A Method for Estimating Fluid Saturations From Well Logs

7/28/2019 A Method for Estimating Fluid Saturations From Well Logs

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.9SOCIETY OF PETROLEUM ENGINEERS PAPER ..- rnne5200 NorthCentralExpresswayDallas,Texas 75206

THIS IS A PREPRINT - SUBJECTTO CORRECTION

A Method for Estimatincj Fluid Saturations fromWdi Logsw

RobertA. Campbell,MemberSPE-HME, Lone Star ProducingCo.@Copyright 1975

American Institute of Mining, Metallurgical, and Petroleum E-ngineers, Inc.This paper was prepared for the Oklahoma City SPE Regional Meeting, to be held in Okla!lomaCity, Okla., March 24-25, 1975. Permission to copy is restricted t~ an abstract of not more then300 words. Illustrations may not be copied. The abstrac.should contain conspicuous a(>knowledg-ment of where and by whom the paper is presented. Publication elsewhere after publication in ttleJOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS .JOURNALis usuall; (!rantedupon request t~ the Edit~r of the appropriate journal provided agreement t~ Eive proper (reditismade. Discussi~n of this ,aperis invited. Three copies of any discussion should l)e:;entto tl)eSociety of Petroleum Engin(ers office. Such discussi~ns may be presented at t}e above meetingand, with the paper, may tJeconsidered for publication in one of the two SPE magazines.

..... . . ...--..-.--.ABSTRACT

Gas and oil saturationare amongthe keyvaluessoughtfrom a loggingsuite. The recentintroductionof the crossplotinterpretationmethodsand the sophisticationf loggingtoolscarIallowbetterestima%esfor theirvalues.Utilizationof standardlog data in theequationspresentedhereina pear to resultin?etterestimatesof oil snd or gas saturations.

INTRODUCTIONWellloggingis by far the mostwidelyusedmethodof evaluatingpotentialreservoirs.Recentimprovementsin loggingtoolshaveresultedin very accurateand reliabledata.1-3However,we are interpretingthis newdatawith the samemethodsdevelopedyears ago. Thisis not to say that the ~old~ethodsare withoutmerit,but ratherthe accuracyend sophistica-tion of the presentloggingtoolsofferbetterinterpretationmethods.The purposeof thispaperis to developaninterpretationmethodthat yieldsestimatesofhydrocarbon,saturationsithoututilizingcomplexcomputerprograms. Water saturation,ascalculatedby conventionalmethods,is verysensitiveto porosity,formationwaterreeistivity,and saturationexponents.4t5Havingcalculateda valueforwater saturation,we may then oniy say that hydrocarbonsoccupythe remainingpore space. But this-valuedoesnot indicatewhetherthe hydrocarbonmaterial

Referencesand illustrationsat end of paper.

,-+.-.,---.. .. . . .. ,. , .. ... . .. .. . . .- .-.,.. ....-is gas or oil nor the percentageof each.

Rodermundet al,6 were the firstho pre-senta methodfor estimatinggas saturations.However,the resultingvalue can onlybe con-sideredas the minimumgas saturationand isonlyvalidunderidealboreholeconditions.The recentcrossplotpresentationof thecompensateddensitylog porositywithporosityfrom eitherthe compensatedneutronlog or side-wall neutronlog has alloweda new approachtopracticallog interpretation.-10 }.mongotherthings,it has suppliedthe well site engineerwith a quicklookmethodfor evaluatingpotentialpay sectionsend indicateslithology,as well as the presenceof gas. However,theconceptsintroducedin thispaperprovideaninterpretationmethodthat can ?eadto bettervaluesfor hydrocarbonsaturationsin sand-stones.THEK)RY

The CompensatedDensityLog,being a wallcontacttool,is not effectedgreatlyby bore-hole conditionsexceptwhen extremewashoutispresent. Apparentporosityin a gas zoneisalwaysoptimisticsincethe calculationisbasedon a formationfluiddensity(Df)rangingfrom 1.0 tc 1.1 gin/cc.The densityof gas,which is compressible?variesLd is usuallybetween0.15 and 0.35 gm[cc. Thus,as gassaturationincreases,the more apparentdensity10gpOrOSitydiver@s fromtruewrosity (@T).


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44 ~ A METHODFOR ESTIMATINGFL1Conversely,neutronlog porositymeasure-ments are alwayspessimisticin gas zonessincethey tendto measureonly fluid-filledporespace-In a gas-waterreservoirthis log would

recorda porosityvaluelowerthantrue poros-ity. As gas saturationapproaches103 percent,the apparentneutronporositywill approachzero. Therefore,neutronlogs shouldbe run inconjunctionwith the densitytool. Otherwise,potentialgas producingreservoirseasilycouldbe overlooked.The CompensatedDensityLog is the primarytoolused in determiningoil and gas saturation,Porosity,as calculatedfrom the densitylog,is knownto be@D=Dgr-DB . . . . . . . , , , (1:

gr - fHistorically,the formationfluid density(Df)has been consideredto be either1.0or 1.1 gm/cc. Thesevaluesare correctonly when the for-mationbeers 100percentwater, When a forma-tion containsoil, gas, and water,the formatiorfluiddensityshouldbe consideredto be

Df=DwSw+DgSg+ DOSO. .,. .(2:2:One neverknowsthe respectivesaturationvalue:so we must eliminatethe formationfluiddensit:termfromour equations. To achievethiswemust first solveEq. 1 for Df. This yields.

f =(DB+$DDgr-Dgr)/$D. .~*(3Elimination of D is accomplishedby substitut-ing Eq. 2 into E&3 and obtairdrqg

