P98-O8

The Effect of Trapped Critical Fluid Saturationson Reservoir Permeability and Conformance

D.Brant Bennion, F. Brent Thomas, A.K. M. Jamaluddin, T. MaHycal Energy Research Laboratories Ltd.

Junel998CIMATM

wettability1,4,S, initial phase saturations6, temperature7, viscosityof fluids', interfacial tension9 and hysteresis effectslO,II,12.

Abstract

Hysteretic effects refer to dte difference between relativepenneability and residual saturation values as a given fluid phasesatW'ation is increased or decreased. The difference betweeninitial, trapped, mobile and irreducible saturations are clarified.Hysteretic effects can impact a number of reservoir productionscenarios in both favourable and unfavourable fashions.Hysteretic effects can operate positively in such processes asanti-water ooning technology (A W AT), mobility control incyclic projects, such as a water alternating gas treatment orcyclic dtermal stimulation operations, or in heterogeneouscarbonates in a process known as the successive displacementprocess (SOP). Adverse effects include phase trapping andaitical vs trapped saturation hysteresis effects. Discussion ondie favourable use ofhystaetic effects for conformance controlprocesses, such as gas or water shut-off, are also presented.

What is Hysteresis?

Hysteresis refers to the directional saturation phenomenaexhibited by many relative permeability and capillary pressurecurves. In many porous media, relative penneability values area non-unique function of saturation, having different valueswhen a given phase saturation is being increased than when it isbeing reduced. This phenomena is illustrated for an oil-watercase in Figure 1. Commencing with a condition of 100% watersaturation ("A"), an oilflood is conducted. reducing die watersaturation to point "B" along relative penneability pad1 A-B. Ina water-wet porous media, this process is often referred to asprimary drainage (drainage referring to a process where diewetting phase saturation is being reduced). Reflooding withwater, referred to as an imbibition process in a water-wet porousmedia (a situation where the wetting phase saturation is beingincreased), we move to point "C" along relative permeabilitypath B-C. It can be ~ that the initial condition of 100% watersaturation is not re-achieved due to capillary trapping, resuhingin a residual oil saturation being obtained. A subsequentdrainage watertlood (C-B) results in different relativepenneability paths being traced, in comparison to the equivalentimbibition process (B-C). This phenomena is known ashysteresis. In general, hysteresis is more pronounced in thenon-wetting phase than in the wetting phase, but may occur inboth phases with up to two orders of magnitude difference inrelative permeability at equivalent saturations being observed.In most cases, the relative permeability for a given phase isgreater when its saturation is increased rather than decreased.This phenomena can be used to advantage in situations such as

Introduction

The concept of relative permeability was introduced tomodify Darcies Law, describing single phase flow in a porousmedia, for the extremely complex multiphase flow effectsoccurring when more than a single immiscible phase is present.Relative penneability values strongly control the flowmechanics, pressure and production response of virtually everyproducing oil or gas property and. therefore, a properunderstanding of how they are influenced is important in theprocess of reservoir optimization. Relative penneabilities areexpressed as functions of water (for water-oil systems) or totalliquid saturation (for gas-liquid systems), and have been weltdocumented in the literature! to be strong functions of suchparameters as pore system geometry and tortousity2 ,

1 mE EFFECT OF TRAPPED CRmCAL FLUID SA nJRA 1lONS ON RESERVOIR PERMEABn.IrY AND CON FORMANCE P98-O8- --- --- --

a cyclic stearn injection process, since it will enhance oilmobility and retard high water production rates on a return flow

cycle.

Critical Fluid Saturations. This represents the minimum phasesaturation which must occur when the minimum phase saturationis being increased the first time such that connectivity of thephase is established and fmite relative permeability exists, sothat the phase can begin to flow in the porous media. This shouldnot be confused with the trapped or irreducible saturation.Examples of a critical fluid saturation would be the point of firstfree gas mobility in a sub- bubblepoint depleted black oil system,the point of first liquid hydrocarbon condensate production in asub-dewpoint reb"ograde condensate gas system, or the point offIrSt water production as water saturation is increased in adesiccated or subirreducible water saturation system (Figure 4).

