here, indicate excellent cement bondto the casing. The fourth track displays classiccement bond log information, including ampli-tude (solid purple), transit time (blue and red dotted) and casing collar locations (black). Additionally, the orange and green solid linesrepresent the expected amplitude for 100% and 80% bond (as predicted by the CemCADEsimulator). The amplitude values are higherfor LiteCRETE cement than for standard, heavier cements, which typically have greaterattenuation. Finally, the Variable Density cementbond log (VDL) in track 5 provides informationabout the quality of the cement-formation bondby displaying a color-coded traveltime trace atevery depth. The relatively low color contrast(low amplitudes) at early times indicates weakcasing arrivals, which is to be expected for agood bond between the casing and a relativelylow acoustic impedance cement. (A high acoustic impedance cement under the same circumstances would give lower amplitudes and weaker casing arrivals, if any.) The highercolor contrast (high amplitudes) at later times represents arrivals from the formation, whosevelocity varies with lithology, and correlatesroughly with lithology indicated in the gamma ray log.

7. For more on single-stage cementing operations inAlgeria: Toukam E: “New Cement Improves Costs,Operations In Northern Africa,” Petroleum EngineerInternational 72, no. 3 (March 1999): 23-29.

8. Mukhalalaty T, Al Suwaidi A and Shaheen M: “IncreasingWell Life Cycle by Eliminating the Multistage Cementerand Utilizing a Light-Weight, High-Performance Slurry,”paper 53283, presented at the SPE Middle East Oil Show,Bahrain, February 20-23, 1999.

Whipstock Plugs and Liner CementingIn Mexico, Petróleos Mexicanos (PEMEX) hasused LiteCRETE cement for whipstock plugs andliner cementing. PEMEX initially used thelightweight optimized blend for whipstock plugsto kick off deviated wells past irretrievable fish.The success ratio of kickoff plugs has beenimproved greatly by using the new technology ina low-density environment. The matched densi-ties of the drilling fluids and cement slurries pre-vented swapping and mixing of fluids duringplacement and ensured development of therequired compressive strength.

In a field with a low fracture gradient in theVillahermosa region, CemCRETE technologyproved to be the best answer for cementing deep (4500- to 5000-m) [14,760- to 16,400-ft],depleted, fractured, dolomitic Mesozoic carbon-ate reservoirs. Lightweight cement is employedbecause the reservoirs have a low fracturegradient.9 In one deviated well, PEMEX elected tokick off in order to reach a better part of thereservoir (above left). A special 15-lbm/gal opti-mized whipstock plug material designed forPEMEX reached a compressive strength of 3750 psi [26 MPa] within eight hours and a finalcompressive strength of 4203 psi [29 MPa] in12.5 hours, allowing the sidetrack to be com-pleted successfully.

Liner cementing has also been improvedthrough the use of new cementing technology.Because of the low formation pressure and sus-ceptibility to fracturing, a low-density slurry wascritical to success. In one case, an 11.1-lbm/gal[1330-kg/m3] LiteCRETE slurry was used tocement a 5-in. production liner from 13,399 ft to 15,095 ft [4084 to 4600 m]. The cementdeveloped a compressive strength of 1200 psi[8273 kPa] after eight hours. A cement bond logconfirmed a good seal between the liner cementand formation.

The overall cost of using LiteCRETE technol-ogy, including service, products and rig time, islower than the cost of using traditional technol-ogy. PEMEX reduced the cost of rig time duringcementing by 30% because new lightweight slur-ries develop compressive strength rapidly. Byusing optimized cement for kickoff plugs, PEMEXsaved 45% of the total operation cost comparedwith the use of conventional cement, which com-monly entailed repeating the cement plug. Also,remedial squeeze operations have not been nec-essary. Conventional jobs commonly required oneor two squeezes.

Cementing Shallow, Low-Pressure WellsHunt Petroleum Corporation has used LiteCRETEcement to complete five wells in the Olla field,LaSalle Parish, Louisiana, USA. Shallow Wilcoxoil wells, with total depths of 3500 ft [1067 m]and bottomhole static temperatures of 129°F[54°C], have low bottomhole pressures and lowfracture gradients, so getting a column of cementhigh enough in the annulus has proven difficult.In the past, as many as three block squeezes perwell were performed to remediate poor primarycement jobs in 51⁄2-in. casing (left).

