depressurization under tertiary conditions in the near-wellbore region: experiments, visualization...

8/14/2019 DEPRESSURIZATION UNDER TERTIARY CONDITIONS IN THE NEAR-WELLBORE REGION: EXPERIMENTS, VISUALIZATIO

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DEPRESSURIZATIONUNDERTERTIARYCONDITIONSINTHENEAR-WELLBOREREGION:EXPERIMENTS,VISUALIZATIONAND

RADIALFLOWSIMULATIONS

P.Egermann,S.BaniniandO.Vizika

InstitutFranaisduPtrole(IFP)

ABSTRACT

Depressurizationcanbeaveryinterestingprocesstorecoverhydrocarbonsfromwaterflooded

oil reservoir with high gas-oil-ratio. Most of the published results are related to depletion

experimentsundersecondaryconditions(virginreservoir).Dataaremorescarceundertertiary

conditionsafterwaterflooding[1,2,3].

Thepresentpapertreatstheissueofthedepressurizationundertertiaryconditionsinthenear-

wellbore region. Practically, this has been achieved by combining experimental results,

obtainedonbothcoreandtransparentmicromodel,withradialflowdepletionsimulationsthatare representative of the conditions prevailing in the near-wellbore region. A validated

methodologypreviouslypresented[4]hasbeenusedtodesigntheexperimentsoncore(the

coreundertertiaryconditionswascontinuouslyflushedwithwateratafixedrate,whilethe

pressureattheoutletwasdecreasedtoreproduceadrawdown).Thesaturationprofiles,the

pressuresandthefluidproductionsasafunctionoftimewererecordedinthetwoexperiments

that are presented. From the core experiments, critical gas saturation (Sgc) and relative

permeability (kr) have been determined using a specific simulation code that takes into

accountnucleation,diffusionandmobilizationprocessesrelatedtotheappearanceofthegas

phasefromsolution.Constantdepletionrateexperimentswerealsoconductedonatransparent

micromodeltostudytheprocessofconnectionofthegasphaseundertertiaryconditions.The

specificityofthenear-wellboreregion(highdP/dtand radialflow) was then investigatedby

conductinganumericalexperimentwithradialgeometry.

Itisshownthatgasrelativepermeabilitiesobtainedundertertiaryconditionsarelowerthan

the ones obtained under secondary conditions. This behavior is attributed to the double

connectionprocessthatisneededforthegastogetmobilized:connectionofthedisconnected

oilphaseandconnectionofthegasphaseitself.Thisphenomenonisalsoconfirmedbythe

visualizationexperimentsconductedonmicromodel.Thenumericalexperimentrepresentative

ofthenear-wellboreconditionswitharadialgeometrydemonstratesthatSgcisafunctionof

thedistancetothewellbore,whichcanhavealargeimpactonreservoirsimulationresults.

INTRODUCTION

Depressurization under tertiary conditions is considered as an interesting process to extend

economicallythelifeof maturewaterfloodedreservoirs[5, 6].Thesuccessof theprocess is

mainlydependentontherecoverablefractionofthesolutiongascontainedinthetrappedoil

and on the quantity of incremental oil that is possible to produce consecutively to the

developmentofthegasphase[7].

Realisticestimationsofthistertiaryrecoverycanonlybeachievedwithrelevantvaluesofboth

Sgc (critical gas saturation) and kr (relative permeability). Unfortunately, most of the

published data on depressurization are related to depletion experiments under secondaryconditions and are mainly focused on the evolution of the Sgc value as a function of the

SCA2003-15


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depletion rate (dP/dt). Secondary depressurization has been looked at with coreflood

experiments[8,9,10],withporenetworkmodeling[11],orwithvisualizationontransparent

micromodel[12,13].Theavailableresultsprovethatkrgvaluesunderinternalgasdriveare

verylowevenatsignificantSgvalues.Thistrendwasobservedforbothlightandheavyoils

[14,15].Morerecently,ithasbeenshownthatinternalgasdrivekrgvaluescanbeseveral

ordersofmagnitudelowerthantheexternalgasdrivekrg[4,16].