Two dominant mechanisms cause die saturation hysteresis.In die primary and secondary drainage case, a portion of diehysteresis is due to die disparity between die initial condition of1000/0 water saturation and die trapped irreducible oil saturation.This is commonly refelTed to as trap hysteresis. The differencein relative penneability curves caused by die motion between diesame endpoint saturation states is due to microscale hystereticeffects, or sometimes called drag hysteresis. It is believed to beprimarily due to a phenomena known as contact angle hysteresis.Contact angle hysteresis is pictorially illustrated in Figure 2. Itrefers to die fact that, as immiscible interfaces advance in aporous media, die effective angle of die advancing interface,which is related by wettability and capillary dynamics to dierelative ease of the fluid displacement in die porous media, isdiff~t between advancing and receding phase conditions. Thisdifference, which appears to be a Strong factor of die degree ofsurface roughness and tortuosity which exists in die system. isbelieved to be die root cause of hysteretic microscale relativepenneability effects.

Trapped or Irreducible Fluid Saturation. This represents d1esaturation value obtained when a fluid saturation is reduced froma large mobile value to an immobile value. This is also d1eirreducible saturation as is commonly detennined from a primarydrainage capillmy pressure test or a waterflood to ultimate 8M

(Figure 4).

Mobile Fluid Saturation. This value is subtly different from thecritical saturation as it is the value to which the saturation mustbe increased after a b'apped or iITeducible saturation is obtainedby a displacement process. The value is generally identical to (inan ideal situation) or larger than the b'apped or irreduciblesaturation (Figure 4).

To further complicate the issue, not only is the relativepermeability value a function of the direction of the saturationchanges, it is also a strong function of the terminal saturationendpoint reached before the direction of the saturation reversaloccurs. This is illusb"ated in Figure 3. It can be seen that therelative penneability path, if a saturation reversal occurs at anintermediate saturation level (and not an endpoint saturation),will result in the relative penneability curves tracing intemIediatepaths between the secondary drainage and imbibition curvescalled a scanning curve. Since these curves have virtually aninfinite number of values, measurement would be expensive andtechnically difficult. They are usually detennined usinganalytical models such as those developed by Killough'}.

How Hysteretic Effects Can Affect Reservoir Confonnanceand Production

Hysteretic effects may positively or negatively influencereservoir performance. Some examples of each situation aregiven for illustrative purposes.

Positive Effects

Mobility Control. Hysteretic relative permeability effects haveoften been used as a mobility control agent to preferentiallyreduce d1e mobility of one phase over another to achievesuperior conformance control and ultimate sweep efficiency,particulary in the presence of adverse viscosity ratios. A primeexample of this technology is the water alternating gas treatmentor WAG process used to reduce the mobility of injected gas ina horizontal gas injection project. The interfering effectsbetween the gas and liquid phases are used to selectively retardthe speed of gas migration. Since the water, due to its higherviscosity, tends to preferentially channel into the higherpenneability channels of the reservoir, it tends to screen offthese better quality zones and selectively reduces thepermeability to gas. Due to hysteresis and mobility effects, it ismore difficult for the gas to displace the water from this zonethan to be preferentially redirected into zones of lowerpenneability, which tends to improve the overall conformanceand sweep efficiency of a horizontal gas injection project,particularly in thick pay zones or zones containing highly

Saturation Definitions

To provide a discussion on hysteretic effects, a properdefinition of a number of commonly confused saturationconditions is ~~ry.

Inilial Fluid Saturalio1lS. This represents the hue fraction of therock actually occupied by oil, gas and water at initial reservoirconditions. There are a number of methods for detenniningthese values'4, which are crucial to proper evaluation of relativepermeability. It should be noted that the initial saturationconditions do not, in many situations, represent the irreduciblesaturation conditions with which they are often confused. Insome reservoirs, the initial water, gas, or condensate saturationmay exist at some value which is considerably less than theirreducible or mobile value's (Figure 4).

p.B. BENNION F.LrnoMAS A.K.M~D~ T. ~

variable permeability or high penneability streaks (Figure 5) (i.e. 5% water, 95% oil)

Water Coning Reduction. Hysteretic relative penneabilityeffects are the basis of anti-water coning technology used insome heavy oil reservoir situations. Due to the extremelyadverse viscosity ratio between many heavy oils and activebottom water present in some of these systems, rapid waterconing, high water cuts and marginal or \meconomic productionoccurs. It has often been found in such situations that thepresence of a mobile gas phase saturation appears topreferentially reduce the water phase penneability over the oilphase pemteability. This tends to selectively reduce the watercut, and may improve the economics of a marginal well by thesimple injection of a slug of inert gas in the near wellboreregion. A particular application of this tedmology was patentedin the 1980's by the Alberta Oil Sands Technology ResearchAuthority (AOSTRA) under the trade name A W AT (Anti-WaterConing Technology). Figure 6 provides an iUustrative set ofgas-water relative penneability curves showing the basis for this

technology.

