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1995 SCA Conference Paper Number 9503

RESIDUAL OIL SATURATION AND MULTIPHASERELATIVE PERM EABILITY M EASUREM ENTS ON AWATER-WET SANDSTONE RESERVOIR AND ALIMESTONE RESERVOIR OF MIXED WETTABILITY

P. Grivot, L. Bouvier, J. Fournier, A. Silverii, TOTAL S.A. (France)

A B S T R A C TThis paper presents three phase data regarding a water-wet sandstone reservoir and alimestone reservoir of mixed wettability. These data originate from a laboratory methodset up for a direct and comprehensive determination of tertiary recovery by gas injection,using both centrifuge and steady-state techniques.The three-phase relative permeability determination experiments are performed using insitu saturation measurements by dual energy gamm a-ray. T he steady-state technique w asused to derive water, oil and gas oil relative permeability isoperms at high oil saturations.Automated centrifuge tests were carried out to determine residual oil saturationsresulting from both secondary and tertiary gas floods and oil and water three-phaserelative perm eabilities at high gas s aturations.The experiments show that residual oil saturations reach a very low value both insecondary and tertiary gas floods for the studied water-wet reservoir sandstone. It is alsopossible from reliable experimental three-phase oil relative permeability data to ascertainthe choice of an analytical three-phase model for numerical modelization. In the case ofthe carbonate reservoir of mixed wettability, the residual oil saturations are not always aslow as expected from secondary gas flood. Results indicate that permeability may bemore important than wettability to influence the total recovery in the studied limestonereservoir.The residual oil saturations and three-phase relative permeability data for a variety ofwettability and permeability values reported in this paper are expected to provide thebasis for ad ded insight into tertiary gas injection phenom enology.I n t r o d u c t i o nResidual oil saturation and time required to reach it are key factors in designing tertiarygas injection recovery projects. The three phase oil relative permeability appears to beone of the crucial variables determining the kinetics of different improved oil recoveryprojects. Accurate predictions of oil recovery in processes that exhibit three-phase flow(Water, Oil and Gas) need rigorous model for three-phase relative permeability.Therefore, reliable acquisition da ta is essential for acc urate prediction of oil recovery.

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1995 SCA Conference Paper Num ber 9503

To warranty the choice of a three-phase relative permeability (kr) model for field scalesimulation, a comprehensive laboratory program should require :1) to perform automated centrifuge tests and multiphase relative permeabilitydetermination using the steady-state technique to derive respectively the three-phase krat low gas saturation and high gas saturations ;2) to carry out an appropriate set of samples gas injection experiments at reservoirconditions using in-situ gamma-ray saturation monitoring and analysed it using anumerical simulator as described by Chalier et al (1). By matching the profile saturationsand fluids productions during gravity drainage, it will be possible to derive a three-phasemodel which have to be compared with ambient results. This final comparison willvalidate and confirm the cho ice of a three-phase k r model for full scale simulation.The results obtained on two different reservoirs and related to ambient conditionsexperiments are reported in this paper. Experiments were carried on numerous smallplugs (4 cm to 10 cm length, 4 cm diameter) at ambient conditions to describe all theencountered flow units in relation with the geological model.Rec overy m echa nism s into the reserv oir for lean gas injection pro jectDuring tertiary gas injection, additional oil is recovered as a result of several individualmechanisms mainly component exchanges and gravity drainage. At ambient laboratoryconditions, only gravity dominated mechanisms will be considered. This is the case oflean gas injection at fairly low pressure where gas is injected continuously to replace thevoidage. It should be noted that gravity drainage still acts in the gas invaded zone even ifgas injection is stopped. The rate at which tertiary oil is draining will depend on thepermeability of the rock, oil relative permeability and the oil viscosity.Residual oil SaturationsMany laboratory experiments suggest that the process can lead to very low oilsaturations. Dumore et al. (2) measured 3% residual oil saturation. There is alsoconsiderable field evidence (3) to show that in region containing gas, gravity drainagecan be comp lete with residual oil saturations tending to ze ro. T o be sure o f mobilizing oilin tertiary conditions, experimental measurements are designed. These experiments havethe objective to determine if additional oil could be recovered from different initialsaturations : virgin area at Swi, waterflooded zone at Sorw or intermediate waterfloodedzone. It is possible to compare and to evaluate the efficiency of gas injection for thedifferent flow units by performing centrifuge experiments on samples representative ofeach unit. Displacing oil and water by centrifugal forces in a centrifuge make it possibleto reach low liquid satu rations, which is the region of interest in gravity drainage.Relative PermeabilitiesRelative permeabilities have to be measured at high gas saturations (around thesecondary gas cap) and at low gas saturation (in the oil producers area). To cover thewhole range of gas saturation, at ambient conditions, two different laboratory techniqueswere used : centrifuge technique for high gas saturations and steady-state technique for