Data aremore scarceon tertiary depressurization process.Lighthelm et al.[1] have mainly

studiedtheevolutionofSgcasafunctionofdP/dtusingcoresandfluidsfromtheBrentfield.

TheyshowedthatSgcincreaseswithdP/dt(sametrendasundersecondaryconditions),but

theSgcvaluedependsontheinitialsaturationstateforagivencoreandagivensetoffluids.

The connection process of the gas phase under tertiary conditions was also studied in

micromodel by Hawes et al. [17]. Only Grattoni et al. and Naylor et al. [2, 3] tackled

simultaneously the issues of Sgc and kr under tertiary conditions. Grattoni et al. [2, 18]

conducteddepletionexperimentsonamodelporousmediumcomposedoftransparentpacked

beadsthatenabledalsoflowvisualization.Themainresultofthisstudyistheextremelylow

valuesofkrgmeasured,whichwerefoundaround10-5

-10-3

evenat23%Sg.Thisparticular

behaviorwasattributedtotheintermittentflowofgasthatwasobservedexperimentallyduring

thedepletionprocess.Naylor etal.[3] conducted several depletion experiments atdifferent

ratesonrealagedcores.Thekrgcurveswereobtainedbynumericalhistorymatchingtaking

into accountthe specific shapeof the localSg profiles. The results obtainedwere in good

agreementwiththeexistingliterature.SgcincreaseswithdP/dtandkrgvaluesareextremely

low,around10-4

.

Inthispaper,wewillfocusonthetertiarydepressurizationinthenear-wellboreregion.Thispartofthereservoirisparticularlyimportantfor hydrocarbonproductionbecauseitcontrols

the well productivity / injectivity, whereas the far field region accounts for the overall

recovery.MostofthepublisheddepressurizationexperimentsareconductedatafixeddP/dt,

sothatthepressuredropalongthecoreremainsmoderate,evenathighdP/dt.Thismakesthis

kindofexperimentsonlyrepresentativeofthefarfieldregion.Inthefirstpartofthepaper,we

presenttwocorefloodexperiments using a specificdesignand methodology that enablesto

reproducetheconditionsthatprevailinthenearwellboreregion[4]:highpressuregradient.

Theexperimental resultsareinterpreted using a tertiary depletionsimulator. The related kr

curvesarepresentedandcomparedwithotherpublishedresults.Thegasmobilizationprocess

undertertiary conditions is studied in the second part usingvisualizations obtained from a

highpressuremicromodel.Inthethirdandlastpart,thespecificityofthenear-wellboreregionisinvestigatedbyperformingnumerical experiments witharadialgeometry toevaluatehow

theflowingpropertiesvaryasafunctionofthedistancefromthewell.

COREEXPERIMENTS

Rock/fluidsystem

Alltheexperimentswereperformedona Vosgessandstonecore,whosemainpropertiesare

gatheredinTable1.Thischoicewasmainlymotivatedbythehighdegreeofhomogeneityof

thissandstoneandalsobyitsfairabsolutepermeability.


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Table1:Vosgessandstonecoreproperties

PermeabilitymD Porosity% Lengthcm Areacm2

PVcm3

58 24 34.5 14.5 120

ThesamebinarymixtureC1C7wasusedfortheoleicphaseinbothexperiments.Thebubblepointofthemixturewasequalto21bar,whichcorrespondstoamolarfractionofC1equalto

0.107.Thetablehereafterprovidestheassociatedpropertiesin termsof IFTandviscosityat

Pband20C.DatawerederivedfromIFPdevelopedPVTsoftware.

Table2:fluidpropertiesofthebinarymixtureC1C7

Pbbar %C1molar IFTmN/m OilviscositycP Oildensitykg/m3

21 10.7 17.25 0.402 672

Thebrineiscomposedof50g/lNaCland5g/lCaCl2.Thiscompositionprovedtobeefficient

instabilizingclayscontainedinthesandstoneandingettinga goodcontrastbetweentheoilandthewaterphaseduringtheacquisitionoflocalsaturationmeasurements.