Figure 8(b) Immiscible gas cap encroachment as gas injectionfor pressure support continues. This results in stable gravitydrainage of oil, in some cases down to fairly low oil sablrationvalues of20-2S%. This establishes a roDe of high gas saturationin d1e gravity drained rone.

Figure 8(c) Gascap b/owdown and oil sandwich displacemenJ.The depressurization of the gascap results in the active aquiferdisplacing up a sandwich of unrecovered oil from the base of thereservoir. As the oil bank penetrates the highly gas saturatedzone, hysteretic saturation and relative penneability effects resultin a very high trapped gas saturation (generally in excess of500/e) being retained in the oil encroached zone.

Figure 8(d) Aquifer encroachment. Following the displaced oilsandwich is the active water front. Due to the high trapped gassaturation. die encroaching water appears to be redirected byhysteretic relative penneability effects to peneb"ate portions ofthe pore space not previously accessed during the gas drainageprocess. This results in a measurable reduction in die residual oilsaturation (to perl1aps as low as 100/0) over that obtained duringthe conventional gas drainage process, and represents significantincremental recovery which may be obtained from what wasdiought to be a depleted reservoir during the blowdown cycle.

Enhanced Cyclic Production. The use of a simulation modelwith hysteretic relative permeability capability is sometimes theonly method of accurately predicting the perfOmlance of somecyclic projects, particularly cyclic steamfloods in heavy oilapplicationsl6. This is illustrated in relative penneability curvesas pictured in Figure 7. It can be seen that the higher waterphase relative penneability on the water injection cycle aids inincreasing the ease of injectivity of the hot water and steamcondensate into the fonnation. The lower oil phase permeability,as its saturation is being reduced, allows the hot water/steam tobypass some oil and penetrate deeper into the formation whichimproves the contact and size of the heated zone. Conversely, onthe production cycle, oil production rate is enhanced as the watermobility is reduced, since its saturation is being reduced, and theoil phase relatively permeability may be significantly increased.This results in enhanced production of oil rather than rapidproduction of the less viscous water phase.

This phenomena has been documented in a number ofreservoir applications in the literature 17.18,19,20.

Negative Effects

Phase Trapping. Phase b'apping has been well documented inthe literature as a mechanism of substantially reducedproductivity in many reservoir applications20.21. Phase trappingis caused by a combination of both adverse saturation hysteresiseffects and associated adverse relative permeability effects asillustrated in Figure 9. Common situations where phase b'appingmay occur are the use of water-based drilling, completion.stimulation or kill fluids in overbalanced conditions in low initialwater saturation condition gas reservoirs or strongly oil wet oilreservoirs (bod1 of which exhibit extremely low initial watersaturations). Hydrocarbon phase traps may be establishedthrough the use of oil-based drilling, completion or stimulationfluids in gas reservoir applications or retrograde condensatedropout effects during the production of rich gas condensatereservoirs.

Successive Displacement Process (SDP). The successivedisplacement process is a recently researched process whichappears to utilize saturation hysteresis effects as a basis forenhanced oil production as a by-product of an immiscible gasand water displacement process. The mechanism is not entirelyunderstood, but is pictorially illustrated in Figures 8(a) to 8(d).The prime application for the SOP has been observed in verticaldisplacements in heterogenous carbonate formations with strongbottom water drives with immiscible gas injection projects forpressure support. The mechanism of the process is illustrated asfollows:

Masshle Critical and Trapped Saturation Hysteresis. In somereservoirs, dte difference between dte initial critical mobilesaturation level, when the phases saturation is being increasedand dte npped saturation value, when that phases saturation isbeing reduced, can be significant. This is schematicallyillustrated as Figure 10. An example would be in a black oildepletion process where dtc reservoir pressure is dropped a