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low gas saturations. As described by Oak et al (4) relative permeability is not a uniquefunction of fluid saturation, but varies with the saturation history. In this study only onecas was considered, gas saturation is always increasing while the liquid saturation(water+oil) is decreasing.Measurements techniquesThe most important consideration in designing the experimental equipment was thereliability o f the measurem ents.Steady-state technique using gam ma-ray in situ saturation m onitoringThe steady-state method only requires establishment steady-flow of three fluids throughthe porous medium and measuring the pressure drop and flow rate for each fluid. Todetermine the relationship between relative permeability and saturation, necessary tomeasure the saturation of each fluid under steady-state conditions for each set of flowrates is measured using a dual energy gamma-ray attenuation technique bench. The in-situ saturation monitoring technique is similar to the one described by C. Barroux et al(5). Water, oil and gas are simultaneously injected into the sample at constant rates, andpressure and fluid phases are measured at steady-state. The three-phase test is conductedat gas-oil and water different flow rate ratios. One tried to increase the gas saturationcontinuously either by increasing the gas rate or by decreasing the total injection rate ofwa ter and oil at a constant w ater-oil injection ratio.According to the experiment different couple of oil-water systems were used. Oil orbrine may be recombined with dopant to enhance the constrast for gamma-ray readings.The synthetic formation brine corresponds to the reservoir formation water. To eliminateend-effects, the pressures are measured using transducers located at some distance fromthe plug ends.The saturation history is operated as follows :Initially, the c ore is clean and dry when set up h orizontally in the Hassler c ore holder cell.The dry profile of the set up (i.e. Set-up + Core Assembly) is then determined precisely.After saturation of core with brine, the saturated profile is determined. The establishmentof Swi is done by centrifugation or displacement. At this stage, profile at immobile watersaturation can be compared to the material balance Swi.The three-phase test is conducted at gas-oil and water different flow rate ratios. At theend of the flooding, the core is removed from the core holder to determine final watersaturation. A flooding by isopropanol is used to extract the water from the core and atitration is done. This checking confirms the saturation readings by gammagraphy.The centrifuge techniqueThis technique for measuring three-phase relative permeability is the systematicapplication of the centrifuge method described by Van Spronsen (6). Relative

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permeabilities are calculated from the observed desaturation rates at a given rotationnalspeed. To ensure accurate fluids production data, centrifuged samples were of largedimensions (i.e. utilisation of high capacity centrifuges) and the technique has beenautomatised using video camera.Procedure of centrifuge techniqueThe procedure for preparing samples for a centrifuge experiment varied somewhataccording to the rock wettability. However, the main procedure to establish the initialconditions is as follows. The samples are cleaned, dried and completely saturated withthe water phase. Irreducible water saturation (Swi) is established either by displacing thewater with oil in flood apparatus or by centrifuging in an oil filled core holder. Ifnecessary, samples are ageing 20 days to restore the wettability. The establishment ofintermediate saturations is done by waterflooding at very low flow rate. Anothertechnique is to inject simultaneously at a constant high rate until a steady-state conditionis reached. Subsequently, the samples are placed in air-filled core holder and three-phaserelative permeability experiment is started. The initial and final sample saturations arechecked by weighing before and after th e centrifuge run.Determination of triphasic end-pointsThe average oil saturation at a gas-oil capillary pressure have to be corrected usingForbes' method (8) to obtain the saturation at the end of the plug. It means that for eachpetrophysical rock group a representative gas-oil capillary pressure curve is determinedby the centrifuge method. For triphasic end-points, samples have to be fully or partiallywaterflooded before centrifuging into air.Determination of oil relative permeability in triphasic conditionsA three-phase relative permeability experiment entails running the centrifuge at a singlehigh speed in an attempt to overwhelm capillary pressure effects. The conventionalanalysis of Hagoort (9) extented by Van Spronsen (6) assumes that first the centrifugalacceleration is constant along the core and second the invading phase is infinitely mobilecompared to the phases it displaces. These assumptions can be reasonnably approached.The first assumption is satistified as soon as the distance between the rotational axis andthe middle of the sample exceeds twice the sample length. This is accomplished by usinghigh capacity centrifuges. The mobility assumption is certainly not valid in the beginningof the test when the gas is just entering the core,its saturation and, hence its permeabilityis low. But the high viscosity constrast insure the mobility assumption is valid throug houtmost of the experiment.

