-
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
operationscasing 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 practicedensity control,mud removal and
slurry designare criti-cal, and here is why.
Density: Controlling the driving forceGas 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 gasIfchannels 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 rightstuffFluid-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 forcebuoy-ancyfor 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 pressurebetween
allow gas to invade. The resistance of anexternal filter cake to
gas flow is controlledby the cakes 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
disruptingitmicropercolation. 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 cements state, the pressures it
experi-ences and other wellbore factors.
Cement state 1: Dense granular fluidWhen 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
processespri-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 slugseen in experimentscarried out
atSchlumberger Cambridge Researchin Englandaschannels
alongcement-formationand cement-casinginterfaces, or as a rising
plumewhere a nearlyspherical chamber is linked to the formation by
a narrow umbilicalconduit.
Rising plumeInterface flowSlug flowBubble flow
-
annulus and formationcombines 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 gelstrengthand resultant
wall shearstressdevelop sufficiently to support theweight of the
cement column. The cementhas now reached its second state.
Cement state 2: A two-phase materialOnce 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 solidOncehydration 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 cements 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
stressesfrom cement hydration,
steam or cold fluid injection hydraulic pressure stressescaused
by
fluid density changes in the casing, communication tests, casing
pressuretests, squeeze pressure or stimulationtreatment
pressures
mechanical stressescaused 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
cementwhich may beclose to zero if shrinkage-induced cracksalready
exista 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 storagewellswhich endure some of
the mostextreme swings in conditionsdeterminedthat 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
materials 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
workandtheir limitations.
Physical techniquesA 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 cementsGas 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 cementwhich may beclose to zero if there are
also shrinkagecracksa 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 materials 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 riseeven
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 filmswhen
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 cementsRight-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
SurfactantsSurfactants 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 cementsCompressiblecements 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 minimal50 ml/30 min at the rec-
ommended latex concentrationdue 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 250F [121C] 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-temsother than concerns
about the safetyof those that generate hydrogenis 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 cementsFractures occur ingelled cement according to
the distributionof stress in the annulus. Eliminating thisstressand
avoiding fractureslimits 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 ettringitea highly hydrated form ofcalcium
sulfoaluminateduring 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 170F [77C], 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 cements16During cementstate 1when cement is a liquid
suspen-siongas 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 slurrys 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 cementsProperties 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 (seeGas
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 migrationeither poor,
moderate orexcellent. 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
removaltechnologywhich 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 ).20Today, 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 factorAnalysis 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 factorAs 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 factoran estimate of
whether enough of the mud in the well isin circulation prior to
cement placement.
WELLCLEAN factorthe 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 factorassigns 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 factordepends on thenumber of bottom plugs used to
reducethe degree of contamination occurring asfluids are
circulated.
Fluids-compatibility factorrelates topossible chemical
interaction betweenvarious fluids.The final mud-removal factor is
then com-
puted by summing these five factorsthegreater the final value,
the better the antici-pated result.
Postplacement factorPostplacementanalysis 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 modifierssuch as ECPsmay 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 factorOnce 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 testthat 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 factorThe 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.