Figure 8(a) Initial saturation conditions. The oil wet carbonatemedia. generally a reef type sb"Ucture with considerable verticalrelief, exhibits oil wet behaviour with a low initial watersaturation encapsulated in the central portion of the pore system

mE ~ OF TRAPP~ CRmCAL ~ SA TURA TK>NS ON RESERVOIR PERMEABIUTY AND CONFORMANCE4 P98~8

significant degree below die satwation pressure. This will resultin die liberation of a large amount of free solution gas. Thecritical gas saturation will rapidly be exceeded and gas will beginto flow but, as pressure continues to drop, die value of diemobile gas satwation will also tend to rise. If die pressuredecline is dlen halted, and we attempt to flow back into diehighly gas saturated zone, a much higher "b'apped" gassatl.uation dian die initial critical value will usually be obtained.This may significantly reduce die mobility of die oil phase,resulting in a large loss in potential productivity.

plotted in a similar fashion and appears as Figme 12.

Using Hysteretic Relative Permeability EffectsforConfonnanceControl Purposes. Some examples have already been presentedas to how hysteretic relative penneability effects may influenceconfonnance control. Potential applications include:

Gas Shut Off Gas phase relative permeability may bepreferentially affected by selective b"eatments with an immisciblefluid. The selective injection of water based fluid, possibly asurfactant, into the upper portion of a high gas cut well mayresult in the hysteretic trap of a higher water saturation in thezone of high gas saturation and penneability, and result in atransient or permanent reduction in gas-oil ratio. Inclusion of asurfactant may generate a high viscosity stable foam systemwhen contact is made with zones of high gas saturation. Thiswill result in a large portion of the pore system which isavailable for mobile gas to flow being occluded by the immobilefoam phase and, once again. result in a preferential reduction inthe gas phase penneability. Treatments of d1is type tend to besomewhat transient in effect due to gradual degradation of thefoam system over time by adsorption (particularly in clasticformations) and dispersion effects. This process is illustratedschematically as Figure 11.

Laboratory Case Studies lIustrating Water, Gas andCondensate Trap Phenomena

Table I provides a summary of basic water trap tests whichwere conducted on two low penneability core samples whichexhibited sub-ireducible initial water saturations ofapproximately 23%. It can seen that a significant phase trap wasestablished in the low penneability porous media by theintroduction of a non-damaging inert 3% KCI phase and 3500kPa of dlreshold pressure was required to cause initial gasmobilization through samples of approximately 5 cm in length.This resulted in regain penneabilities at the original thresholdpressure with reductions ranging from 78 to 85% of the originalgas phase penneability measurements. It provides a goodindication of the severe type of damage which can be associatedwid! significant invasion depd1S or minimal drawdown gradientsassociated wid1 water-blocking phenomena that may occur inlow permeability porous media.

WaJer Shut Off Hysteretic effects can also be used to aid inwater shutoff. Reference has already been made to the A W ATprocess when non-condensible gas injection is used topreferentially reduce water cuts by a selective reduction in therelative penneabilitY to water over the oil value. In zonescontaining pure water. which cannot be readily isolated bycasing and selective completion. a similar effect can beaccomplished by the direct zone specific injection of animmiscible non condensible gas into the water saturated zone.This will establish a zone of high trapped gas saturation andmay. depending on the rock geometry and relative penneabilitYcharacteristics. significantly reduce the inflow characteristics ofwater from die affected zone. The establishment of a trapped gassaturation in the oil saturated strata is obviously undesirable andshould be avoided as this may substantially also impair thepenneabilitY to oil.

Table 2 provides analogous examples for d1e depletion of alow penneability black oil reservoir. As gases evolve fromsolution as pressure is effectively dropped. the b'8pped aiticalgas saturation increases until the mobile critical gas saturation isacltieved. In the higher permeability samples observed in Core# I, it can be seen that the aitical gas saturation is achieved at amuch lower saturation value of approximately 5.5% incomparison to the lower permeability sample suite where aiticalgas mobility was not obtained until a saturation in excess of 16%had been physically realized. Once again, significant reductionsin apparent oil phase permeability are noted due to theestablishment and entrapment of a trapped critical gas saturationvalue in the near wellbore or frac face region. Oil injection (although opposite of what we want to

accomplish is most producing wells) may also be an effectualWOR reducing technique in certain situations where free bottomwater with an active drive in present. The objective of thistechnique is to inject produced oil, or some other low viscosityhydrocarbon, directly into the wet zone underneath theproducing zone. Hysteretic effects will trap an irreducible oilsaturation in this zone which may reduce the effectivepermeability to water by as much as 95%, depending on therelative permeability characteristics of the porous media.