Experimental resultsHere we give some examples of triphasic end-points and three-phase relativepermeability measurements on samples from a water-wet reservoir and a limestonereservoir of mixed wettability. The case study of the water-wet reservoir was chosen toillustrate how from centrifuge and stead y-state technique it is possible to define a reliablethree-phase oil relative permeability model. Mixed wettability reservoir have much morecomplex flow behaviour. The chosen case shows that within relatively short time residual

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oil measurements in secondary and tertiary conditions bring an evaluation of the potentialof gravity drainage for all the flow units of a limestone reservoir. A summary of theresults together with the core properties is given in Tables 1&2. Another point worthnoting is that, in centrifuge experiments residual water saturations (Swig) may besometimes lower than Swi.A water-wet sandstone reservoirSamples were from an oil field that has nearly produced its secondary conventionalreserves. The reservoir is a sandstone reservoir known as being water w et. Bo th steady-state and centrifuge mesurements were carried out which enables to cover the wholerange of ternary diagram. As expected in a water-wet system, the water relativepermeability is found to be the same function of water saturation in both the two-phaseand drainage three-phase system. This was confirmed by the two different techniques athigh gas saturations and low gas saturations. This will validate the consideration of thewhole set of data (both steady-state and centrifuge) to calculate oil isoperms. Thissignificant result is shown on Figure 1.The saturation paths during centrifuge and steady-state experiments are shown in Figure2 in a ternary diagram. The data interpretation in terms of oil isoperms involves twosteps of analysis. First, "experimental" isoperms are determined by using a griddingmethod directly from the raw data. The algorithm is based on a classical inverse-distance,radial searching technique. In the second step, the curvature of the minimum oilsaturation (Som) curve is set from the "experimental" isoperms. Then, an analytical tree-phase kr model is searched to fit the whole experimental data set by varying adjustableparameters of the Som curve. The fitting of Fayers (9) or modified Stone 1 (10) modelenables us to have meaningful results for future modelization. "Experimental" andmodified Stone 1 kro isoperms are reported respectively in Figure 3 and 4.A lim estone reservoir of mixed w ettabilitySamples were from a limestone reservoir which is currently producing under naturaldepletion with an active bottom water drive. The wettability of this limestone reservoirchanges from water-wet low on structure near the water-oil contact to mixed-wetbehavior up to oil-wet higher on structure. The effect of these wettability variations isclear on the w ater-oil forced imbibition capillary c urve shape at low oil capillary p ressurebut hidden on the values of residual oil saturation (Sorw) which depends mainly onpermeability.Diphasic results (i.e. gas-oil capillary pressure at Swi and water-oil forced imbibition)make it possible to compare the efficiency of gas with that of water in secondaryconditions. It shows as illustrated by Figure 5 that in all cases oil recovery is better bygas than by water for equivalent hydrocarbon column, except for very low heigths andsamples with lower permeability.Recovery by gas injection in secondary conditions is very high for high permeabilitysamples, Sorg is low even at low capillary pressures.

