Five years ago, an article in Oilfield Reviewstated, “Understanding gas intrusion is anevolutionary process that has not yet run itsfull course.”1 Since then, the evolution hascontinued, providing a more detailed pic-ture of the downhole phenomena activeduring gas migration. Although many possi-ble solutions are similar to those available in1991, increased knowledge of gas entrymechanisms means that these solutions cannow be deployed in a more logical andcost-effective way.

Gas invasion occurs when pressure islower in the annulus than at the formationface. Gas then migrates either to a lowerpressure formation or to the surface. Theseverity of the problem may range fromresidual gas pressure of a few psi at thewellhead to a blowout. Whatever the sever-ity, the major factors contributing to gasmigration are common. Successfully achiev-ing a long-term annular cement seal beginsby understanding these contributing factors

36 Oilfield Review

Getting to the Root of Gas Migration

Art BonettCambridge, England

Demos PafitisSugar Land, Texas, USA

Of the two principal objectives facing primary cementing operations—casing support and zonal isolation—

the latter usually raises the most concern, and is perhaps the hardest to achieve when there is potential for

formation gas to migrate into the cement sheath. The challenge for industry is to achieve a long-term annular

cement seal and prevent formation gas entry. Successful handling of gas migration is an evolving science.

This article looks at causes, consequences, predictive methods, new solutions and the latest state of play.

For help in preparation of this article, thanks to ArtMilne, Dowell, Clamart, France and Tom Griffin, Dowell, Sugar Land, Texas.CemCADE, GASBLOK, GASRULE , VIP Mixer andWELLCLEAN are marks of Schlumberger. MicroVAX is atrademark of Digital Equipment Corp.

fluid densities are too high. Also, considera-tion must be given to the free-fall or U-tub-ing phenomenon that occurs during cementjobs.3 Therefore, cement jobs should bedesigned using a placement computer simu-lator program to assure that the pressure atcritical zones remains between the pore andfracture pressures during and immediatelyafter the cement job.

Any density errors made while mixing aslurry on surface may induce large changesin critical slurry properties, such as rheologyand setting time. Inconsistent mixing alsoresults in placement of a nonuniform col-umn of cement in the annulus that may leadto solids settling, free-water development orpremature bridging in some parts of theannulus. This is why modern, process-con-trolled mixing systems that offer accurate

37Spring 1996

and knowing what can be done to minimizeor counteract their effects.

In the past, various techniques have beendeveloped to tackle individual factors thatcontribute to gas migration. However, gasmigration is caused by numerous relatedfactors. Only by addressing each factor sys-tematically can a reasonable degree of suc-cess be expected. There is no single “magicbullet” for gas migration.

This article summarizes the current stateof knowledge about gas migration, drawingon field expertise from Dowell, and onexperimental work carried out predomi-nantly at Schlumberger Cambridge Research(SCR) in England. Much of this experimentalwork is unpublished.

Setting the SceneSuccessfully cementing a well that haspotential for gas migration involves a widerange of parameters: fluid density, mudremoval strategy, cement slurry design(including fluid-loss control and slurry freewater), cement hydration processes,cement-casing-formation bonding and setcement mechanical properties (above).

Although gas may enter the annulus by anumber of distinct mechanisms, the prereq-uisites for gas entry are similar. There mustbe a driving force to initiate the flow of gas,and space within the cemented annulus forthe gas to occupy. The driving force comeswhen pressure in the annulus adjacent to agas zone falls below the formation gas pres-sure. Space for the gas to occupy may bewithin the cement medium or adjacent to it.

To understand how, and under what cir-cumstances, gas entry occurs, a review ofthe main mechanisms, including cementhydration and resultant pressure decline,follows. First, however, no cementing articleis complete without emphasizing that goodcementing practices are vital.2 To effectivelycement gas-bearing formations the centralpillars of good practice—density control,mud removal and slurry design—are criti-cal, and here is why.

Density: Controlling the driving force—Gas can invade and migrate within thecement sheath only if formation pressure ishigher than hydrostatic pressure at the bore-hole wall. Therefore, as a primary require-ment, slurry density must be correctlydesigned to prevent gas flow during cementplacement. However, there is a danger oflosing circulation or fracturing an interval if

1. Bol G, Grant H, Keller S, Marcassa F and de RozieresJ: “Putting a Stop to Gas Channeling,” Oilfield Review3, no. 2 (April 1991): 35-43.

2. Bittleston S and Guillot D: “Mud Removal: ResearchImproves Traditional Cementing Guidelines,” OilfieldReview 3, no. 2 (April 1991): 44-54.

3. Cement free-fall or U-tubing occurs when the weightof the slurry causes it to fall faster than it is beingpumped. This must be considered when designingdisplacement rates and pumping schedules.

Wrong density Poor mud/filter-cake removal Premature gelation Excessive fluid loss

Highly permeable slurry High shrinkage Cement failure under stress Poor interfacial bonding

nMajor contributing parameters during the cementing process, in the order that they typically occur. Incorrectcement densities can result in hydrostatic imbalance. Poor mud and filter-cake removal leaves a route for gasto flow up the annulus. Premature gelation leads to loss of hydrostatic pressure control. Excessive fluid loss con-tributes to available space in the cement slurry column for gas to enter. Highly permeable slurries result inpoor zonal isolation and offer little resistance to gas flow. High cement shrinkage leads to increased porosityand stresses in the cement sheath that may cause a microannulus to form. Cement failure under stress helpsgas fracture cement sheaths. Poor bonding can cause failure at cement-casing or cement-formation interfaces.

density control are proving popular for criti-cal cement operations (left).

A cement slurry will not transmit hydro-static pressure forever. The transition from aliquid that controls formation pressure to animpermeable solid is not instantaneous. Con-sequently, there is a period during whichcement loses the ability to transmit pressure.No matter how carefully a slurry has beendesigned to counterbalance formation pres-sure, it will not necessarily resist gas invasionthroughout the hydration process.

Mud removal: No easy paths for gas—Ifchannels of mud remain in the annulus, thelower yield stresses of drilling fluids mayoffer a preferential route for gas migration.Furthermore, water may be drawn from themud channels when they come into contactwith cement. This can lead to shrinkage-induced cracking of the mud, which alsoprovides a route for gas to flow. If the mudfilter cake dehydrates after the cement sets,an annulus may form at the formation-cement interface, thus providing anotherpath for gas to migrate. For example, a 2 mm[0.08 in.] thick mud filter cake contractingby 5% will leave a void 0.1 mm [0.004 in.]wide that has a “permeability” on the orderof several darcies.

