Gas Cap Drive Reservoirs - Petrowiki

Gascapdrivereservoirs

Insomeinstances,oilreservoirsarediscoveredwithasegregatedgaszoneoverlyinganoilcolumn.Theoverlyinggaszoneisreferredtoasaprimarygascap.Inadditiontofreegas,gascapsusuallycontainconnatewater(/Glossary%3AConnate_water)andresidualoil.Theunderlyingoilcolumnissometimesreferredtoasanoilleg.Inotherinstances,asreservoirpressuredeclineswithproduction,gasevolvesinthereservoir(seeSolutiongasdrivereservoirs(/Solution_gas_drive_reservoirs))andmigratestothetopofthestructuretoaddtoanexistingprimarygascaportoformagascap.Ifproperlyharnessed,gascapscanenhanceoilrecoveryconsiderably.Thedegreewithwhichtheyimproverecoverydependsmainlyontheirsizeandontheverticalpermeabilityand/orformationdip.Producingwellsusuallyarecompletedonlyintheoillegtominimizegasproduction.

Contents

1Properties2Performance

2.1Nonsegregationdrivegascaps2.2Segregationdrivegascaps

2.2.1Gasreinjection3Materialbalanceanalysis4Nomenclature5Subscripts6References7NoteworthypapersinOnePetro8Externallinks9Seealso

Properties

Broadly,gascapsareclassifiedassegregatingornonsegregating.Table1summarizesthedistinguishingcharacteristicsofeach.

(/File%3AVol5_Page_0921_Image_0001.png)

Table1

Segregatinggascapsaregascapsthatgrowandformanenlargedgascapzone.Fig.1showsaschematicofasegregationdrivereservoir.Twodifferentsegregationmechanismsarepossible:

1. Expansionofandfrontaldisplacementbypreexistinggascapgas2. Upwardmigrationofoilcolumngasassolutiongasisliberatedandafreegasphaseforms

Thesecondmechanisminvolvesthesimultaneousdownwardmovementofoiltobalancetheupwardflowofgas.Thisdiametricflowpatternisreferredtoascounterflow.[1]

Pirson[2]referstothefirstmechanismaspassivesegregationandthelattermechanismasactivesegregation.Hall[1]referstothefirstmechanismassegregationdrivewithoutcounterflowandthesecondmechanismassegregationdrivewithcounterflow.Bothmechanismsaretimedependent,andtheirdisplacementefficiencydependsonthegas/oildensitydifference,theproducingrate,andtheverticalpermeability.

Bothsegregationmechanismsyieldaprogressivelydescendinggasoilcontact(/Glossary%3AGasoil_contact)(GOC).Thesegregationdrivemechanismscanbeaugmentedbycrestalgasinjection.

Ifneitherofthesesegregationmechanismsispresent,thegascapiscalledanonsegregatinggascap.Nonsegregatinggascapsdonotformanenlargedgascapzone,andtheirGOCappearsstationary.Thegascapgasexpandsbutthedisplacementefficiencyissopoorthattheexpandinggasappearstomerelydiffuseintotheoilcolumn.Fig.2illustratesthedistributionofwater,oil,andgasinanonsegregationdrivegascapreservoir.


Fig.1Distributionofwater,oil,andgasandpositionofgas/oilcontact(GOC)inasegregatinggascapreservoir:(a)beforeproductionand(b)duringdepletion.


Fig.2Distributionofwater,oil,andgasinanonsegregatinggascapreservoir:(a)atdiscoveryand(b)duringdepletion.