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In Figure 6 are reported the tw o correlations, Sorw w ith gas permeability and Sorg withgas permeability corresponding to the same hydrocarbon column (80m), and the resultsfrom triphasic end-points experiment on the same samples.Differences are identified from secondary to tertiary conditions. Tertiary end-pointsSorgw are on the whole located between Sorg and Sorw. This means that gas injection isbetter in secondary conditions than in tertiary conditions. The wettability may be anexplanation. After waterflooding, gas did not have access to the oil present in small po resor on the po re walls of larger pores.D i s c u s s i o nCentrifuge and steady-state experiments as described above were designed to acquiremaximum reliable data in a relatively sho rt time. Because of the assumpti ns taken fordata interpretation, this method is preferably applied to permeable reservoirs. The twotechniques can also be used for three-phase relative permeability measurements on lowpermeability samples if a numerical correction procedure is applied to take into accountthe error due to capillary end-effect.It should be reminded that no mass transfer between oil and gas is involved in theseexperiments. Under certain conditions depending on gas and reservoir oil compositionand reservoir pressure, high microscopic displacement efficiency may be achieved bythermodynamical dominated mechanisms that can be estimated only by additionalreservoir conditions experiment.The main feature of the method is to combine two different experiments to have fullycoverage of ternary diagram from high to low gas saturations at ambient conditions.Unless the whole set of tests is performed on the same sample, which is actuallyunpractical, some scattering occurs in three-phase data. But in this water wet reservoircase study it was possible to ascertain a Fayers (9) or a modified Stone I model (10)from the experimental oil isoperms. Since the fitting between the estimated values andthe measured ones are based on the Som curvature, the best fitting of kro is obviouslyobtained for low oil saturation region.In a mixed wettability system, Stone (10) conjectured that the respective oil and waterequations may be used in the different saturation ran ges wh ere they are applicable. Thu s,for example, oil permeability may be computed at the lower ranges depending on thedistribution of wettability according to pore sizes, thus on the rock. In that case, nothree-phase model is directly applicable without fitting some experimental data,enhancing the need of a methodology such as that reported. It is hoped that it will also bevalidated by the additional data, in terms of three-phase kr, being currently determinedon the limestone reservoir although the wettability aspect implies more difficultacquisition.

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C o n c l u s i o n s Combining in situ-saturation monitoring steady-state and centrifuge experiments is anefficient method for measuring water and oil relative permeability and residual oilsaturations in triphasic conditions. Rese rvoir charac terization is as crucial as flow mechanisms modelling for gas injectionprojects. Steady-state and centrifuge advanced techniques run at ambient conditionsmake it possible to describe in relatively short time, the whole studied reservoir.Q) For a water-wet reservoir, it is possible to identify a reliable three-phase model fromlaboratory experimental data. The efficiency of gravity drainage mechanism as an oil recovery p rocess m ay be less intertiary conditions than in secondary conditions for a limestone reservoir of mixedwettability.NomenclatureKgKrgKrwKroKeoKwPcPVSgSoSoiSorgSorwSorwgSwSwiSwigH

Gas permeabilityRelative permeability to gasRelative permeability to waterRelative perm eability to oilEffective oil permeabilityBrine permeability at Sw=1.0Capillary pressurePore volumeGas saturationOil saturationInitial oil saturation before gas displacementResidual oil saturation to gasResidual oil saturation to waterThree phase residual oil saturationWater saturationIrreducible water saturationFinal immobile water saturationViscosity

AcknowledgementsThe authors wish to thank TOTAL Exploration Production for permitting publication.Thanks are also due to M. P. PEZERIL for performing 3D visualisation.R e f e r e n c e s1. G.Chalier, S. Sakthikumar, V.Giry and Ph.Maquignon, "Dual Energy gamma-rayabsorption technique applied to oil relative permeability determination during atertiary gas gravity drainage experiment", to be presented at the SCA Conference, SanFrancisco, September 12-14, 1995.

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2. Dumore, J.M. and Schols, R.S., "Drainage capillary pressure functions and theinfluence of connate water", SPEJ, Vol. 14, n 5, October 1974, P437.3. King, R.L. and Lee, WJ, "An engineering study of the Hawkins Woodbine reservoir",SPE 5528, Presented to the 50th Annual Fall Meeting of SPE, Sept 28-Oct 1, 1975.4. Oak, M.J., Baker, L.E. and Thomas, D C : "Three-Phase Relative Permeability ofBerea Sandstone", J. Pet. Tech (Aug. 1990) 42, 1054-1061.5. Barro ux, C , Bouv ier, L., Maquignon , P., Thiebot, B. , "Alternate dual energy gamm aray attenuation technique : a new tool for three fluid saturation measurements in IORflood experiments", Proceedings of the Sixth European Symposium on Improved OilRecovery, Stavanger, May 1991.6. Van Spronsen, E., 1982, "Three-phase relative permeability measurements using thecentrifuge method", SPE/DOE 10688, presented at the SPE/DOE Third Symposiumon Enhanced Oil Recovery, Tulsa, Oklahoma, April 4-7.7. Cuiec, L.E. : "Evaluation of Reservoir Wettability and its Effect on Oil Recovery",Chapter 9, in Interfacial Phenomena in Oil Recovery, N.R. Morrow, ed., MarcelDekker, 1990.8. Forb es P.L., "Simple and accurate metho ds for conv erting centrifuge d ata intodrainage and imbibition capillary pressure curves", SCA 9107 presented at the SCAconference, San Antonio, August 20-22, 1991.9. Fayers, F.J. and Matthews, J.D., "Evaluation of normalized Stone's methods forestimating three-phase relative permeabilities", Soc. Pet. Eng. Jour., April 1984,P.224-232.10. Stone, H.L., "Probability model for estimating three-phase relative permeability",

Jour, Pet. Tech., Feb 1970, p.214-218.11. Stone, H.L., "Estimation of three-phase relative permeability and residual oil data",Jour, Can. Pet. Tech., Oct-Dec 1973, p.53-59.