Cement slurry design: Mixing the rightstuff—Fluid-loss control is essential. Understatic conditions following placement,uncontrolled fluid loss from the cementslurry into the formation contributes to vol-ume reduction. This reduces pressurewithin the cement column and allowsspace for gas to enter.

Before the cement slurry sets, interstitialwater is mobile. Therefore, some degree offluid loss always occurs when the annularhydrostatic pressure exceeds the formationpressure. The process slows when a low-per-meability filter cake forms against the forma-tion wall, or can stop altogether when annu-lar and formation pressures equilibrate.Once equilibrium is reached, any volumechange within the cement will cause a sharppore-pressure decline in the cement slurry orthe developing matrix, and severe gas influxmay be induced. Poor fluid-loss control infront of a gas-bearing zone may acceleratethe decrease in cement pore pressure. It isequally important to have a cement slurrywith low or zero free water, particularly indeviated wells. As cement particles settle tothe low side, a continuous water channelmay be formed on the upper side of thehole, creating a path for gas migration.

Oilfield Review

0 bar 300 0 sgu 3 00

m3L/min

95950

Time

hh:mm:ss

07:12:00

06:18:00

05:24:00

04:30:00

End displacementBump top plugBleed off pressure

End cement slurryStart displacement

End spacerStart cement slurry

Start pumping spacerPressure test lines

Pressure Fluid Density Tot. FlowrateCumVolume Messages

nProcess-controlledmixing. The VIPMixer delivershighly consistentcement slurries(top). The comput-erized log showsconsistent slurrydensity throughoutthe job (bottom).

38

by disrupting the gel structure in the form ofbubbles or elongated slugs, in channelsalong the interfaces with the casing and for-mation or as bubbles which adhere to oneof the surfaces of the annulus. If rising gasremains connected to the influx source itmay form a plume as it moves through thecement slurry (above).

The size of gas bubbles entering the annu-lus is governed by the size of the cementpore throats and the surface tensionbetween the gas and the slurry. Once bub-bles have invaded the annulus, their lowerdensity provides a driving force—buoy-ancy—for them to move up the annulusthrough any available path. Bubble flow iscontrolled by slurry gel strength, and isrestricted to early in slurry development.When cement shear strength is greater thanabout 25 Pa, bubble flow ceases.5

At higher yield stress values, slurry behav-ior changes from that of a viscous fluid to aviscoelastic fluid, and the possibility of flowby viscous fingering or viscoelastic fracturesarises.6 The differential pressure—between

allow gas to invade. The resistance of anexternal filter cake to gas flow is controlledby the cake’s strength and adhesion to therock face, which both have relatively lowvalues for drilling fluids and neat cements.

This explains the driving force of gas inva-sion, however, there must also be spacewithin the cemented annulus for gas tooccupy. Space is provided by shrinkage,which occurs because the volume of thehydrated phase is generally less than that ofthe initial reactants. This total shrinkage issplit between a bulk or external volumetricshrinkage, less than 1%, and a matrix inter-nal contraction representing 4 to 6% by vol-ume of cement slurry.4

Permeability is a more complicated issue.Once gelation begins, a cement slurry canbe considered as a pseudoporous mediumas long as the stress that it must withstandfrom formation fluid is less than its intrinsicstrength. Thus, even though only a partialstructure has been formed and the cementcolumn is not yet fully self-supporting, withregard to its flow capacities, it can be said tohave permeability.

Cement slurries display an evolving yieldstress that must be overcome before gasentry and flow can occur. Depending on thestate of the slurry, gas can migrate bymicropercolation, bubbles or fractures.Opportunity for gas entry decreases as thecement cures. The rate and degree of yieldstress development at the time of invasionwill influence the form in which gas flows.Gas may enter and flow through the poros-ity of the gelling structure without disruptingit—micropercolation. Gas may also move

How Gas Gets into the AnnulusUnderstanding the mechanisms of gasmigration is complicated by the evolution ofthe annular cement column with time. Theslurry begins as a dense, granular suspen-sion that fully transmits hydrostatic pressure.As the slurry gels, a two-phase materialcomprised of a solid network with pore fluidforms. Finally, the setting process reaches apoint where the cement is for all intents andpurposes an impermeable solid. After slurryplacement, gas may enter through differentmechanisms according to the evolution ofthe cement’s state, the pressures it experi-ences and other wellbore factors.

Cement state 1: Dense granular fluid—When pumping stops, the cement slurry inthe annulus is a dense, granular fluid thattransmits full hydrostatic pressure. If forma-tion pore pressure is not greater than thishydrostatic pressure, gas cannot invade.However, almost immediately, pressurewithin the annulus begins to fall because ofa combination of gelation, fluid loss andbulk shrinkage.

This pressure reduction is best describedby the evolution of a wall shear stress (WSS)that begins to support the annular columnas the cement slurry gels. In order for astress to evolve to counteract the hydrostaticpressure, there must be a vertical or axialstrain at the annulus walls. This strain iscaused by the removal of material duringthe hydration and setting processes—pri-marily through fluid loss and shrinkage.

If it is assumed that WSS equals the staticgel strength (SGS) of the slurry and there issufficient axial strain, the following simplifiedexpression can be used to describe hydro-static pressure reduction during gelation:

where ∆P = hydrostatic pressure change across column length

SGS = static gel strength Dh = hole diameterDc = casing outside diameter (OD)L = cement column length.

As the cement sets, static gel strength con-stantly increases, with the rate of increasedependent on the nature of the slurry. Thereis potential for gas invasion once pressure inthe annulus falls below the pressure in thegas-bearing formation. Even with a mud fil-ter cake between the formation and cement,a differential pressure of less than 1 psi may

∆P = SGS 4LDh - Dc

39Spring 1996

4. Parcevaux PA and Sault PH: “Cement Shrinkage andElasticity: A New Approach for a Good Zonal Isola-tion,” paper SPE 13176, presented at the 59th SPEAnnual Technical Conference and Exhibition, Hous-ton, Texas, USA, September 16-19, 1984.