Table 3 provides a summary of the illustration of the criticalcondensate saturation accumulation phenomena. It can be seenthat for the higher quality sample tested that critical condensatesaturation values and permeability reduction effects arerelatively insignificant in comparison to a similar effect observedin lower quality porous media, where large scale reductions ineffective gas phase permeability and relatively high values oftrapped critical condensate saturation are achieved. Thepermeability vs gas saturation data of Table 2 has been plottedon Cartesian coordinates and appears as Figure 11. Theoenneabilitv vs condensate saturation data of Table 3 has been

D.B. BENNION. F.B nIOMAS. A.K.M. JAMALUDDIN. T. MA 5

Salathiei. R.A., "Oil Recovery by Surface Film Drainage inMixed Wettability Rocks", SPE 4014 presented at die SPE47d1 annual meeting, San Antonio, California, Oct 8, 1972.

3Conclusions

A discussion on cyclic hysteresis effects in the exploitationof oil and gas producing properties indicates that:

Leach, R.O., et aI, "A Laboratory and Field Study ofWettability Adjustment in Waterflooding", JPT, 44, 206,1962.

4Significant saturation and relative permeability hysteresisoccurs in many reservoir systems. The degree of hysteresisis usually more pronounced in the non-wetting phase, butmay be significant in both phases. Research studies suggestthat the degree of hysteresis is related to the magnitude ofcontact angle hysteresis which is, in turn, a function of theamount of surface roughness in a given reservoir system.Therefore, tight, low penneability rocks with high surfaceroughness may exhibit more hysteresis than their moreunifonn higher permeability counterparts, although specificdetennination on a reservoir by reservoir system is required.

s Denenkas, N.O., "The Effect of Crude Oil Components onRock Wettability", Trans AIME, 216,330,1959.

Caudle, B.H., et aI, "Further Developments in dieLaboratory Determination of Relative Permeability", TransAIME. 192,145, 1951.

6

,. Edmonson, T.A., "The Effect of Temperature onWaterflooding", JCPT, 10,236, 1965.

2. The difference between initial, irreducible, critical, mobileand trapped fluid saturations has been defmed. 8.

Lefebvre du Prey, E, " Deplacements Non-Miscibles dans

les Milluex Poreux Influence des Parameters Interfaciax surles Permeabilities Relatives", C.R. IV Coloq, ARTFP Pau,1968.

3. Hysteretic effects may be advantageous in reducing water

coning problems, gas coning problems, enhancingproduction from some cyclic projects (such as steaminjection) and reducing gas phase mobility in someprocesses such as water alternating gasfloods (WAG) orco-current injection projects. Residual oil saturation may besubstantially reduced in some heterogenous carbonateformations due to hysteretic effects in what is known as thesuccessive displacement process (SDP).

Leverett, M.C., "The Flow of Oil Water Mixnua ThroughUnconsolidated Sands", Trans AIME, 132,149, 1939.

9.

10. Geffen, T.M., "Experimental Investigation of FactorsAffecting Laboratory Relative PermeabilityMeasurements", Trans AIME, 192,99,1951.

Osaba, J.S., "The Effect ofWettability on Rock Oil-WaterRelative Penneability Relationships", Trans AIME,192,91,1951.

4. Hysteretic effects may also reduce productivity andincrease problems with water and gas coning in certainsituations due to adverse effects associated with "trap"saturation hysteresis or what is more commonly referred toas "phase trapping". 12. Donaldson. E.C.. "Microscopic Observations of Oil-Water

Displacement in Water Wet and Oil Wet Formations". SPE3555. Presented at die 46th annual SPE fall meeting. NewOrleans. Oct 3-6. 1971.

Acknowledgments

The authors wish to express appreciation to Vivian Whitingfor assistance in the preparation of the manuscript and figuresand to the management of Hycal Energy Research Laboratoriesfor the funding of this work and permission to present the data.