Experimentalset-up

It is mainly composedof a Hassler type core holder made with composite material, which

enables acquisitionof localsaturationmeasurementsalongthecore with CT-scanner.In the

inlet,thebinarymixtureorthebrinecanbeinjectedintothecoreatafixedratewithapump.

Downstream,theoutletpressureiscontrolledbyabackpressurevalveremotelyoperatedbya

PC,sothatthedrawdowncanbemonitoredveryeasily.Attheoutlet,adensimeterisused

mainlyduringthecorepreparationtocheckthecoresaturationwiththemixture.Duringthe

experiment,thedensimeterisalsousefultodetecttheveryfirstapparitionofthegasphasedownstream.Fluidsarecollectedintoaseparatorsothatbothliquidsandgasquantitiescanbe

measured.Evolutionoftheoutletpressureisdirectlyrecordedbyapressuregauge.Theinlet

pressureisdeducedfromadifferentialpressure gaugemeasurement.Only thetotalpressure

dropalongthecoreisavailable(nopressureports).Allthedataareautomaticallyrecordedon

aPC.

CTscanner

acquisition

C1C7reservoir

PumpDensimeter

DP

PCAutomaticacquisition

Outletpressurecontrol

Gasometer

Separator

Pressureregulator

Brinereservoir

Figure1:corefloodexperimentalset-up

Experimentalprocedure

Residualoilsaturation(tertiaryconditions)wasestablishedwiththefollowingprocedure: PreliminarysettingofS wiconditionsusingaviscousoil(Marcol52,10cP)


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MisciblefloodingofMarcolbyC7 MisciblefloodingofC7bythebinarymixtureC1C7underpressure(abovePb) SettingofSorwbyinjectionofbrineatfixedrateTheprincipleofthedepletionexperimentisdescribedinearlierpublication[4].Undertertiary

conditions, the brineis injected upstream ata fixed rate with the outlet pressure originally

abovethebubblepoint. Pressureat theoutlet isprogressively decreaseddownto a definedvaluebelowthebubblepointofthemixture.Alltheexperimentswereperformedwitharate

of30cc/htosimulateconditionsinthewellboreofaproductionwell.Thedrawdowncaused

theformationofthegasphasestartinginthedownstreampartofthecore.Transientevolution

ofgassaturationthroughthecoreisfollowedbyCT-scannerandimpactofgasapparitionon

thepressuredropisdetectedontheinletpressurevariations.

Productiondataresults

Twoexperimentswereconducted(M1andM2).Theoutletpressurewasdecreasedbelowthe

bubblepressureintwosteps(17and14bar)inthefirstexperiment,whereasitwasdecreased

directly down to 14 bar in the second experiment. For both experiments, the pressureevolutionsareverysimilarandalsoingoodagreementwithpreviousexperimentsperformed

undersecondaryconditions(Figure2).Atthebeginning,theinletandoutletpressuresremain

parallel.Thispartcorrespondstoaconstantpressuredropalongthecoreduringtheflowof

waterwhentheoutletpressureisabovethebubblepoint(residualoil immobile).Assoonas

thegasappears downstream, thepressure dropincreaseswith increasinggas saturation,and

then it stabilizes, while the outlet pressure is maintained constant (17 or 14 bar). The

experimentalresultsaresummarizedinTable3.

Secondaryconditions

65

70

75

80

85

90

0 2 4 6 8Timehour

Pressurebar

Inletpressure

Outletpressure

Tertiaryconditions

10

15

20

25

30

35

40

0 1 2 3 4

Timehour

Pressurebar

Outletpressure

Inletpressure

17ba rste p 14ba rste p

Figure2:inletandoutletpressureevolutions(secondaryconditionsandM1)

Oneinterestingfeatureconcernstheevolutionoftheinletpressure.Areboundwasmeasured

under secondary conditions and was associated to a recompression along the core

consecutively to the development of the gas phase and its effect on kro (the oil was the

injectedfluid).Theinletpressureonly decreasesorstabilizesundertertiary conditions.This

result suggests that the development of the gas phase has a less pronounced effect on the

permeabilityoftheinjectedfluid(krw).Thismaybeaconsequenceofthetertiaryconditions,

wherethegasappearswithintheoilphase,whichisnonmobileandtrappedinthelargepores

sincetherockiswater-wet.