Broadly,gascapsacttomitigatethepressuredecline,extendthelifeofthereservoir,andultimatelyimprovetheoilrecovery.Thedegreeofoilrecoveryimprovementdependsonthefollowing:

SizeofthegascapWhetheritisasegregationdriveornonsegregationdrivegascap

Tounderstandthemechanicsofgascapreservoirs,numericalsimulationresultsofsegregatingandnonsegregatinggascapsarepresented.EachexampleusesthefluidpropertydatainTable2.EachexamplealsousesthereservoirdatasummarizedinTable3,exceptthattheinitialpressureis1,640psiainsteadof2,000psiaandthegascapthicknessis10ft.Thegas,oil,andwatersaturationsinthegascapare60,20,and20%,respectively.Thegascapinitiallycontains270,000STBofoiland816MMscfofgastheoilleginitiallycontains210millionSTBofoiland1.718Bscfofgas.Thetotaloriginaloilinplace(OOIP)=2.37millionstocktankbarrels(STB),andoriginalgasinplae(OGIP)=2.534Bscf,andm=0.33.Forreference,thesegregatingandnonsegregatinggascapcasesarecomparedwithanidenticalreservoirwithoutagascap(basecase).


Table2


Table3

Performance

Nonsegregationdrivegascaps

Fig.3plotspressureasafunctionofcumulativeoilrecoveryforanonsegregationdrivegascapreservoir.Forcomparison,thisfigureincludestheresultsofthenogascap(base)case.Thisfigurealsoincludestheresultsofothercases,whicharediscussedlaterinthispage.AllrecoveriesarereportedasafractionoftheoriginaloillegOOIPtomakedirectcomparisonsvalid.Thenonsegregationdrivegascapcaseconsistentlyyieldshigheroilrecoveriesatagivenpressurethanthenogascapcase,whichillustratesthesuperiorrecoveryperformanceofgascaps.Viewedanotherway,thenonsegregationdrivegascapcaseconsistentlyyieldsahigherpressureatagivenoilrecoverythanthenogascapcase,whichillustratesthesuperiorpressuremaintenanceabilityofgascaps.


Fig.3TheeffectofgascapandgasreinjectiononoilrecoverasafunctionofpressureforawestTexasblackoilreservoir.

Fig.4isacompositefigureandshowstheperformanceasafunctionoftime.Thisfigureincludesthe:

Gas/oilratio(GOR)GassaturationOilrateOilrecoveryhistories

TheGORhistoryshowsthatnonsegregatinggascapseventuallyyieldhigherproducingGORsthanthenogascapreservoir.ThehigherGORiscausedbyhighergassaturationintheoilleg.Thehighergassaturationiscausedbythegascapgasmigratingfromthegascapintotheoillegasthepressuredeclines.


Fig.4Performanceofagascapreservoir:(a)instantaneousproducingGOR,(b)gassaturation,(c)oilrate,and(d)oilrecoveryhistoriesforawestTexasblackoilreservoir.

Fig.4alsoshowstheeffectofanonsegregatinggascapontheoilratehistory.Thenonsegregatinggascapconsistentlyyieldshigheroilratesthanwithoutthegascap.Ifaneconomiclimitcorrespondingtoaminimumoilrateof20STB/Disarbitrarilyassumed,thenthenogascapcaseisterminatedafter13.8yearswhilethenonsegregationdrivegascapcaseisterminatedafter15.2years.Thiscomparisonshowsthatthepresenceofagascapextendstheprimaryrecoverylifeofthereservoir.Thecurveendpointsdenotethetimeoftermination.Thenogascapcaseisslightlydifferentfromtheblackoilcasediscussedearlierduetotheassumedloweroriginalpressure.

Fig.4includesthefractionaloilrecoveryhistoryFig.5showsthegasrecoveryhistory.Thecurveendpointsdenotethetimeoftheeconomiclimit.Table4summarizestheconditionsattheeconomiclimit.Thenogascapandnonsegregationdrivegascapcasesrecover23.7%and26.8%oftheoillegOOIP,respectively.Thus,thenonsegregatinggascaprecoversmoreoilthanwithoutthegascap.Thenonsegregatinggascapalsoisterminatedatahigherpressure,producingGOR,gassaturation,andgasratethanwithoutthegascap.Thenonsegregatinggascaprecovers74.9%oftheoillegOGIPwhilethenogascapcaserecovers52.3%oftheoillegOGIP.Thenonsegregatinggascaprecoversmoregasbecausesomeofthegascapgasinfiltratestheoillegandisproduced.Inconclusion,thepresenceofanonsegregatinggascap:

YieldshigherultimateoilandgasrecoveriesAcceleratesrecoveryExtendstheprimaryrecoverylifeofareservoir


Fig.5TheeffectofgascapandgasreinjectionongasrecoveryhistoryforawestTexasblackoilreservoir.