The Wilcox reservoir in Olla field has a strongwaterdrive. Productive zones are completed byperforating the top of the productive intervalabove the oil-water contact. Offset wells com-monly produce high volumes of water at watercuts greater than 95%. The wells completedwith LiteCRETE cement produce at water cutsless than 85% water, but, more importantly, thetotal volume of water produced is significantlyreduced. Hunt Petroleum interprets the reducedwater production as verification of proper isola-tion of the producing zone from nearby zonesthat contain 100% water. The additional waterproduction in the offset wells has beenattributed to water channeling from nearbywater zones; radioactive tracer injection logshave verified this. None of the wells in whichHunt Petroleum used LiteCRETE cement hasrequired remedial work.

Besides reducing the need for remedial work,Hunt Petroleum has lowered total well costs onWilcox completions by avoiding the mechanicalrisks associated with squeezing operations. Suchrisks include the possibility of setting the cementretainer incorrectly, drilling a hole in the casingwhen drilling out the cement retainer, splittingcasing during the squeeze, cementing the work-string if cement sets up early, or fracturing into awater zone. Because LiteCRETE cement columnsextend higher in the annulus, upper zones of theWilcox may be completed without additionalcementing to cover these zones, which generallyare not covered during conventional operations.

26 Oilfield Review

Cement plug

2878 m

4150 m4160 m

Optimized CemCRETE Plug for Sidetracking

> High-performance lightweight slurry. Optimized,low-density blends are used for whipstock plugsand liner cements in depleted reservoirs with low fracture gradients. In this example, PEMEXdecided to sidetrack to reach a better part of the reservoir. By using CemCRETE technology,PEMEX has improved its success ratio for kickoff plugs and minimized WOC time.

8 5/8-in., 24-lbm/ft surfacecasing at 1712 ft

Weak zone

5 1/2-in., 15.5-lbm/ft production casingat 3150 ft

Cementing Low-Pressure Zones

> Cementing in a low-pressure gradient. Theuse of conventional cements in the Olla fieldtypically required two or three block squeezesafter each primary cementing operation. UsingLiteCRETE slurry systems on five wells improvedzonal isolation without block squeezes. It alsomakes it possible to complete shallower zoneswithout additional cementing work. In thisexample, the LiteCRETE slurry column could be placed high enough in the annulus to coverthe weak zone.

9. Pérez Mejía G, Ramírez Martínez I and Prado-Velarde E:“Optimización de los Tapones de Desvío y Liners,Utilizando un Sistema de Cemento de Baja Densidad yAlta Resistencia a la Compresión (LBDARC), en laRegión Sur de Pemex, México,” presented at the XICongreso Latinoamericano de Perforación, BuenosAires, Argentina, October 25-29, 1998.

10. In the North Sea, a LiteCRETE blend remained on a sup-ply boat for several days in bad weather. Nevertheless,the blend did not segregate during its rough journey tothe wellsite.

Spring 1999 27

Cementing High-Pressure WellsHigh-pressure wells benefit from the use ofreduced-water cements. Petroleum DevelopmentOman (PDO) first adopted DensCRETE technologyto address numerous challenges in fields such asthe Al Noor and Sarmad fields of southern Oman.While adjustments to the mud system and casingprogram can reduce the cost and risk of drillingoperations, the use of new cementing technologywas the most important factor in improving oper-ations for PDO.

In the southern Oman fields, PDO produces oilfrom stringers of tight Cambrian Athel silicilyteembedded in salt. The Athel reservoir, which isalso a world-class hydrocarbon source rock, is upto 400 m [1312 ft] thick and contains 80 to 90%microcrystalline silica, with an average porosityof 22% and permeability below 0.05 mD. Highdrawdown pressures are applied to produce oilfrom such a tight reservoir, so it is crucial tomechanically isolate the individual stringers ofreservoir rock.

Drilling and completing such wells suc-cessfully are challenging. At depths of 3500 to4800 m [11,483 to 15,748 ft] and temperatures of90°C [194°F], pressure control dictates a high-density slurry. Segregation of the weightingagent, hematite, from conventional dry blendsduring transport across graded roads led to diffi-culty mixing and pumping slurries and up tothree hours of lost time to clean pluggedcementing lines. Displacing heavy muds withhigh rheologies was inefficient. There was a nar-row window between the formation pore pres-sure of 16.2 lbm/gal [1941 kg/m3] and formationfracture pressure of 20.4 lbm/gal [2444 kg/m3],as well as a low differential pressure betweenthe 17-lbm/gal [2037-kg/m3] mud system, 18.3-lbm/gal [2193-kg/m3] spacer and 19.6-lbm/gal[2348-kg/m3] cement. There was little leeway toadjust densities and displacement rates.