5. Beris AN, Tsamopoulos JA, Armstrong RC and BrownRA: “Creeping Motion of a Sphere Through a Bing-ham Plastic,” Journal of Fluid Mechanics 158(September 1985): 219-244.

6. Geometry separates fingering and viscoelastic frac-tures. A fracture has a sharp tip; a finger has a smoothtip. This difference is determined by a fractal lengthscale that is associated with the fracture or fingergeometry.

nGas migration in aviscoelastic fluid.Gas may flowthrough cement in anumber of differentways in addition tobubble flow. It canrise in the form of an elongated slug—seen in experimentscarried out atSchlumberger Cambridge Researchin England—aschannels alongcement-formationand cement-casinginterfaces, or as a rising plume—where a nearlyspherical chamber is linked to the formation by a narrow umbilicalconduit.

Rising plumeInterface flowSlug flowBubble flow

annulus and formation—combines with thedeveloping elasticity of the cement to deter-mine the rates of deformation and internalrelaxation. The relative values of thesedetermine the transition from fingering tofracture.7 The transition to fracture is exacer-bated if the cemented annulus contains aninternal tensile stress caused by the strain ofshrinkage, fluid loss or pressure fluctuationsin the casing. Gas may then drive the propa-gation of fractures and lead to a rapidlyextending gas channel. Hydrostatic pressurewill continue to decline as static gelstrength—and resultant wall shearstress—develop sufficiently to support theweight of the cement column. The cementhas now reached its second state.

Cement state 2: A two-phase material—Once a cement column becomes fully self-

supporting, it may be considered to act as amatrix of interconnected solid particles con-taining a fluid phase. Setting continues andhydration accelerates. Pressure, now a porepressure, decreases further as cement hydra-tion consumes mix water. This leads to anabsolute volume reduction or shrinkage ofthe internal cement matrix by up to 6%.Furthermore, the majority of shrinkageoccurs at this stage, leading to tangentialtensile stresses in the annulus, which mayassist the initiation of fractures and disruptbonding between the cement and the casingor formation.

Internal shrinkage creates a secondaryporosity in the cement composed mainly ofconductive pores. At the same time, the vol-ume of water continuously decreases due tohydration, and its ability to move within the

Degree of hydration, %

Frac

tion

of c

onn

ecte

d p

ore

s, %

0 10 20 30 40 50 60 70 80 90 1000

20

40

60

100

80

nConnected pores versus hydration of a 0.45 water-to-cement ratio slurry. From this curve, the degree of hydrationneeded to achieve capillary pore discontinuity for cementpaste can be calculated. In this case, it was found that asolids fraction of about 82% was required for discontinuity.A solids fraction of this level is typically not achieved untilwell after the cement has solidified. Hence, at most stagesof setting, some connected paths remain within the porespace. [After Bentz PB and Garboczi EJ: “Percolation of Phases ina Three-Dimensional Cement Microstructural Model,” Cementand Concrete Research 21 (1991):325-344.]

40 Oilfield Review

pores is reduced by chemical and capillaryforces. Shrinkage and water reductionsharply decrease the hydrostatic pressurethat cement exerts on formations.

There are two essentially different mecha-nisms for gas invasion at this stage, depend-ing on the strength of the solid structure andthe ease with which pore fluid can be forcedthrough the cement pores by invading gas.Early in the setting process, while the cementstill has a weak solid structure, the possibilityof creating fingers or viscoelastic fracturesremains. Later, the solid network becomessufficiently stiff and strong to withstand thiseffect, and gas invasion and subsequent floware limited by the impermeability of the solidnetwork to pore fluids. Now, the flow of gasthrough a channel of connected, fluid-filledcement pores is limited by the flow of thatpore fluid as it is displaced through theporous structure and by the connectivity ofthe channel (left).

Once gas has invaded the porous struc-ture of the cement, it may rise due to buoy-ancy forces. Alternatively, if the invadinggas remains connected through the cementpore space to the gas-bearing formation, thehigher pressure in the formation may forcegas farther into the annulus. If gas pressureis higher than the minimum compressivestress in the cement and the permeability istoo low to allow significant flow, then thecement may fracture. However, this is likelyto occur only where residual tensile stressesin the annulus are sufficiently high to allowcracks to open under the influence of thegas pressure.

During the latter stages of this phase,there is a significant and rapid decrease inpore pressure as water is further consumedby hydration. If this occurs while the porestructure is still interconnected, gas mayinvade and flow rapidly through this porespace (next page). Gas flow may also dis-place fluid remaining in the pores and pre-vent complete hydration that would eventu-ally block pore spaces with reactionproducts.

Cement state 3: An elastic solid—Oncehydration is complete, cement becomes anelastic and brittle material that is isotropic,homogeneous and essentially impermeable.8In most cases, gas can no longer migratewithin the cement matrix and can flow onlythrough interfacial channels or where therehas been mechanical failure of the cement.

Regardless of the cement system used, gascan still migrate at the cement-formation orcement-casing interfaces if a microannulusdevelops, or along paths of weakness wherethe bond strength is reduced. Both shearand hydraulic bond strengths vary as a func-tion of the same external parameters. Bondstrengths increase with effective mudremoval, and with water-wet rather than oil-wet surfaces.

Researchers at Schlumberger CambridgeResearch (SCR) have characterized thenature of hydraulic bonding by measuringshear bond stress and interfacial permeabil-ity. This work showed that lower chemicalshrinkage and higher cement deformabilitypromote better bonding.9 In addition, SCRresearchers found that bonding is not influ-enced by the cement’s compressivestrength.10

Although cement shrinkage leaves par-tially unbonded areas, it does not by itselflead to the development of a microannulus.Development of a true microannulus morelikely results from stress imbalances at theinterfaces due to: • thermal stresses—from cement hydration,

steam or cold fluid injection• hydraulic pressure stresses—caused by

fluid density changes in the casing, communication tests, casing pressuretests, squeeze pressure or stimulationtreatment pressures

• mechanical stresses—caused by drillpipeand other tubulars banging in the casing.The second potential conduit for gas in set

cement is the mechanical failure of thecement sheath due to propagation of radialfractures or cracks across the annulus. Thesecracks may be due to shrinkage-inducedstresses, thermal expansion and contractionof the casing, and pressure fluctuationswithin the casing.