Table4

Theeffectofagascaponoilrecoveryisrelateddirectlytoitssizerelativetothesizeoftheoilleg.Thesizeofthegascapisdescribedeffectivelyintermsofthedimensionlessvariablem,whichisdefinedastheratiooftheinitialfreegasandfreeoilphasevolumes(seeEq.5).Ifallthefreegasislocatedinthegascap,allthefreeoilislocatedintheoilleg,andtheoillegandgascapporositiesandconnatewatersaturationsarethesame,thenmrepresentstheratioofthegascapandoillegporevolume(PV).Fig.6showstheeffectofmonthefinalfractionaloilrecoveryforannonexpandinggascapreservoir.TheresultsinFig.6usethesamereservoirdataasintheprevioussimulationsexceptdifferentgascapsizesareconsidered.Otherreservoirconditionsmayyieldslightlydifferentresults.Themostnoticeableimprovementinoilrecoverycomesasmincreasesfrom0to2.0.


Fig.6Theeffectofdimenstionlessgascapsize(m)onfinalprimaryoilrecoveryandpeakproducingGORforawestTexasblackoilreservoir.RecoveriesreportedaspercentofoillegOOIP.

ThegascapsizealsoaffectsthepeakGOR.Asthegascapincreases,thepeakGORincreases.Fig.6showsthepeakGORasafunctionofmforthewestTexasreservoirproperties.ThepeakGORincreaseswiththegascapsizebecausemoregascapgasmigratesintotheoilcolumnasthegascapincreases.Insummary,nonsegregationdrivegascapreservoirstendtoyieldfinalfractionaloilrecoveriesintherangeof15to40%oftheOOIP.Segregationdrivegascapreservoirstendtoyieldevenhigherfinaloilrecoveries.

Segregationdrivegascaps

Segregatinggascapsarecharacterizedbyprogressivelydescendinggas/oilcontacts(GOCs).ThemovementoftheGOCiscausedbyactiveorpassivegravitysegregation.Activegravitysegregationisthesimultaneousmigrationofgasupwardanddrainageofoildownward.Passivesegregationisthenaturalexpansionofthegascapgas.BothoftheseprocessesinvolvefrontaldisplacementofoilattheGOC.Frontaldisplacementhelpsdriveoiltotheproducingwells.Frontaldisplacementdoesnotdominateinnonsegregationdrivegascapreservoirs.Theextenttowhichgravitysegregationoccursdependsontheverticalpermeabilityandtherateatwhichfluidsarewithdrawnfromthereservoir.Thegreatertheverticalpermeabilityandslowerthefluidwithdrawal,themorepronouncedtheeffectsofgravitysegregation.

Figs.3through5includesimulationresultsofasegregationdrivegascapreservoir.Thesesimulationsassumepropertiesidenticaltothoseofthenonsegregationdrivegascapsimulationsexceptgravitysegregationisincluded.Thesimulationsassumenofreegasproductionfromthegascap.

Fig.3showsthepressureasafunctionofcumulativeoilrecovery.Thisfigureshowsthatoilrecoveryinasegregationdrivegascapreservoiratagivenpressureisconsistentlygreaterthanthatinanonsegregationdrivegascapornongascapreservoir,especiallyatlowpressureswhentheeffectsofgasexpansionbecomepronounced.Theoilrecoveryperformanceisdiscussedbelow.