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Table3:tertiarydepletionexperimentsresults

Exp Sorw% DPwater/oil Step1/DP1 Step2/DP2 KrwatSorw So%OOIP

M1 28.1 6.1bar 17bar/8.7bar 14bar/9.8bar 0.057 42

M2 29.0 6.4bar 14bar/9.1bar 0.054 35

Figure3showstheevolutionofthedifferentialpressureandtheincrementaloilproductionas

afunctionoftime.Inbothexperiments,theappearanceofthegasphasemakesthepressure

dropalongthecoreincrease,andisassociatedwiththeproductionofasignificantquantityof

oil,whichrepresentsaround10%PV(34-42%OOIP).Inthetwostepexperiment(M1),the

break in the slope of the oil production curve strongly suggests that this incremental oil

productionisdirectlyconnectedwiththeextensionofthethree-phaseareaintothecore.

M1

0

2

4

6

8

10

12

14

16

0 1 2 3 4

Timehour

Oilproductioncm3

0

2

4

6

8

10

12

Differentialpressurebar

Experimentaloilproduction

ExperimentalDP

M2

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6

Timehour

Oilproductioncm3

0

1

2

3

4

5

6

7

8

9

10

Differentialpressurebar


ExperimentalDP

Figure3:Experimentalpressuredropandoilproduction(M1andM2)

Saturationprofilesresults

Theaboveobservationsmadeontheproductiondataarecompletelyconfirmedbythein-situ

saturation measurements. Figure 4a illustrates the extension of the gas region upstream

dependingontheoutletpressurevalue(17and14bar).TheshapeoftheSgprofilesisvery

typicalwithagassaturationjumpaheadthegasregionfollowedbyalesspronouncedincrease

of Sg. This shape was found common in all experiments and very similar with the results

obtainedundersecondaryconditions[4].AnotherinterestingfeatureconcernstheevolutionoftheSwprofilesduringM1(Figure4b).

TheSwvalueremainsveryuniformandconstantwithinthethree-phaseareasuggestingthat

the non wetting phasesaturation (Sg+So) value isaround 35%, close to the original Sorw

value,whatevertheexperimentalconditions.

17barstep 14barstep

Gasphase

a earance


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0

0.05

0.1

0.15

0.2

0.25

0.3

0 50 100 150 200 250 300 350

Distancefromtheinletmm

Gassaturation%

17barstep:experimentalM1



0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200 250 300 350

Distancemm

Watersaturation%

Swbeforedepletion

Sw:end17barstep

Sw:end14barstep

Figure4:(a)Sgprofiles(M1andM2);(b)SwprofilesM1

The conservation of the overall non-wetting phase saturation also explains why the

developmentofthegasphasehasalesspronouncedimpactontheinletpressure,andon the

injectedphasepermeabilitythanundersecondaryconditions.

Interpretation

Numericalsimulator

Thein-housedepletioncodethatwasdevelopedtosimulatetheexperimentsundersecondary

conditionswasused[19].ThegeneralstructureofthecodeisremindedinFigure5.Themain

advantagesofthiscodearelistedbelow:

Account of the physical phenomena involved in the gas formation (nucleation,diffusion) Controlofthegasmobilizationprocess.Dependingontheexperimentalconditions,thecodecanhandlebothdispersedorcontinuousgasflow.

Initialization

t=t+dt

Waterrate

Pressureprofile

Nucleation

Bubblegrowth

Gasmobilization Connection Dispersed

Oilsaturation

Solutiongasconcentration

Upstream

Qw

Downstream

Poutlet

Figure5:codearchitecture

Somemodificationshavebeenintroducedintheoriginalcodeinordertotakeintoaccountthe

specificityofthetertiaryconditions.