Radial expansion at the cement-casinginterface, due to increased pressure in thecasing, creates a stress that compresses thecement radially and eventually induces ten-sile tangential stress in the cement. When

nChanges in slurrypermeability, porepressure and temperature versushydration time.These graphs showthat cement porestructure is still inter-connected whenpore pressure beginsto decrease rapidly.In this Dykerhoffclass G plus 1% cal-cium chloride slurry,pore pressure beginsto drop after about 5hours, just before thepeak temperature ofhydration is reached.When cement porepressure drops belowformation gas pres-sure, it is likely thatcement permeabilitywill still be in themillidarcy range,potentially allowingsignificant gas flowby micropercolation.

11

12

13

14

15

16

17

18

19

20

21

0 5 10 15 20 25

10-8

10-7

10-6

10-5

10-4

10-3

10-2

0 5 10 15 20 25

Slurry Permeability

Per

mea

bilit

y, d

arci

es

28

29

30

31

32

33

34

35

36

0 5 10 15 20 25

Slurry Pore Pressure

Por

e pr

essu

re, b

ar

Slurry Temperature

Slu

rry

tem

pera

ture

, °C

Time, hr

41Spring 1996

7. Lemaire E, Levitz P, Daccord G and Van Damme H:“From Viscous Fingering to Viscoelastic Fracturing in Colloidal Fluids,” Physical Review Letters 67(October 1991): 2009-2012.

8. A limited exception to this may occur in the case ofcement systems with high water-cement ratios,resulting in fairly high innate permeabilities (0.5 to 5 md). However, these are exceptional and not con-sidered significant among those cements generallyplaced when a potential gas migration problem isthought to exist.

9. Deformability is the reciprocal of elastic modulus.10. Parcevaux and Sault, reference 4.

this tangential stress reaches the tensilestrength of the cement—which may beclose to zero if shrinkage-induced cracksalready exist—a crack initiates at the casing-cement interface (below).

Cracks change the stress distribution inthe cement sheath. Once a crack is initi-ated, tangential stress in the cracked sectionis reduced to zero. Conversely, stress inadjacent uncracked cement eventuallyincreases because of stress redistribution.This process helps the crack propagate radi-ally outward and eventually reach thecement-formation interface. Stress is nowfully transferred to the cement-formationinterface. If this cracking occurs over a sig-nificant axial distance, a channel is formedthrough which gas can readily flow.

Long-term cement durability is importantif a well is to remain safe throughout its life-time. During its active life, a cementedannulus may be subjected to wide varia-tions of temperature and stress from pres-sure testing, workover operations and varia-tions in producing conditions.

However, field surveys on gas storagewells—which endure some of the mostextreme swings in conditions—determinedthat annular gas leakage occurs early, withinthe first few cyclic fluctuations in tempera-ture and pressure, rather than over a longperiod. This implies that leakage occurs dueto failure induced by static loads rather thanlong-term, low-cycle fatigue crack growth.Deeper and higher-pressure wells showedthe greatest tendency to leak.11

The propensity of a particular cement tocrack and for that crack to propagate hasoften been equated with compressivestrength. In fact, work carried out at SCRshows that a property termed toughnessdetermines the extent to which a cementslurry fractures under stress. Toughness isgenerally described in terms of the ability ofa material to resist the initiation and subse-quent propagation of a fracture. However,the situation is somewhat more complicated,since initiation and propagation of fracturesare controlled by physical phenomena thatdiffer, depending on the material’s structure(see “Compressive Strength Versus Tough-ness: A Brief Overview,” next page).

Using Theory to Define Best PracticeOver the years, a number of solutions to gasmigration have been proposed by the indus-try. Theoretical understanding helps toexplain how these solutions work—andtheir limitations.

Physical techniques—A number of physi-cal techniques are available to combat gasentry. Annular pressure can be applied atsurface to keep formation gas from entering,and external casing packers (ECPs) can beemployed to mechanically seal off the annu-lus at intervals and prevent gas migration.

Each of these techniques may sometimesbe valid, but well conditions often limit theirapplication. Annular pressure may berestricted by the risk of inducing lost circula-tion in weak zones and, once the cementstarts to set, surface pressure is not transmit-

ted to the formation. Alternatively, hole con-ditions and type of formation may not allowECPs to seal the annulus. Furthermore,reduction of hydrostatic pressure throughuse of ECPs may enable more gas to imme-diately enter the slurry than would havebeen the case without ECPs (above).

Impermeable cements—Gas migrationmay be prevented by reducing the matrixpermeability of cement systems during thecritical liquid-to-solid transition. There aretwo approaches to achieving this: stop fluidfrom moving through the pores or close offthe pores themselves.

The use of water-soluble polymers thatviscosify cement interstitial water andreduce permeability within setting cementfalls into the first category. Since at least apart of gas migration involves displacementof cement pore fluid, this viscosification canlimit gas mobility. Unfortunately, the pro-cess also tends to affect slurry rheology,making it more viscous and raising the dis-placement pressure. This method is alsousually limited to low-temperature applica-tions because efficiency of viscosifiersdecreases with temperature.

The second strategy of reducing thespaces in the cement matrix, preventingbubble entry and locking the fluids withinthe cement pore spaces, has proven morefertile. As a solid structure develops in set-ting cement, the smaller pore throats reduce

42 Oilfield Review

Cement

(Fully cracked)

Casing

Rock

Displacement

Cement

P

Tensilestress

(Partially cracked)

nCasing under pressure. Radial expan-sion at the cement-casing interface due toincreased pressure (P) in the casing dis-places the cement sheath creating stress.This stress compresses the cement radi-ally and eventually induces tensile tan-gential stress in the cement (top). As soonas the tangential stress reaches the tensilestrength of the cement—which may beclose to zero if there are also shrinkagecracks—a crack initiates at the casing-cement interface. This crack propagatesradially outwards and may eventuallyreach the cement-formation interface(bottom). If this occurs over a significantaxial distance, a channel is formedthrough which gas can flow.

n Mechanical barrier limitations. Exter-nal casing packers (ECPs) may fail to sealagainst some types of formation. Alterna-tively, the reduction in hydrostatic pres-sure due to the ECP may allow gas toenter the annulus, leaving the packer asthe only barrier to gas movement.