Fig.4showstheeffectofasegregatinggascapontheGORhistory.OnlyamarginalincreaseintheGORisnotedafter15years,theGORactuallydecreasesslightly.ThistypeofGORbehaviorischaracteristicofsegregationdrivegascapreservoirs.[3][4][5]Thesegregatinggascapeffectivelydrivesandconcentratesoilintotheshrinkingoilleg.TheoillegshrinksastheGOCdescendsthus,thesegregatinggascapminimizesthegassaturationintheoilleg.TheGORreversalcoincideswithareversalinthegassaturation.Fig.4includesthegassaturationhistory.Thegassaturationsteadilyincreasesuntilitpeaksatapproximately0.25PVthenitdecreases.TheGORandgassaturationreversalsoccuratamoderatetolowpressurewhentheexpansionofthegascapgasbecomespronounced.ThechangeinthepositionoftheGOCyieldsameasureoftheoillegshrinkage.Attermination,theGOChasdescendedapproximately9.3ftintotheoriginal20ftoilcolumn.

Fig.4includestheoilratehistory.Theoilrateforthesegregatinggascapisconsistentlyhigherthanforthenonsegregatinggascaporwithoutthegascap.Theoilrateeventuallyflattensouttobetween20and50STB/Dandstayswithinthisrangefor15to31years.Thismoderatebutsteadyoilrateexplainsthesuperiorperformanceandlonglifeofsegregationdrivegascapreservoirs.Table4summarizesandcomparestheprimaryrecoverylifetimesofthevariouscases:

Segregatinggascaphasalifeof31.3yearsNonsegregatinggascaphasalifeof15.2yearsSolutiongasdrive(basecase)hasalifeof13.8years

Fig.4includesthecumulativeoilrecoveryhistory.Thesegregatinggascapreservoirrecovers38.7%oftheoillegOOIPwhilethenonsegregatinggascapandsolutiongasdrivereservoirsrecover26.8and23.7%oftheOOIP,respectively.Suchahighrecoverylevelforasegregationdrivereservoirisnotuncommon.Itisnotuncommonforgravitydrainagereservoirstorealizerecoveriesashighas60to70%oftheOOIPhowever,theygenerallyrequirealongtimetodoso.ThecurveendpointsinFig.4denotethetimeoftheeconomiclimit.Thesegregatinggascapreservoirterminatesatapressureof508psia.

Fig.5showsthegasrecoveryhistory.Thesegregatinggascapreservoirrecovers91.1%oftheoillegOGIP.Thisrecoverylevelisconsiderablygreaterthanthenonsegregatinggascaporsolutiongasdrivereservoirs(74.9and52.3%,respectively).Onereasonsegregatinggascapreservoirstendtoyieldsuchhighgasrecoveriesisthattheyoftenrecoversomeoftheoriginalgascapgas,whichmigratesintotheoilleg.Inaddition,theygenerallyrealizelowerterminationpressures.

Thefinalfractionaloilrecoveryinasegregatinggascapreservoirisastrongfunctionoftheverticalcommunicationwithinthereservoir.Verticalcommunicationdictatestheextentofsegregation.Ifverticalcommunicationisgood,thenmostofthegascapgaswillbeavailableforsegregation.Itwillalsobeavailabletohelpdriveoilthroughfrontaldisplacementtotheproducingwells.Ifverticalcommunicationispoor,thenverylittle,ifany,ofthegascapgaswillsegregate.Insummary,segregationiscontrolledprincipallybythreevariables:

VerticalreservoirpermeabilityProducingrateWellspacing

Aswellspacingandverticalpermeabilityincreaseandastheproducingratedecreases,theeffectofgravitysegregationincreases.Fortheeffectsofgravitysegregationtobeimportant,however,thewellspacingmayneedtobeprohibitivelylargeortheproducingratemayneedtobeprohibitivelylow.Insuchreservoirs,theverticalpermeabilityisnothighenoughtopermitmuchgravitysegregation.