Contamination of fluids by salt-saturated mudled to instability. Bulk shrinkage of set cementoften resulted in microannuli. In at least onewell, a microannulus was not detectable with acement bond log, but was discovered when pres-sure in the annulus rose. Finally, when comparedwith conventional cements, CemCRETE slurriesset faster at the top of the liner, which reducesthe risk of fluid migration. In one well, a gas kickoccurred 14 hours after conventionally cementingthe liner and it took four days to control the welland avoid a blowout.

Before approval for the initial use ofDensCRETE cement by PDO, numerous tests byPDO and by Dowell in Oman and at theSchlumberger-Riboud Product Center in Franceconfirmed that the advanced technology wouldsurpass critical performance requirements. Inaddition to exceeding the performance of tradi-tional cements in 8-hour compressive strength,24-hour compressive strength, stability andshrinkage, DensCRETE cement offered greaterability to optimize slurry rheology and density(below). A yard trial in early 1998 also demon-strated that the DensCRETE blend would not seg-regate during transport, remained mixable aftertransport and passed relevant API tests, such asrheology, compressive strength and fluid loss.10

The first DensCRETE operations in Omanwere performed during the second quarter of1998 on the Sarmad-1 well, placing cement plugsat 4100 m [13,451 ft] and 4300 m [14,108 ft] with 21.5-lbm/gal [2576-kg/m3] slurry and a 7-in.liner at 3850 m [12,631 ft] with 19.5-lbm/gal[2337-kg/m3] slurry (above). Because the wellencountered a fault and fluid losses occurred justabove total depth, PDO decided to set plugsabove the fault and then cement the liner usingDensCRETE cement for both operations. Theplugged interval exceeded 200 m [656 ft] in thick-ness, so the plug was set in two stages.

To date, seven DensCRETE cement jobs havebeen performed in the area for PDO, includingthree liner jobs and four plugs for abandonmentof high-pressure wells. The slurry is less sensi-tive to salt-saturated mud contamination thanordinary cement. As the optimized high-densitycement sets, it is less prone to forming amicroannulus because it suffers less bulk shrink-age. Even in long liners, no density gradient isobserved in the set cement column in the annu-lus. The column is uniform and stable, even asthe cement is setting, so the risk of a blowout isreduced. The top of DensCRETE plugs is closer tothe theoretical top than that of conventionalplugs because the rheology of optimized high-density slurries allows more efficient removal ofdrilling fluids.

Compressive strength at 8 hr

Initial set 50 psi

Compressive strength at 24 hr

Shrinkage

Stability of set cement(BP settling test)

Tolerance to density variations

Separation of heavy particlesfrom blend during transport

0 kPa

After 20 hours

18,275 kPa [2651 psi]

1.5% after 24 hours

0.35 kPa/m [0.297 lbm/gal]top to bottom

Low

High risk

18,616 kPa [2700 psi]

Properties Conventional slurry DensCRETE slurry

After 4 hours

24,132 kPa [3500 psi]

0% after 24 hours

0.20 kPa/m [0.169 lbm/gal]top to bottom

High

Very low risk

< Laboratory testing. In testsconducted before the firstuse of high-performanceheavyweight slurry byPetroleum DevelopmentOman (PDO), DensCRETEslurries outperformed conventional heavyweightslurries. This superior performance carried overto field applications.

DensCRETE Plug and Liner Cementing

7-in. liner3850 m

4100 m

4300 m

Cement plug(21.5-lbm/gal)

Liner cement(19.5-lbm/gal)

> High-pressure cementing. In the deep, high-pressure Sarmad-1 well, PDO set 21.5-lbm/galDensCRETE cement plugs to counter fault-related fluid losses near total depth and then cemented the liner using a 19.5-lbm/galDensCRETE slurry.

> Water-control diagnosticplots. Log-log plots of theactual water-oil ratio (WOR)and its derivative (WOR’)versus time help differentiatebetween water-control problems, including waterconing and channeling, dur-ing production. Systematicflow model numerical simulations produced characteristic curves. Thesecurves are used to diagnoseproblems and then decidethe appropriate remedy. Thetheoretical representation ofbottomwater coning (upperleft) is similar to the actualfield example below it. Simulated multilayer channeling (upper right)also mimics actual multilayerchanneling observed in thefield (lower right).