ExternalcasingpackerECP

Cement

Gas flowsaroundECP sealbecause ofincompetentformation

Annulus

More gasentersbecauseECP reduceshydrostaticpressure

11. Marlow RS: “Cement Bonding Characteristics in GasWells,” Journal of Petroleum Technology 41, no. 11(November 1989): 1146-1153.

(continued on page 44)

The compressive strength of a material describes

the stress at which a material fails when a com-

pressive load is applied (top right). When a com-

pressive load is applied to a sample of brittle,

elastic material such as cement, stress generally

increases linearly with strain (displacement) until

small microcracks and flaws in the sample begin

to grow.

This is a progressive mechanism and manifests

itself on the stress-strain plot by the change from

linear proportionality between stress and strain to

a softening section of the curve near the failure

point. Once the cracks coalesce and reach a criti-

cal size, the sample will fracture via a complicated

mechanism, which is determined by the boundary

stress conditions and geometry of the sample.

Compare this with a description of cement

toughness. Simplistically, toughness describes the

property of the material to resist the initiation and

propagation of a crack in a particular orientation.1

Fracture toughness is quantitatively defined as the

energy required to propagate a fracture of unit

width by unit length.

Without considering mathematical details, a

reasonable indication of toughness for similar

materials is given by the area (A) under the

stress-strain curve to the failure point. This area

varies according to the toughness of the material

being tested.

For example, consider two materials X and Y

that have the same compressive strength. The

material X has a much smaller strain to failure

than material Y, which contains latex. Therefore,

material Y can deform further and absorb more

energy before it fractures. Material Y is tougher

than material X.

Data like these were gathered at Schlumberger

Cambridge Research using three-point bend test

equipment (right). The cement sample is placed

on two lower static knife edges and the upper

moveable knife edge is moved downward until the

cement fails. The equipment is designed so that

the sample always fails in tension. Strain (dis-

placement) and load (stress) are recorded using

computerized data recording systems.

1. The situation is somewhat more complicated, since initia-tion and propagation of fractures are controlled by physicalphenomena that differ depending on a material’s structure.

nCement behaviorunder compression. The load or stress atwhich complete failureoccurs defines the ulti-mate compressivestrength of a material.Toughness, on the otherhand, is an indication ofthe ability of a materialto deform and absorbenergy before fracturesinitiate and propagate.

43

Compressive Strength Versus Toughness:A Brief Overview

Microfracturesdevelop undertensile stress andresult in failure ifallowed to grow andcommunicate

Area indicates toughness

Compressivestrength

A’

Strain

Str

ess X Y

Strain

Str

ess

Homogeneouscement (X)

Cement withlatex (Y)

Compressivestrength

Cement

Compressive load

Failure

A

Displacement transducer

Three-Point Bend Test Equipment

Upper moving knife edge

Sample

Staticknifeedges

nThree-point bend test.This equipment isdesigned so that cementsamples always fail intension. Strain (displacement) and load(stress) are recordedusing computerized datarecording systems.

the size of bubbles that enter, slowing theirsubsequent rise—even when the yield stressof the cement is relatively low.

Polymer latex additives are effective inresisting gas migration. A latex is an aque-ous dispersion of solid polymer particles,including surfactants and protective colloidsthat impart stability to the dispersion. In thepast, the gas-blocking mechanism of latexadditives was attributed to a capability toform films—when latex particles come incontact with a gas or when their concentra-tion exceeds a given threshold value, theycoalesce to form an impermeable polymerbarrier to gas.

However, new work has revealed thatlatex particles are also able to block gaswhen the cement slurry has developedsome structure or some compressivestrength. This demonstrates that the primaryeffect of latex particles is matrix permeabil-ity reduction by plugging spaces betweencement particles, rather than by the forma-

tion of an impermeable plastic film. Due toits smaller size and lower density comparedto cement particles, latex reduces cementslurry porosity, improves fluid-loss control,reduces relative permeability to water andlimits gas migration (above). Latex particlesreduce slurry porosity by 10 to 15%,depending on slurry density and composi-tion (see “A Robust System to Cement Gas-Bearing Formations,” next page).12 Latexadditives also affect the properties of thecement when it is set (see Tough cements,page 46).

The addition of other types of fine fillerswith particle size in the micron range maydecrease permeability throughout the rapidhydration stage by quickly decreasing porecontinuity. For example, if 30% by weightof these fine particles is added to a slurrywith a water-cement ratio of 0.45, the poresbecome discontinuous about 30% morequickly. In addition to latex additives, silicafume and microsilica have been used suc-cessfully in the field.

Right-angle-set cements—Right-angle-set(RAS) cement slurries are well-dispersed

systems that show no progressive gelationtendency, yet set rapidly. Before setting, RASsystems maintain a full hydrostatic head ongas zones, developing a low-permeabilitymatrix with sufficient speed to prevent sig-nificant gas migration.

It is important to differentiate betweentrue RAS systems and cement slurries thatonly build a gel strength. The high-gel-strength systems quickly revert to a waterhydrostatic gradient and, since their gelstrength development is not related to actualsetting, permeability can remain high for aconsiderable time. This may allow gas toenter the cement matrix many hours beforethe cement sets. On the other hand, RAScement systems rapidly build consistency asa direct result of the setting process.13

Surfactants—Surfactants may be includedin cement slurries and preflushes. Under theright circumstances, they entrain invadinggas downhole and create a stable foam. Thisfoam offers significant resistance to flow,limiting upward gas migration.14

Compressible cements—Compressiblecements are sometimes used in an attemptto maintain the cement pore pressure aboveformation gas pressure. These slurries fallinto two main categories: foamed cementsand in-situ gas generators.

Foamed cements work by expanding tooccupy the reduction in slurry volume dueto fluid loss or chemical contraction. This

44 Oilfield Review

12. Appleby S and Wilson A: “Permeability and Suctionin Setting Cement,” Chemical Engineering Science51, no. 2 (1996): 251-267.

13. Rang CL: “Evaluation of Gas Flows in Cement,”paper SPE 16385, presented at the SPE CaliforniaRegional Meeting, Ventura, California, USA, April 8-10, 1987.

14. Stewart RB and Schouten FC: “Gas Invasion andMigration in Cemented Annuli: Causes and Cures,”paper SPE 14779, presented at the 1986 IADC/SPEDrilling Conference, Dallas, Texas, USA, February10-12, 1986.

nLatex particles incement slurry. After some struc-ture or compressivestrength develops,the primary latexgas-blockingmechanism ismatrix permeabil-ity reduction byplugging of porespaces betweencement grains.Because of itssmall size andlower density compared tocement particles,latex reducescement slurryporosity, improvesfluid-loss control,decreases relativepermeability towater and limitsgas migration.