Thelikelyroleofgravitysegregationcanbemeasuredintermsofagravitynumber,Ng.Ngisdefinedastheratioofthetimeittakesafluidtomovefromthedrainageradiustothewellboretothetimeittakesafluidtomovefromthebottomofthereservoirtothetop.Inoilfieldunits,thegravitynumberis

(/File%3AVol5_page_0927_eq_001.png)....................(1)

where:

kv=verticalpermeability,md=densitydifference,lbm/ft3re=drainageradius,ftq=producingrateatreservoirconditions,RB/Do=oilviscosity,cp

GravitysegregationislikelypronouncedifNg>10gravitysegregationislikelyunimportantifNg

Fig.3showstheeffectofgasreinjectiononpressureasafunctionofoilrecovery.OilrecoveryatagivenpressureisconsistentlyhigherforthegasreinjectioncasethanfortheothercasesinFig.3,exceptatverylowpressuresatwhichthesegregatinggascapcaseyieldssuperiorperformance.Gasreinjectionleadstohigheroilrecoveriesbecausethecompressedreinjectedgaseffectivelyaddsextraenergytothereservoir.

Fig.4showstheeffectofgasreinjectionontheGORhistory.GasreinjectionleadstoveryhighproducingGORs,significantlyhigherthantheothercases.TheGORishigherbecausethegassaturationishigher.Thegassaturationishigherbecausereinjectedgasandinitialgascapgasmigrateintotheoillegduringpressuredepletion.Thisoccursbecausethegascapisnonsegregating.HighproducingGORsareacharacteristicfeatureofreservoirssubjecttogasreinjectionifthereislittleornoactivegravitydrainage.HighproducingGORsmeanthatlargevolumesofproducedgaswillhavetobehandledandprocessedatthesurface.

Fig.4includestheeffectofgasreinjectionontheoilratehistory.Thisfigureshowsthattheoilrateishigherforthefirst81/2yearsforthegasreinjectioncasethanforanyoftheothercases.After81/2years,theoilrateforthesegregatinggascapcaseisslightlygreaterthantheoilrateforthegasreinjectioncase.Theseresultsdemonstratethatgasreinjectionisaneffectivemeanstoarrestthenormaloilratedeclinedramatically.

Fig.4alsoshowstheeffectofgasreinjectiononthefractionaloilrecoveryhistoryandthatthegasreinjectioncaseissuperiortotheothercases.Thegasreinjectioncaserecovers36.7%oftheoriginaloillegOOIPatitseconomiclimitof181/2years.Onlythesegregatinggascapreservoirrecoversmoreoil(38.7%)however,thesegregatinggascapreservoirrequiresmoretimetorecovertheadditionaloil.

Fig.5showstheeffectofgasreinjectiononthefractionalgasrecoveryhistory.ThefractionalgasrecoveryisthecumulativeproducedwellheadgasnormalizedbytheoriginaloillegOGIP.Thegasreinjectioncaserecovers177%oftheoillegOGIP(seeTable4).Morethan100%oftheoillegOGIPisproducedbecausesomeofthereinjectedgasisproduced.Because30%oftheproducedgasisnotreinjected,0.30177or53.1%oftheoillegOGIPisavailableforgassales.Thissalesgasrecoveryiscomparabletothecasewithoutgasreinjection(52%OGIP).

Reservoirssubjecttogravitydrainageareespeciallyattractiveforgasreinjection.Crestalgasinjectionintothedevelopinggascapisthepreferredstrategybecausegravitydrainagehelpscontrolthemovementoftheinjectedgas.Excellentsweepanddisplacementefficienciesandhighoilrecoveriescanberealized.TheTensleeppoolintheElkBasinfieldinWyomingisagoodexample.[6][7][8]Thispoolwasprojectedtorecoverapproximately64%oftheOOIP.Seetheimmisciblegasinjectioninoilreservoirs(/Immiscible_gas_injection_in_oil_reservoirs)pageformoreinformationongravitydrainage.

Materialbalanceanalysis

Thepurposeofamaterialbalanceanalysisincludesconfirmingtheproducingmechanismandestimatingthefollowing:

Originaloilinplace(OOIP)Originalgasinplace(OGIP)Sizeofthegascap

Theapplicablematerialbalanceequationforinitiallysaturatedoilreservoirsis[9][10][11]


Thisequationisapplicabletoallinitiallysaturatedreservoirsregardlessofthedistributionoftheinitialfreegas.Forexample,thisequationisapplicabletoreservoirswhethertheinitialfreegasissegregatedintoagascaporuniformlydispersedthroughoutthereservoir.Eq.2alsoappliestowaterdriveshowever,ifthefollowingmethodsareappliedtowaterdrives(/Waterdrive_reservoirs),thewaterinfluxhistorymustbereliablyknown.Ifthewaterinflux(/Water_influx_models)historyisunknown,thenthemethodsinmaterialbalanceinwaterdrivereservoirs(/Material_balance_in_water_drive_reservoirs)mustbeapplied.