13. Killough, J.E. et aI, "Reservoir Simulation With HistoryDependent Saturation Functions", Trans SPE of AIME,261, pp. 37-45.

References 14. Bennion. D.B., et al, "Detennination of Initial FluidSaturations - A Key Factor in Bypassed PayDetennination", Presented at the PNEC 2nd annualconfermce on Profile Modification and ConfonnanceControl, August, 1996, Houston, Texas.

Honarpour, M. et ai, "Relative Penneability of PetroleumReservoirs", CRC Press, 1986, Boca Raton, Florida.

Arps, J.J., et aI, "The Effect of Relative Penneability Ratio,dte Oil Gravity and dte Solution Gas-Oil Ratio on thePrimary Recovery from a Depletion Type Reservoir", TransAlME. 204,120, 1955.

2.15. Katz, D.L. et ai, "Absence of Connate Water in Michigan

Gas Reef Reservoirs", AAPG Bulletin, Vol. 66, No.1, Jan.1982.

16. Bennion, D. W., et aI, "The Effect of Relative Penneabilityon the Steam Stimulation Process", JCPT, 1985.

6 mE EFFECT OF TRAPPFD CRmcAL FLUID SA TURA TlONS ON RESERVOIR PERMEABD.JTY AND CONFORMANCE P9I-OI

17. Batycky. J. and Irwin. D.. "Trapped Gas Saturations inLeduc Age Reservoirs", JCPT, Feb. 1998, Vol 37, No.2, pp32.

18. Irwin, d. and Batyclcy, J., "The Successive DisplacementProcess", SPERE, Nov. 1997.

19. Jensen, E.M. and Gillund, G.N., "Windfall D3A BlowdownStudy", Amoco Canada Petroleum. June 1988.

20. Bennion, D.B., et ai, "Water and Hydrocarbon PhaseTrapping in Porous Media, Diagnosis, Prevention andTreatment", JCPT, September, 1996.

21. Bennion, D.D., et ai, "Reductions in the Productivity of Oiland Oas Reservoirs Due to Aqueous Phase Trapping",JCPT,1994.

Table IIllustration of Water-Based Phase Trap Potential

Table 2Illustration of Oil Phase Permeability Reduction Due to

Critical Gas Saturation Accumulation

Sg Core #1(k.a) mD

Core #2(k.J mD

Core #3(k...) mD

0.000.0250.0550.0750.1000.1330.166

4.512.861.99.

.

2.782.512.161.46.

0.06960.06720.06010.05520.04920.03120.0156

I . Criti~ gas saturation_value

Table 3Illustration of Critical Condensate Saturation Accumulation Data

Se Core #1(Gas Perm-mD)

Core #1(Gas Penn-roD)

0.0000.0150.0300.0680.1200.1850.260

369.0346.0322.0.

0.630.490.320.180.100.0600.020.

I . Critical condensate saturation

-

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IN A WA1ER-~ oa...WATER DISP\ACBENT

~2ILJ.USTRATION OF CONTACT ANGlE ~818

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Water SatlK8tion

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~3ItJ.U8TRATION OF A RElATIVE PERMEABILITY "SCANNING"

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~4IllUSTRATION OF VARIOUS SA~T1ON TYPES

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Water Saturatioo

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ON WATER & OIL PHASE Im.ATIVE ~ (AWAT)AGme5

WAG PROCESS FOR MOBIlITY CONTROL

Water Sat\ntlon

~.I.lUSTRATION OF THE SUCCESSIVE DISPlACEMENT PROCESS

AGURE7ILlUSTRATION OF CYCUC HYSTERESIS EFFECTS

~ B8tANCED PRODUCTION RATES

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Water Saturation

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FIGURE 10IU.USTRA T1ON OF CRITICAL AN!) TRAPPED SA TURA TION

HYSTERESIS EFFECTSFIG~9

MECHANISM OF PHASE TRAPPING

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Liquid Satwation

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FIGURE 11 I

ILLUSTRATION OF OIL PHASE PERMEABIUTY REDUCTIONDUE TO CRmCAl GAS SATURATION ACCUMULAnON

FIGURE 12ILLUSTRATION OF CRITICAL CONDENSATE SATURATION

ACCUMULATION DATA

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