(continued on page 46)

The “ideal” slurry properties required to success-

fully withstand gas invasion include:

• favorable rheology to facilitate

efficient placement

• no gel strength development to maintain

hydrostatic balance

• rapid transition to set

• low shrinkage to minimize gas entry

• low fluid loss

• low permeability as the slurry sets

• toughness to absorb stress changes

• good bonding to avoid microannuli.

The Dowell GASBLOK gas migration control

cement system combines specific additives and

strict adherence to good cementing practices,

including spacers and washes, and casing central-

ization. It has a wide range of applications and has

had excellent success. The system is based on

using a well-dispersed, thin, nongelling slurry

with fluid-loss control. The slurry is also imperme-

able to gas in the cement matrix due to plugging of

pore throats during the setting period (above).

In addition to reducing permeability in the pres-

ence of gas, GASBLOK slurries exhibit many other

desirable properties. The main advantages are

ease of design and consistent properties over a

wide range of temperatures.

The lubricating action of the aqueous dispersion

of the latex beads creates low-viscosity slurries.

These thin slurries are beneficial for effective mud

removal, since the friction pressure during place-

ment is reduced and the critical rate for turbulent

flow will be lower. If turbulent flow cannot be

achieved and an effective laminar regime is

chosen, it is necessary to increase the value of the

rheological parameters to satisfy WELLCLEAN

mud removal service criteria. Viscosification of a

GASBLOK slurry is easily achieved.

Fluid loss is minimal—50 ml/30 min at the rec-

ommended latex concentration—due to the plug-

ging of pore throats in the cement filter cake by

latex particles and improved dispersion of cement

grains. Setting and thickening times are straight-

forward and slurries exhibit rapid sets. There is

no premature gelation of the slurry when the

GASBLOK additive is well stabilized. The slurry

remains thin until final setting. The criterion

used is that the slurry should remain below

30 units of consistency for at least 70% of the

thickening time. Above 250°F [121°C] bottomhole

circulating temperature, a right-angle set should

be easily obtained.

The tendencies for free-water development and

settling of GASBLOK slurries are minimal. The for-

mation of water channels or pockets (especially in

deviated wells) is therefore greatly reduced and

slurry density variations, with resulting changes in

slurry properties, are avoided.

Once set, a cement must also possess good

mechanical properties to withstand thermal and

mechanical stresses. Poor shear bond strength

may lead to formation of microannuli through

which gas can migrate. GASBLOK slurries display

increased tensile strength, reduced drying shrink-

age, increased fracture toughness and improved

adhesion or bond strength. Dowell latex slurries

demonstrate all of the necessary properties to

keep gas at bay. In certain cases, other cement

systems used together with proper placement

techniques have been as successful as, or even

better than, latex in achieving particular individual

properties, but none demonstrate the same

complete range of desirable properties as the

GASBLOK slurries.

45Spring 1996

10-2

10-4

10-5

10-6

10-7

Per

mea

bilit

y, d

arci

es

Time from hydration peak, hr

20 30 40 50

Neat Class G Cement slurry

GASBLOK slurry

nComparison of cement permeabilities. The GASBLOKslurry retains lower permeability throughout the hydra-tion process. Compared to a neat cement slurry, afterabout 40 hours of hydration, it has permeability that isan order of magnitude lower.

A Robust System to Cement Gas-Bearing Formations

expansion maintains a higher pore pressurein the slurry for longer than would havebeen the case with incompressible slurries.Foamed cement may be limited by depthbecause in deeper, higher pressure wellsmore gas is needed than is available in thecement to compensate for the chemicalcontraction.

In-situ gas generators are designed tomaintain cement pore pressure by chemicalreactions that produce gas downhole. Thegas produced may be hydrogen or nitrogendepending upon the technique used.15

The principal criticism of these sys-tems—other than concerns about the safetyof those that generate hydrogen—is theinability of a gas at typical downhole pres-sure to achieve the 4 to 6% volumetricexpansion necessary to maintain pore pres-sure. The volume of gas required to offsetchemical shrinkage alone would be exces-sive at high pressure. Also, in unstabilizedgas-generating systems, individual gas bub-bles may coalesce and begin migrating, cre-ating channels for formation gas to follow.

Expansive cements—Fractures occur ingelled cement according to the distributionof stress in the annulus. Eliminating thisstress—and avoiding fractures—limits gasinvasion. Tensile stresses build up in the gelif annular volume increases or cement vol-ume decreases. Thus, designing cement slur-ries with low shrinkage and controlled fluidloss during the gelation stage, and avoidingexcessive pressure fluctuations in the casingare important in preventing fractures.

Designing cement slurries that expand asthey set takes this one step further. The twoprincipal techniques for inducing expansionin oilwell cements are gas generation andcrystal growth. The gas-generating tech-nique operates on the same principle as thatused for compressible cements, except thatthe concentration of gas-generating materialis reduced. Also, expansion can occur onlybefore the cement develops significantstructural strength.

The most common way of inducingexpansion is to encourage the development

of ettringite—a highly hydrated form ofcalcium sulfoaluminate—during thehydration reaction. This is often achieved byadding gypsum or plaster of Paris to thecement powder. Ettringite increases thegrowth of certain expansive crystallinespecies within the set cement matrix. Bulkvolumetric expansion is generally less thanone percent.

Alternatively, oxides of certain alkalineearth metals may be added to achieveexpansion. An advantage of these is that theexpansion occurs above 170°F [77°C], atemperature at which ettringite is unstable.

There is little doubt that controlled cementexpansion by crystalline growth can helpseal small gaps between the cement sheathand the casing or formation, but it is unlikelyto be effective in sealing large channels cre-ated by gas migration. Much of the expan-sion takes place after gas flow has been initi-ated and the size of the created channels issimply too large. Also, these cementsundergo a bulk expansion, but still exhibit anet chemical contraction and experience thesame hydrostatic and pore pressuredecreases as nonexpansive cements.