ThequantitiesGfgiandNfoiarerelatedtoN(OOIP)andG(OGIP)bythefollowingequations:


and (/File%3AVol5_page_0929_eq_002.png)....................(4)

wheremistheratioofthefreegasphaseandfreeoilphasevolumesandisdefinedby:


Thedimensionlessvariablemissometimescalledthedimensionlessgascapvolume.

BecauseGfgiandNfoiareindependent,theymustbedeterminedsimultaneously.Atleasttwosetsoftheindependentvariables(F,We,Egwf,Eowf)mustbeknownattwoormorepressures(otherthantheinitialpressure)todeterminetheset(Gfgi,Nfoi).Ifthreeormoresets(F,We,Egwf,Eowf)areknown,thenmultiplesets(Gfgi,Nfoi)canbedetermined.Theoptimalsetisdeterminedbyoneoftwoleastsquaressolutiontechniques:iterativeordirectmethods.

Intheiterativemethod,Eq.2isexpressedas


whereEtisthetotalexpansivityexpressedperunitvolumeofstocktankoilandisdefinedby


ThesolutionproceduretoestimatetheOOIPandOGIPinvolvesthefollowingsteps:

1. ComputeF,Egwf,andEowfforeachdatapoint(i.e.,averagereservoirpressuremeasurement).2. Guessm.3. ComputeEt(m)withEq.7.4. EstimateNfoiwithaleastsquaresanalysisusingEq.8.


wherejdenotesthedatapointindexandnisthetotalnumberofdatapoints.

5. ComputetheresidualRforeachdatapointwith


6. Computesumofthesquaresofresidual,Rss,as


7. ReturntoStep2andrepeatuntilRssisminimized.8. ComputeG,N,andGfgifromEqs.3through5.

Minimizationalgorithmsspeedsolution.Thisprocedureisideallysuitedforspreadsheetcalculation,especiallyspreadsheetprogramsthatcontainminimizationalgorithms.

TheuseofEq.8inStep4todetermineNfoiisequivalenttotheslopeofa(FWe)vs.Et(m)plot.ThisgraphicalsolutionmethodcanbesubstitutedforEq.8inStep4if

desired.Overall,Steps2through7areequivalenttothegraphicalprocedureofvaryingmuntilthestraightestpossible(FWe)vs.Et(m)plotisrealized.[12]Fig.7showsthequalitativeeffectofmontheshapeofthe(FWe)vs.Etplot.

Ifmistoosmall,theplotcurvesupwardslightlyIfmistoolarge,theplotcurvesdownwardslightly


Fig.7Theeffectofmonaplotof(FWe)vs.Et.

Oncemisdetermined,thefinal(FWe)vs.Etplotisusedtoconfirmtheproducingmechanism.Thelinearityoftheplotisameasureofmaterialbalance(/Material_balance_in_oil_reservoirs)andtheapplicabilityofthepresumedproducingmechanism.Iftheplotexhibitsconsiderablecurvature,theneither:

Thepresumedmechanismisincorrect

or

Additionalproducingmechanismsareactive

Ifcurvatureexists,theshapeofthecurvatureprovidesinsightintothetrueproducingmechanism.Forinstance,iftheplotcurvesupward,thisindicatesthatnetwithdrawalexceedsnetexpansionandthatwaterinflux,forexample,hasbeenignoredorispossiblyunderestimated.