The pressure drop along the core results from the injection of the water phase(krw(Sg))

The three-phase flow was handled like a pseudo two-phase flow. As the localsaturation measurements showed that the non wetting phase saturation remains

a b

Sgjump


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constantwithinthethree-phaseregion,theoilsaturationineachcellwascalculated

from the gas saturation value using the conservation of the non wetting phase

saturation:

SgSSo NWR = , whereSNWRwaskeptconstantandequalto35%

HistorymatchingA very good agreement was reached between the simulation and the experimental data in

termsofproductiondataandSgprofiles(Figure6),whateverthevalueoftheoutletpressure.

Theconsistencyofthishistorymatchingwasalsocheckedbycomparingtheexperimentaland

the simulated gas production a posteriori. Here again, an excellent matching is obtained

(Figure7a).

0

2

4

6

8

10

12

14

16

0 1 2 3 4

Timehour

Oilproductioncm

3

0

2

4

6

8

10

12

Differentialpressure

bar

Simulatedoilproduction


SimulatedDP

ExperimentalDP

0

0.05

0.1

0.15

0.2

0.25

0.3

0 50 100 150 200 250 300 350

Distancefromtheinletmm

Gassaturation%

17barstep:experimental

14barstep:experimental17barstep:simulated

14barstep:simulated

Figure6:historymatchingofM1(productiondataandSgprofiles)

0

50

100

150

200

250

300

350

400

450

500

0 1 2 3 4

Timehour

Gasproductioncm3

Experimentalgasproduction

Simulatedgasproduction

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 0.1 0.2 0.3 0.4 0.5Sg

krg

Tertiarykrg

Secondarykrg

Grattonietal

Drummondetal

Figure7:gasproductionhistorymatching(a)andkrgresults(b)

Krgcurveshape

The tertiary krg curve deduced from the history matching procedure is compared with the

secondarykrgcurvethatwasobtainedon anotherrocksampleinFigure7b[4].Bothcurves

show that the mobility of the gas phase is limited under depletion, but this trend is more

pronounced under tertiary conditions. It suggests that one part of the gas formed into theporousmediumdoesnotparticipateinthegasflowundertertiaryconditions,whichleadstoa

a b


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lowerkrgvalueatagivenSg,comparingtothesecondaryconditionscase.Asthegascomes

fromsolutionwithinthetrappeddisconnectedoil,itissuspectedthattheonepartof thegas

formedinnonaccessibleoilgangliacanmisstheconnectionprocessandexplainthisresult.

Althoughtheexperimentsundertertiaryandsecondaryconditionshavenotbeenconductedon

the same rock, it is interesting to observe that a lower Sgc value is found under tertiary

conditionsforsimilardrawdownrate(evenifkrgarelower).Thismaybeaconsequenceofthewaterwettabilityoftherock,whichfavorstheentrapmentoftheoilphaseinthelargest

pores.Hence,itmakesthegasphaseappearsundertertiaryconditionsintheclassofporesthat

accounts for the porous mediumconnectivity leading to lower Sgc value comparing to the

secondarycase,wherethegasbubblescanbecreatedineveryclassesofpores.

Thetertiarykrgcurveisalsoingoodagreementwiththeotheravailablepublishedresultsfrom

Grattonietal.[2]andNayloretal.[3](correspondingkrgcurveinDrummondetal.[6]).All

curveshaveacommonpartaround10-4

,whichconfirmsthegasmobilityisverylimitedunder

tertiaryconditions,whatevertheexperimentalconditions.