Thixotropic cements16—During cementstate 1—when cement is a liquid suspen-sion—gas bubbles can move within acement column only if cement yield stressremains below a critical value. Designing aslurry with a rapid increase in gel strengthhelps trap invading gas before it can rise inthe form of a bubble, preventing zonal com-munication or gas flow to surface. Somethixotropic slurries offer such a rapidincrease in gel strength.17

There are two ways to induce thixotropicbehavior in a cement slurry. The firstinvolves creation of a microcrystalline net-work of mineral hydrates throughout theslurry by adding a small amount of plaster,bentonite or silicate materials. This friableand temporary microstructure supports thebulk of cement solids from an early stage inthe slurry’s life. The second techniqueemploys polymers (dissolved or dispersed inthe interstitial water), which are crosslinkedto create a self-supporting viscous gel bychemical reaction.

The transmitted hydrostatic pressure ofthixotropic systems should revert to the gra-dient of the interstitial water and remain assuch until the setting period begins. How-

ever, at this point hydrostatic pressure maybegin to decrease and gas may enter bysome other mechanism.

Tough cements—Properties of set cementmay also be modified by inclusion of vari-ous additives. Once again, attention hasturned to polymeric latex additives thathave had widespread use outside the oilfield, largely because of their ability to actas tougheners. Latex-modified cements haveincreased tensile strength, reduced shrink-age during hydration, increased fracturetoughness and improved adhesion or bond-ing (see “Compressive Strength VersusToughness: A Brief Overview,” page 43).18

Predicting Gas Migration and Designingan Appropriate SolutionArmed with an understanding of the phe-nomena, completions engineers face thechallenge of finding the right solutions (see“Gas Migration Mechanisms and ControllingFactors,” next page, bottom). Predicting like-lihood of postplacement gas migration allowsthe design of cost-effective remedies basedon the relative risk of gas migration.

Modeling gas migration is difficultbecause it represents a series of complexphysical processes. Furthermore, it is a non-steady-state phenomenon involving varyingpressure fields, changing fluid saturationand an evolving matrix structure. Hetero-geneity within the cement paste or bound-ary effects at the casing or formation caninduce events such as nonuniform gasbreakthrough which are, by definition,unpredictable. Therefore, it is not possibleto predict gas migration with absolute relia-bility. The following section describes howone company, Dowell, has developed mod-eling and software techniques to assess gasmigration risk.19

The Dowell methodology for predictingpotential gas migration began in 1989 withthe GASRULE gas migration predictive sliderule. This simple slide-rule-based methoduses well data, gas-zone permeability andheight, gas pressure, hydrostatic conditions,mud spacer and cement characteristics,fluid volumes and mud-removal efficiencyto estimate four dimensionless factors: for-mation factor, mud-removal factor, hydro-

46 Oilfield Review

nQualitative gas-migration prediction.The GASRULE slide-rule-based method of working out theoptimal cementingsolution has beenrefined and incorpo-rated into a quantita-tive design approach.

47Spring 1996

15. Fery JJ and Romieu J: “Improved Gas Migration Con-trol in a New Oil Well Cement,” paper SPE 17926,presented at the Middle East Oil Technical Confer-ence and Exhibition, Manama, Bahrain, March 11-14, 1989.Richardson EA: “Nitrogen Gas Stabilized Cementand a Process for Making and Using It,” US PatentNo. 4,333,764 (1982).Burkhalter JF, Childs JD and Sutton DL: “WellCementing Process and Gasified Cements UsefulTherein,” US Patent No. 4,450,010 (1984).

16. Thixotropic gels are viscous when static, but becomemore fluid-like and less viscous when disturbed ormoved by pumping.

static factor and slurry-performance factor(above ). Each factor may be optimizedindependently and combined into an indexthat classifies the possibility of controllinggas migration—either “poor,” “moderate” or“excellent.” While strictly qualitative, theseclassifications do allow testing of differentcompletion strategies against one another.

Three developments have helped refinethe GASRULE approach. First, in 1990, theempirical mud-removal factor was replacedwith a more complete approach, based onthe Dowell WELLCLEAN mud removaltechnology—which helps choose washes,spacers and slurry types, while indicatingwhether a turbulent or laminar displace-ment regime is the most favorable. Second,the hydrostatic factor used in the GASRULEsystem has been replaced by a more rigor-ous postplacement analysis.

The third development marks a majoradvance. A quantitative design approachhas now been incorporated in the newCemCADE cement job computer-aided

State Mechanism Limiting parameters Potential gas flow rate

Viscoelastic fluid Bubble flow Yield stress, gap width 10-9m3/secTube flow Yield stress, gap width 10-6m3/secViscous fingering Plastic viscosity, 10-7m3/sec

viscosity ratioFracture Elasticity, 10-6m3/sec

stress in annulus,Relaxation Time

Porous solid Fingering Fluid viscosity 10-6m3/secFracture Elasticity, darcy drag, 10-5m3/sec

stress in cement,elasticity

Permeation Permeability, 10-9m3/secdarcy drag,capillary pressure

Elastic solid Fracture Fracture toughness, 10-1m3/secinterfacial toughness,stress state

Gas Migration Mechanisms and Controlling Factors

17. Sutton DL, Sabins F and Faul R: “Annular Gas-FlowTheory and Prevention Methods Described,” Oiland Gas Journal 82 (December 10, 1984): 84-92.

18. Ohama Y: “Polymer-Modified Mortars and Con-cretes,” in Ramachandran VS (ed): Concrete Admix-tures Handbook: Properties, Science & Technology.Park Ridge, New Jersey, USA: Noyes Publications(1984): 337-429.

19. The prediction methodology outlined is based onexperiment, engineering and statistical analysis. Thisapproach assumes gas flow through the evolvingcement matrix. The model cannot predict theappearance of gas flow weeks or months after thecement job.

nDesigner cementjobs. CemCADEsoftware improvesthe design andevaluation ofcementing joboperations. In thefirst step of a CemCADE session,well geometry and casing config-uration to becemented aredefined (top). The composition,sequence, volumeand final positionsin the wellbore ofthe fluids that willbe pumped (mud,wash, spacer, and lead and tailslurries) are thendefined, andhydrostatic pres-sures are checked(middle). ThePlacement Simula-tor module is usedto determine nec-essary centraliza-tion and to selectthe pump rate formud removal; fric-tion pressures andflow regimes arecalculated (bot-tom). Finally, thejob is simulatedusing the U-TubeSimulator module,indicating the ratesat which fluidsmust be pumped.

design and evaluation software (right ).20

Today, the CemCADE gas-migration mod-ule assists in design and assesses alternativesolutions. This methodology is a consider-able improvement over the GASRULEapproach, but it does retain four similardesign factors: formation factor, mud-removal factor, postplacement factor andslurry-performance factor.