Asanalternativetotheiterativemethod,Walsh[11][13]presentedadirectmethod.Thismethodisbasedonleastsquaresmultivariateregression.Theleastsquaresequationsaresimplebutlengthy.Thetechniqueisideallysuitedforspreadsheetcalculation.Walshsmethodisespeciallyattractivebecauseitavoidsiterationandthecomplicationsofattainingandjudgingconvergence.

HavlenaandOdeh[12]proposedanothersolutionmethodinwhich(FWe)/Eowfisplottedvs.(Egwf/Eowf)theslopeoftheplotisequaltoGfgiandtheyinterceptisequaltoNfoi.Thismethodispopularandattractivebecauseityieldsadirectsolution.Intheory,thismethodisperfectlyacceptable.Inpractice,however,ithasshowntobeunreliable

becauseitsuffersfromhypersensitivitytopressureuncertainty.[13][14]ThemethodhasbeenshowntoyieldhighlyerroneousGfgiandNfoiestimatesinthepresenceofonly

smallamountsofuncertainty.Forinstance,Walsh[13]showsthatonlya5psipressureuncertaintyyieldedanerrorofmorethan150%inNfoiandanerrorofmorethan250%inGfgi.Thehypersensitivityiscausedbythefactthatthedivisor(Eowf)approacheszeroasthepressureapproachestheinitialpressure.SmallerrorsinEowf,inturn,producelargeerrorsinthequotients:(FWe)/Eowf(Egwf/Eowf)

Tehrani[15]callsthisproblema"lossinresolvingpower."Becauseofthishypersensitivity,thismethodshouldbeusedcautiously.

Walsh[13]testedthedirectanditerativemethodsfortheirtolerancetouncertainty.Heobservedsensitivity,butthedegreeofsensitivitywaslessthanthemethodofplotting(FWe)/Eowfvs.(Egwf/Eowf).Heconcludedthatmaterialbalancemethodsforgascapreservoirsshouldbeusedcautiously.

Nomenclature

Bg = gasFVF,RB/scfF = totalfluidwithdrawal,L3,RB

G = totaloriginalgasinplace,L3,scfGfgi = initialfreegasinplace,L3,scfEgwf = compositegas/water/rockFVF,RB/scfEowf = compositeoil/water/rockFVF,RB/STBEt = totalexpansivity,RB/STBkv = verticalpermeability,L2,mdN = totaloriginaloilinplace,L3,STBNfoi = initialfreeoilinplace,L3,STB

Ng = dimensionlessgravitynumberq = producingrateatreservoirconditions(RB/D)orsurfaceconditions(STB/D),vL3/tre = reservoirdrainageradiusRj = residualforpointj,L3,RBRs = dissolvedGOR,scf/STBRss = sumofsquaresoftheresidual,L6,RB2

Rv = volatilizedoil/gasratio,STB/MMscfVfgi = initialvolumeoffreegas,L3,RBVfoi = initialvolumeoffreeoil,L3,RBWe = cumulativewaterinflux,L3,RB = densitydifference,m/L3,lbm/ft3andg/cm3

o = oilviscosity,m/Lt,cp

Subscripts

i = initialconditionj = index

References

1. 1.01.1Hall,H.N.1961.AnalysisofGravityDrainage.JPetTechnol13(9):927936.SPE1517G.http://dx.doi.org/10.2118/1517G(http://dx.doi.org/10.2118/1517G)2. Pirson,S.J.1958.OilReservoirEngineering.NewYorkCity:McGrawHillBookCo.Inc.3. Muskat,M.1949.PhysicalPrinciplesofOilProduction.NewYorkCity:McGrawHillBookCo.Inc.4. Katz,D.L.1942.PossibilitiesofSecondaryRecoveryfortheOklahomaCityWilcoxSand.Trans.,AIME146:28.5. Hill,H.B.andGuthrie,R.K.1943.EngineeringStudyoftheRodessaOilFieldinLouisiana,Texas,andArkansas.ReportInvestigation3715,USBureauofMines,

Washington,DC,87.6. Garthwaite,D.L.andKrebill,F.K.1962.Supplement1962:PressureMaintenancebyInertGasInjectionintheHighReliefElkBasinField.InFieldCaseHistories,Oil