VISUALIZATIONEXPERIMENTS:GASMOBILIZATIONPROCESS

Apressurizedmicromodelwasusedtovisualizethegasmobilizationprocessundertertiary

conditionsandexplainthelowvaluesofkrgmeasuredoncorefloodexperiments.Thegeneral

principleoftheexperimentalset-upispresentedinFigure8.Themicromodelcanbeoperated

up to 100 bar and 60C using a circulating device of the heated confining fluid. The

establishmentofthetertiaryconditionswithC1C7wasachievedusingtheprocedurefollowed

withcorefloodexperiments.Thedepletionwasconductedatfixeddepletionrate(dP/dt)by

closing the inlet valve and decreasing progressively the pore pressure through the back

pressurevalve.

Pressurepumps

(doublebody)

Saphircell

N2

Pressure gauges

Oven

Highpressuremicrodel

(100b,60C)

Accumulator

Monitor

Video

recording

Vizualizationdevice

Transfercells

C1C7

Confining

fluid

Pump

N2

Water

Videocamera

Figure8:Experimentalset-upforMicromodelvisualization

AsequenceofthegasphasedevelopmentisgiveninFigure9.Onthefirstpicture,thegas

bubbles(white)startappearingfromtheoilganglia(blue)disconnectedbythewaterphase

(red).Dependingonthesizeofthegangliaandtheavailablequantityofsolutiongasinthe

vicinity of the bubbles, the growingratecan bevery different from one bubble toanother.

Several connection events between two adjacent bubbles are also observed, whereas somebubblesremaindisconnectedduringthewholeexperiment.Attheendofthesequence,alarge


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gas bubble isobtained, whereas a significantfractionof thegas phase is still disconnected

withinthedisconnectedoilganglia.Fromtheseobservations,thelowvaluesofkrgmeasured

from coreflood experiments can be qualitatively explained by the double mechanism of

connectionneededforthegastogetmobilized:reconnectionoftheoilphaseandconnection

ofthegasitself.Thismechanismisalsoverysimilartowhatisobservedundertertiarygravity

assistedinertgasinjectionasshownbyKantzasetal.[20].

Figure9:Developmentofthegasphaseundertertiaryconditions

RADIALGEOMETRYEFFECTINTHENEARWELLBORE

Principle

Thenearwellboreregionischaracterizedbyahighdrawdownandradialgeometry.Thehigh

drawdown is reproduced in our experiments by injection of the producing fluid at a

sufficiently high rate, but the flow is linear, which makes the pressure gradient (dP/dx)

uniformalongthecore.Astheflowisradialnearthewell,bothdP/dxandU(fluidvelocity)

increasefromthefarfieldtothenearwellboreregion,sotheequivalentdepletionratedP/dt,

equal to UdP/dx, also increases [4]. As the flowing parameters (Sgc, kr) are strongly

dependent on dP/dt, we have designed a numerical laboratory experiment with a radial

geometrytoinvestigatetheevolutionoftheflowingpropertiesasafunctionofthedistance

fromthewell.

Rmax=60cm

Rmin=10cm e=10cm

QoilPhi=20%

Angle=30

Qoil=800cm3/h

Figure10:principleoftheradialnumericalexperiment

Gas

Oil

Water

Connection

events

1 2 3

4

987

65


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The simulations were performed under secondary conditions. The numerical experiment

principle is shown in Figure 10 and is the same as the one followed for the coreflood

experiments. The depletion simulator was first calibrated on an existing experiment with a

linear geometry to fix the parameters controlling the nucleation, the diffusion and the gas

mobilization.ItisworthremindingthatourcodeenablestoevaluatethelocalSgcvaluefrom

thebubble population properties (size,number)anda shape factor totake intoaccountthebubble ramifications for connection criterium. After calibration, the code was slightly

modifiedtohandleradialgeometryandthenumericalexperimentwaslaunched.

Results

Figure11showsthepressureprofileswithbothgeometries.Asexpected,thepressuregradient

dP/dxincreasesneartheoutletwhenaradialgeometryisconsidered,whereasdP/dxremains

uniformalongthecorewithalineargeometry.TheassociatedSgprofileshaveasimilarshape

with a gas saturation jump followed by a less pronounced increase of Sg, whatever the

geometryconsidered(Figure12).Themobilizationofthegasphasecanbedetectedfromthe

break in the slope of the Sg profiles (when the increase is less pronounced). With linear

geometry, the break occurs always at the same saturation, whereas the break is clearly a

functionofthedistancewithradialgeometrysuggestingSgcvaluevariesfromtheoutletpart

ofthecoretotheinlet.