Formation factor—Analysis begins withcharacterizing all possible gas-bearing for-mations in terms of position, height, pres-sure and permeability. An accurate descrip-tion of pore pressure versus depth isrequired to optimize hydrostatic parameters.Good descriptions of the pore pressure ofother permeable layers and the fracture gra-dient are also required. The formation fac-tor, indicating the risk of gas flow, is calcu-lated from these formation parameters.

The more information about the formationthat is available, the greater likelihood of agood design. Trying to understand the gasmigration problem is quite difficult usingonly an average pore-pressure gradient forthe entire openhole section.

Mud-removal factor—As mentioned, aprimary goal when cementing across a gaszone is optimum mud removal. The correctapplication of WELLCLEAN technology ismandatory for gas-migration control. Forpractical purposes, good zonal isolationover a 600-ft [180-m] section above the topof a gas zone should be achieved. In thegas-migration module, information aboutseveral factors is required to determine thequality of mud removal, including:• Mud-circulation factor—an estimate of

whether enough of the mud in the well isin circulation prior to cement placement.

• WELLCLEAN factor—the factor chosen iseither the turbulent or laminar flow resultfor a given simulation, whichever isappropriate for the well conditions anddelivers the required mud removal. Timeof turbulence across the zone is calcu-lated, along with effective volume ofspacer to displace the mud in laminarflow, and effective volume of cement todisplace the spacer in laminar flow, asestimated from the U-tube simulation.

48 Oilfield Review

• Pipe-movement factor—assigns a positivevalue for pipe movement, which aids inbreaking the gel strength of the mud andmakes it easier to remove. This factordepends on whether reciprocation, rota-tion or both are used to enhance mudmobilization.

• Bottom-plug factor—depends on thenumber of bottom plugs used to reducethe degree of contamination occurring asfluids are circulated.

• Fluids-compatibility factor—relates topossible chemical interaction betweenvarious fluids.The final mud-removal factor is then com-

puted by summing these five factors—thegreater the final value, the better the antici-pated result.

Postplacement factor—Postplacementanalysis is used to evaluate the severity of apotential gas migration problem and toquantify the influence of simple solutionssuch as applying annular pressure. As previ-ously discussed, gas migration is generallycaused by a loss of hydrostatic pressure.First-level understanding of this may bederived from gelation alone.

To characterize gelation, the notion ofwall shear stress (WSS) has been introduced(see “How Gas Gets into the Annulus, page39). As WSS increases, annular hydrostaticpressure falls. When hydrostatic pressureequals formation gas pressure, WSS istermed “critical” WSS (CWSS). Furtherincrease in WSS beyond this critical valuewill allow gas to enter the annulus. WSSdepends on parameters such as formationgas pressure, openhole diameter, and den-sity and position of fluids. It is also sensitiveto any extra annular pressure, the presenceof external casing packers or techniques liketwo-stage cementing that may sometimes beemployed to improve gas control.

CemCADE software calculates WSS andassesses how use of hydrostatic modifiers—such as ECPs—may be adjusted to maxi-mize the critical WSS, delaying gas entryand allowing more time for cement toharden uninvaded. However, the calcula-tion does not take into account possiblefluid loss that may accelerate annular pres-sure decrease.

Slurry-performance factor—Once gasenters the cement column, it may migrate toa point of lower pressure. Resistance to gas

depends on slurry composition. For everyslurry there is a minimum wall shear stress(MWSS) above which gas can no longermigrate. The MWSS depends mainly on thechemical composition of the slurry as wellas bottomhole static temperature.

For every design there is a critical rangefor WSS and, therefore, a critical timeperiod during which gas can migrate in theslurry. This period extends from the time atwhich the slurry reaches critical WSS to thetime it becomes impermeable to gas. Opti-mizing a design consists of reducing thistime period by increasing critical WSS,decreasing MWSS or shortening the time togo from the CWSS to the MWSS.

The two parameters used by the DowellCemCADE system to calculate the slurry-performance factor are transition time andfluid loss. The faster the slurry developsimpermeability to gas, the lower the proba-bility that gas migration will occur. Themeasure of the evolution of the relative per-meability of a cement slurry to gas duringthe hydration period determines whether acement slurry can control gas. The rate ofcement-slurry permeability decline is diffi-cult to measure. But it is possible to corre-late permeability decline to the rate ofchange in consistency of a cement slurryduring an API thickening time test—that is,the transition time.

During cement hydration, a major cause ofpore-pressure loss is the loss of fluid to sur-rounding formations. The propensity for gasto percolate may thus be related to the fluid-loss potential of the slurry. Transition timeand fluid loss have been incorporated into asingle term, the slurry-performance factor.

Gas-migration factor—The formation,mud-removal, postplacement and slurry-per-formance factors are then linearly combinedto give the final index or gas-migration factor.Evaluation of the risk associated with a givendesign is based on the gas-migration factorcompared to a scale ranging from “very criti-cal” to “very low” risk of migration.

Looking Forward to Further ChangeEvery completions engineer knows that gasmigration is a complex problem. Successfulcontrol requires systematically addressingthe gamut of factors that affect final jobquality. Attempting to prevent gas migrationby addressing a single factor chosen fromthe list of possible chemical and mechanicalevents will inevitably result in failure.

This year, CemCADE design software willbecome available on a PC platform. Thetransition from rules-of-thumb governingchoice of solution through a slide-rule sys-tem of assessing gas migration to a com-puter-based design system will be complete.Some of the advances and technology thathave been described contribute not only tocombating gas migration, but also toimproving the quality of all critical cementoperations. Mud removal, correct choice ofslurry type and accurate mixing technologyare key elements in the evolving world ofcementing design and execution. —CF

49Spring 1996

20. Catala G, de Montmollin V, Hayman A, Hutin R,Rouault G, Guillot D, Jutten J, Qureshi U, Kelly B,Piot B, Simien T and Toma I: “Modernizing WellCementation Design and Evaluation,” OilfieldReview 3, no. 2 (April 1991): 55-71.