Reservoirs,Vol.4.Richardson,Texas:ReprintSeries,SPE.7. Garthwaite,D.L.1975.Supplement,1975:PressureMaintenancebyInertGasInjectionintheHighReliefElkBasinField.FieldCaseHistories,OilandGas

Reservoirs,Vol.4a.Richardson,Texas:ReprintSeries,SPE.8. Stewart,F.M.,Garthwaite,D.L.,andKrebill,F.K.1955.PressureMaintenancebyInertGasInjectionintheHighReliefElkBasinField.Trans.,AIME204:49.9. Walsh,M.P.1995.AGeneralizedApproachtoReservoirMaterialBalanceCalculations.JCanPetTechnol34(1).PETSOC950107.http://dx.doi.org/10.2118/9501

07(http://dx.doi.org/10.2118/950107)10. Walsh,M.P.,Ansah,J.,andRaghavan,R.1994.TheNew,GeneralizedMaterialBalanceasanEquationofaStraightLine:Part2ApplicationstoSaturatedandNon

VolumetricReservoirs.PresentedatthePermianBasinOilandGasRecoveryConference,Midland,Texas,1618March1994.SPE27728MS.http://dx.doi.org/10.2118/27728MS(http://dx.doi.org/10.2118/27728MS)

11. 11.011.1Walsh,M.P.andLake,L.W.2003.AGeneralizedApproachtoPrimaryHydrocarbonRecovery.Amsterdam:Elsevier.12. 12.012.1Havlena,D.andOdeh,A.S.1963.TheMaterialBalanceasanEquationofaStraightLine.JPetTechnol15(8):896900.SPE559PA.

http://dx.doi.org/10.2118/559PA(http://dx.doi.org/10.2118/559PA)13. 13.013.113.213.3Walsh,M.P.1999.EffectofPressureUncertaintyonMaterialBalancePlots.PresentedattheSPEAnnualTechnicalConferenceandExhibition,

Houston,Texas,36October1999.SPE56691MS.http://dx.doi.org/10.2118/56691MS(http://dx.doi.org/10.2118/56691MS)14. Wang,B.andHwan,R.R.1997.InfluenceofReservoirDriveMechanismonUncertaintiesofMaterialBalanceCalculations.PresentedattheSPEAnnualTechnical

ConferenceandExhibition,SanAntonio,Texas,58October1997.SPE38918MS.http://dx.doi.org/10.2118/38918MS(http://dx.doi.org/10.2118/38918MS)15. Tehrani,D.H.1985.AnAnalysisofaVolumetricBalanceEquationforCalculationofOilinPlaceandWaterInflux.JPetTechnol37(9):16641670.SPE12894PA.

http://dx.doi.org/10.2118/12894PA(http://dx.doi.org/10.2118/12894PA)

NoteworthypapersinOnePetro

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Externallinks

UsethissectiontoprovidelinkstorelevantmaterialonwebsitesotherthanPetroWikiandOnePetro

Seealso

Primarydrivemechanisms(/Primary_drive_mechanisms)

Solutiongasdrivereservoirs(/Solution_gas_drive_reservoirs)

Waterdrivereservoirs(/Waterdrive_reservoirs)

Oilfluidcharacteristics(/Oil_fluid_characteristics)

PEH:OilReservoirPrimaryDriveMechanisms(/PEH%3AOil_Reservoir_Primary_Drive_Mechanisms)

(https://www.onepetro.org/search?q=Gascapdrivereservoirs) (http://scholar.google.ca/scholar?q=Gascapdrivereservoirs)

(http://www.worldcat.org/search?q=Gascapdrivereservoirs) (http://wiki.seg.org/index.php?

title=Special%3ASearch&redirs=1&fulltext=Search&ns0=1&ns4=1&ns500=1&redirs=1&title=Special%3ASearch&advanced=1&fulltext=Advanced+search&search=Gascap

drivereservoirs) (http://wiki.aapg.org/index.php?

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Gas Cap Drive Reservoirs - Petrowiki