The WOC time for conventional cements todevelop adequate compressive strength underthe conditions in the southern Oman fields is atleast 28 hours. DensCRETE cement achieves highcompressive strength in as few as 15 hours (theworst case to date has been 26 hours) and ulti-mately develops higher compressive strengththan standard high-density cement (right). Thedecrease in WOC time has proven especiallyimportant in drilling exploration wells, and therehas been a decreased need to repeat plugs orremediate liner cements. Thus, PDO plans tocontinue to use DensCRETE cement for high-pressure cementing operations.11

Water-Control ApplicationsSqueezeCRETE technology has been used inAlberta, Canada, for numerous squeeze jobs. Inthe Halkirk field northeast of Calgary, an oil welloperated by PanCanadian Petroleum Ltd. pro-duced 35 m3 [220 bbl] of oil nearly water-freefrom the Upper Manville ”I“ Glauconitic forma-tion upon its initial production in 1995. Within a year, however, water production increased from1 m3 [6 bbl] per day to more than 20 m3 [126 bbl]per day. By late 1998, the well was completelywatered out. From knowledge of reservoir geol-ogy and performance, the water influx wasattributed to layer breakthrough.

The attractive economics for remedial workprompted action. On the basis of known wellboreintegrity, a bridge plug was set above existingperforations at 1266.5 m [4154 ft] and the zoneabove it was reperforated, but water productioncontinued. After reviewing geological, reservoirand completion data, the water influx wasascribed to poor cement behind the bridge plug.Because of a large drawdown and close water

proximity, the Dowell DESC Design andEvaluation Services for Clients engineer wasasked to verify that water coning was occurring.

Water-control diagnostic plots, which displayraw historical production data versus time on alog-log scale, help identify water sources, suchas differentiating bottomwater coning frommultilayer channeling (below). Systematic flowmodel numerical simulations were performed to

28 Oilfield Review

0:00 1:45 3:30 5:15 7:00 8:45 10:30 12:15 14:00 15:45 17:30 19:15

Elapsed time

196

176

156

137

117

98

78

58

39

19

0

Compressive strength

Temperature

Com

pres

sive

stre

ngth

, psi

Tem

pera

ture

, °C

5200

4680

4160

3640

3120

2600

2080

1560

1040

520

0

> Rapid development of compressive strength. High-density optimized slurries develop compressive strength sooner than their conventional counterparts. In this examplefrom the Al Shomou-4 well, the 22-lbm/gal DensCRETE slurry achieved a strength of5000 psi in only 17 hr.

0.00011 10 100 1000

10

1

0.1

0.01

0.001

Time, days

0.0011 10 100 1000 10,000

10

100

1000

1

0.1

0.01

Time, days

0.00110 100 1000 10,000

10

1

0.1

0.01

Time, days

0.00011 10 100 1000 10,000

10

100

1000

10,000

1

0.1

0.01

0.001

Time, days

WORWOR'

WORWOR'

WORWOR'

WORWOR'

WOR

or W

OR'

WOR

or W

OR'

WOR

or W

OR'

WOR

or W

OR'

Spring 1999 29

produce characteristic curves for different typesof water production. On the basis of the water-control analysis for the Halkirk well, the diagno-sis was a high-permeability layer with waterbreakthrough (right). This problem was compli-cated by a microannulus that allowed water flowbehind the casing.

Because of low oil prices and the fact that themature Halkirk field is undergoing waterflooding,workover costs must be minimized to achieveacceptable economic results. Considerable effortis made to mitigate the risk and impact of unsuc-cessful treatments. Therefore, procedures with ahigh probability of success are favored. In thiswell, a conventional cement squeeze wasdeemed too risky. The SqueezeCRETE treatmentwas predicted to have a much higher probabilityof success, so the economics for that treatmentwere acceptable.

SqueezeCRETE slurry was placed across theperforations from 1263 to 1265.25 m [4144 to4151 ft] as a balanced plug, and a hesitationsqueeze was performed.12 After 24 hours, thecement was drilled out and successfully pres-sure- and swab-tested. Following reperforation,the zone is producing 28 m3 [176 bbl] of oil, 3100 m3 [110 Mcf] of gas and 0.32 m3 [2 bbl] of water per day, reversing the water cut from 99.5% before to only 1.1% after thesqueeze operation.