Lineargeometry

70

75

80

85

90

0 100 200 300 400

Distancemm

Pressurebar

Radialgeometry

70

72

74

76

78

80

82

84

86

88

90

0204060

Radiuscm

Pressurebar

Figure11:pressureprofileswithlinearandradialgeometry

Lineargeometry

0.00

0.10

0.20

0.30

0.40

0.50

0 100 200 300 400

Distancemm

Sg

Radialgeometry

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0102030405060

Radiuscm

Sg

Figure12:Sgprofileswithlinearandradialgeometry

SgclineSgcline

increasingtimeincreasingtime

increasingtime increasingtime


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ThisbehaviorisconfirmedinFigure13,wheretheSgcvaluescalculatedbythecodeineach

cell are plotted as a function of the distance. When a linear geometry is considered, the

calculatedSgcvaluesareveryuniformalongthecore,equalto24%,whichisinlinewiththe

experimental data and the theory(high equivalent dP/dt is constant along the core, several

metersawayfromthewell).

0.10

0.15

0.20

0.25

0.30

0 10 20 30 40

Distancefromtheinletcm(linear)

CalculatedSgc

010203040Radius(cm)

Linearcase

Radialcase

1

10

100

1000

0.10 0.15 0.20 0.25 0.30

Sgc(fraction)

dP/dt(psi/hr)

Radialcase

Linearcase

Figure13:calculatedSgcvaluesalongthecoreinlinearandradialgeometry

When a radial geometry is considered, the calculated Sgc value varies from 26% near the

outlet,wheretheequivalentdP/dtismaximum,downto20%atthegasfront,wherethedP/dt

valueislower.ThecalculatedSgcvaluesareclearlya functionofdP/dt(Figure13)andfall

alongalineartrendinsemi-logscaleasalreadypointedoutbyScherpenisseetal.[9].This

studyconfirmstheroleplayedbytheflowgeometryinthespatialdistributionoftheflowing

propertiesunderdepletion.Whensignificantdrawdownvaluesareobservedinthefield(high

productionratesorheavyoilproduction)thisdependencehaslimitedeffectsontheoverall

field recovery [21]but mayimpacttheproductionperformances significantly andshouldbe

takenintoaccountinreservoirsimulations.

CONCLUSIONS

Itisshownthatthegasrelativepermeabilitycurveobtainedundertertiaryconditionsislower

than theone obtainedunder secondaryconditions.This behavior isattributed tothedouble

connectionprocessthatisneededforthegastogetmobilized:connectionofthedisconnected

oilphaseandconnectionofthegasphaseitself.Thisphenomenonisalsoconfirmedbythe

visualizationexperimentsconductedonmicromodel.Thenumericalexperimentrepresentative

ofthenear-wellboreconditionswitharadialgeometrydemonstratesthatSgcisafunctionof

thedistance,whichcanhavelargeimpactonreservoirsimulationresults.

ACKNOWLEDGEMENTS

ThisworkwaspartlyfundedbyARTEP,a researchassociationbetweenTOTAL,ELF,Gaz

deFranceandIFP.

NOMENCLATURE

dP/dt: depletionrate

dP/dx: pressuregradient

IFT: interfacialtension

kri: relativepermeabilityofphasei

OOIP: oiloriginallyinplace

Pb: bubblepressure

Q: totalinjectionrate

Si: saturationofphasei

Sorw: residualoilsaturationtowaterflood

U: fluidvelocity

REFERENCES

Sgc=6%


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This paper was prepared for presentation at the International Symposium of the Society of Core Analysts held in Pau, France, 21-24 September 2003

depressurization under tertiary conditions in the near-wellbore region: experiments, visualization...

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