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TABLE 1 - Water-wet sandstone reservoir - Centrifuge and steady-state relative permeability testsExperimentTypeQw/Qo or accelerat ion^Sample PropertiesSample length, cmSample diameter, cmPorosity, per centSwi, per cent pore spacekeo at Swi, mDTests End-PointsSoi, per cent pore spaceSwig, per cent pore spaceSorwg, per cent pore space

H 51 / 5

7.83.9

25.519.046 346.6--

H31 / 1

7.83.9

21.117.045 652.3--

H5 b1 0 / 1

7.73.9

28.220.355 670.6--

H3 B H6Steady-state

2 0 / 16.83.9

28.123.540 166.5--

Q w = 07.93.9

27.123.238 6

.--

H7Qg=0

8.03.9

26.723.148 476.9--

H 3CQ w = 0

6.23.9

28.323.942 0

.--

c835134.33.9

27.719.175 822.813.86.0

c9 c2BCentrifuge

3515 36084.3 3.93.9 3.9

27.4 27.017.9 21.5752 27849.3 39.715.3 10.86.4 8.6

c2 b32 13.93.9

27.120.828 030.022.113.3

I

0.1

0.01

0.001

0.0001

0.00001

1 E- 0 6

1 F . 0 7

& ft jS

>

fft?o *

50Sw, %

100

Figure 1. Water-w et Sandstone Reservoir.Water Relatrive Permeabilityas a Function of Sw.

D

OAO

k r w c 8krw c2bk r w c 9k rw c2 BK r w H 7k r w H Sk r w H 3krw H5b

-S t, 100*/.

+Swi400%- W i So, urnFigure 2. Water-w et Sandstone Reservoir.

Saturation Paths of Centrifuge and Steady-StateExperiments. Arrows : Direction of Saturation change

+S E, 100% -fSg, 100%

Figure 3. Water-w et Sandstone Reservoir.Experimental Oil Isoperms Estimatedby Gridding Techniques from R aw D ata.

So , 10 Sw, 100% 4*0Figure 4. Water-w et Sandstone Reservoir.

Analytical Oil Isoperms Estimatedby Modified Stone 1 Method.

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1995 SCA C onference P aper Number 9503

TABLE 2 - Limestone reservoir of mixed wettability - End-points in tertiary conditionsSample PropertiesWell numberSample numberPorosity, per centKg, mDSwi establishmentSwi, per cent pore spacekeo at Swi, mD

#110

24.830 919.2127

#111

26.155121.1193

Intermediate saturation before gas displacementSoi, per cent pore space 37.4After eas displacement of oil and water atSwig, per cent pore spaceSorwg, per cent pore space

29.817.2

35.9a constant

28.715.7

#114

20.8118827.087724.1

#19

22.799.221.826.640.8

capillary pressure41.08.3

25.721.2

#113

16.731 733.4134

33.329.716.7

#215

25.721.325.311.138.621.519.3

#29

25.138.529.814.345.421.022.2

#211

25.112.222.88.7941.315.729.9

#213

23.890.234.647.737.921.218.8

#221

15.05.1715.20.9739.018.638.2

#218

17.73.3413.81.5938.120.837.8

%PiSog

50454035302520151050

Sorg, hc=30m

Sorg, hc=60mSor g, hc="90m

Sorw, hc=IOm

Sorw, hc=30mSorw, hc=70mSorw, hc=14Sm

Permeability, mD

Figure 5. Limestone Reservoir of Mixed Wettability - Comparison between Oil Residual Saturationto Gas and to water Displacement. Equivalent Hydrocarbon Column, he.

Correlation, Secondary Sorg, hc=80mCorrelation, Sorw, hc=80mTertiary End-Points, Wells #1 &U1

X Tertiary End-Points, Well #3

100Kg, mD

Figur e 6. Limestone Reservo ir of Mixed Wettabil i ty - Com parison between Oil Residual Saturationafter Gas Displacement in Secondary and Tertiary Conditions.

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