In another well in southern Alberta,PanCanadian wanted to shut off old perforationsand complete a deeper interval. Because theslurry feed rate into the old perforations was lessthan 20 L/min [5.3 gal/min], ordinary slurrieswould not be effective. After acid was spottedacross the perforations to increase the injectionrate, only minor improvement occurred.SqueezeCRETE slurry was then batch mixed and1.2 m3 [8 bbl] placed across the perforations, fol-lowed by a hesitation squeeze. After 48 hours,the cement was drilled out and the perforationswere successfully pressure tested and swabtested. The lower interval was subsequently per-forated and completed. Without a highlyinjectable remedial system like SqueezeCRETE

slurry, the operator might have risked impairingthe additional completion by using a casing patchto shut off the abandoned perforations.

SqueezeCRETE cement has the potential toaddress stringent well plugging requirements assome of the many shallow gas wells in westernCanada are abandoned. Its high injectability andlow permeability can repair gas leaks better thantraditional cementing materials.

Successful water-shutoff jobs have beenperformed using engineered squeeze cementselsewhere. In one case in the North Sea, oil pro-duction increased from 2000 to 4000 bbl per day[317 to 635 m3/d] while water productiondecreased from 7000 to 1500 bbl per day [1112 to 238 m3/d]. This sharp reduction inwater production made gas lift unnecessaryafter production resumed.

Also in the North Sea, BP Amoco plc success-fully abandoned a reservoir section in a well fromits Bruce platform using a single optimizedcement plug. After remedial completion effortsand other attempts to isolate and abandon thereservoir failed, SqueezeCRETE slurry waspumped through coiled tubing across the perfora-tions and then squeezed. BP Amoco plc was thenable to sidetrack an adjacent wellbore to reachthe reservoir.

Merely pumping a superior slurry does notalways effect the desired repair. Sound comple-tion engineering concepts, proper design andexecution are critical ingredients for successfulwell remediation.

Present and Future Value of Optimized CementsCemCRETE technology has proven its value onseveral fronts. Its early development of compres-sive strength saves rig time because drillingoperations can resume sooner. The reliability ofthe technology decreases the need for remedialblock squeezes or repetition of plugs. Repairs offaulty cement and casing are more effective thanever before. The risk and expense of stage-cementing operations are avoided with a single-stage operation using CemCRETE technology.Well designs can be optimized to avoid costlycasing strings.

The lower porosity and permeability of set cements using CemCRETE technology willallow safer abandonment of wells and isolationof aquifers from hydrocarbon zones. Low-permeability cements are more resistant tocorrosive brines and there is less bulk shrinkageas the cement sets, resulting in better zonal iso-lation over time. Studies are nearing conclusionon the enhanced durability of the new systemsover conventional cements when perforating.

The successful application of CemCRETEtechnology in 257 wells during 1998 provides afoundation for expansion of this versatile tech-nology from specialized initial applications tomainstream cementing operations. —GMG

0.001

0.01

0.1

1

10

100

1,000

10,000

10 1000100 10,000

Cumulative production time, days

High-permeability layerwater breakthrough

Initial wellborefluid cleanup

Wellbore waterholdup

Post-treatment WOR

WORWOR'

WOR

or W

OR'

> Halkirk water-control diagnostic plot. The PanCanadian Halkirk well produces oil from several layers. A log-log plot of the actual water-oil ratio (WOR) and its derivative (WOR’) versus cumulativeproduction time illustrates water breakthrough from a high-permeability layer. Increasing hydro-static pressure from wellbore water holdup significantly reduced oil production, and the water cut reached 99.5%. Successful shutoff of the water layer restored the previous oil production andreduced the water cut to 1.1%.

11. For more on the use of high-density slurries in well construction: Adamson K, Birch G, Gao E, Hand S,Macdonald C, Mack D and Quadri A: “High-Pressure,High-Temperature Well Construction,” Oilfield Review 10,no. 2 (Summer 1998): 36-49.

12. In a hesitation squeeze, a portion of the slurry is pumped,then pumping stops to expose the slurry to differentialpressure against the zone of interest in stages over a period from several minutes to several hours. This pressure, higher than necessary for fluid movement, isapplied to force filtrate from the cement slurry, leavingonly solid material in the area requiring repair. This pro-cedure is repeated until all the slurry has been pumped.The dehydrated cement remaining in the zone forms aseal with a higher compressive strength and lower per-meability than the original slurry design.