Wsw + gsg + Oso =(DB+$DDgr -Dgr)/$JD . . . . . . ..(4Bulk density(~) has been directlymeas-ured by the densitylog, Nevertheless,appsrendensityporosityis not correctsinceit hasbeen estimatedfrom erroneou ass

Therefore,true porosity(UITmus~~%d fro,a crossplotporositychart,whichrequirestheuse of a neutronlog.11~12 Substitutionoftrueporosityfor apparentdensityporosityleadstows~ + & + Oso = (DB+ $TDgr- Dgr) / $T

. . ...** . . . . . . . q ***** (5Conventionallog interpretationtechniques

allowcalculationof water saturations(~).The densityof formationwater (~) is foundbyestimatingdensityat standardconditionsfromthe approximationof waterresistitity,dividingby formationvolume factorfor water,and convertingto gramsper cubiccentimeter;

SATURATIONSFROMWELL LOGS SPE $395 or by a correlationthat correctsforpressureand temperature.These valuescan be subtractedfromboth sidesof l?q,5.

g% + Oso = {(DB+ $TDgr - Dgr) / $T}-DWSW ..,...,.. (6)

At thispointvaluesexceptsaturationofall variablescanbe assigneddensityof gas and oil, end thegas and oil.

Gas and oil densitiesare functionsofspecificgravityand API gravity,respectively.Gas densityis alsoa functionof reservoirtemperatureand pressure, Good estimatesoftemperatureand pressureere availablefromdrillstemtests,nearbywells,or if absolutelynecessary,fromregionalgradientrelationships.Once the gravitiesare knownor estimated,thedensitiescenbe calculatedfromthe follotigequations.3

g = (0.04329GgPf) / (Z T~). . . . . . (7)and

Do = {:.~1.5/ (131.5+ API)Bo} .C . . (8)Gas densitywill usuallybe between0.15 and0.35 gin/ccand oil densitywill usuallybebetween0.5 and 0.8

Upon compilingfoundthatonlyoilas unhowns. If noavailable,we would

gin/ccall of the abovedata~it isand gas saturationremainotherrelationshipwerehave an infinitenumberof

solutions;However,it is knownthatSo=l-s-sw. .,. .o. o...(9)gSinceEq. 9 is independentof @. 6, we maysolvesimultaneously,liminateSo as a vartableand obtainour workingequation. Thus,gassaturationmay be calculatedas(Dg - Do) Sg = {(DB + OTDgr - Dgr) / $T}

- DWSW - Do (l-SW) . . ,(10)This generalequationhas been developedscthat gas saturationmay be estimated. Table 1summarizesthe basic algebraicformsof ~. 10.If only two phasesare presentin the formationsurroundingtinewellbore,the equationissimplifiedsomewhat. Table2 is a convenientsummaryfor thesetwo-phasesystems.

ERRORANALYSISThe theoryof the proposedsaturationequa-tionsis mathematicallyvalid. This is not tosey that applicationof theseequationswillalwaysresultin precisesolutions. The two


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t q SPE5395 ROBERTA. CAMPBELL 45-r . biggestdisadvantagesof this approachare (1) methods(Sw),percent 33.8the sensitivityof the equationsto trueporositysnd (2)the influenceof mud filtrate This zonewas perforated,acidized,end hydrau-invasion. I.icallyractured, Upon recoveringthe load

fluids,productionconsistedof smallquantitie:In usingllq.10, one mustbe awareof the of gas end 3 bbl of waterper hour,sensitivityof the calculatedresultsto theaccuracyof the numberssubstitutedtherein. Re-evaluationof thiszonewith theSubstitutionof numbersintoEq. 10 will show authorsproposedsaturationequationfor gas-that sensitivityof the resultsis witkdn water systemsrevealedthat gas andwateracceptablelimitsfor most practicaldecision saturationswere 32.4 percentand 67.5percent,purposes. It would appearfrom testsmadeto respectively.Thus, the reservoirperformancedate thatit is no more sensitiveto errorthen verifiedthatthe saturationequationwas moremost conventionallog interpretationequations nearlycorrectendno furthercompletionend correlations. attemptswere made in theUpperMorrow.In usingl?q.10, one must alsobe aware The resultsof thesecalculationsindicateof the processknown as mud filtrateinvasion. that the assumedformationwaterresistivitywa:Becauseof thisprocessone is never sureof too low for the Upper Morrowsand. Therefore,thepositionof the invadedzonein relationto water saturation,as calculatedby conventionalthe depthof investigationof the porosity methods,was indicatedto be lowerthen truetools. However,it is reasonableto assunethat water saturation.Applicationof the proposedthewatersaturationcalculatedfromthe pro- saturationequationcouldhave savedtheposedsaturationequationsshouldneverbe less completionexpenditurefor testingthe Upperthanthat calculatedby conventionalmethods Morrowsand- a savingsin excessof $50,000.(Fig.1). When this occursthe resultsof the

conventionaltechniqueshouldbe consideredto The valuesmeasuredby the abovementionedbe highlysuspect. logs (Fig.2) in a LowerMorrowsendintervalwere:CASEHISTORIES AssumedformationresistivityTwo case studiesare presentedwhereinthe (~), ohm-m .08methodoutlinedhas been appliedsuccessfully. ApparentdensitypOIOSfLtY(OD),These showthat the methodproposeddoes have percent 10.0merit. Apparentneutronporosity(N)?percent 1.6AnadarkoBasin,Okla. Trueporosity(@T),percent 6.3Water saturationby conventionalThe A. B. No. 1 was a MorrowSand test that methods(~), percent 40.3drilledto a totaldepthof 13~450ft before Water saturationbyprcposedbeing logged. The open-holelogs obtainedfrom equations(S,l),ercent L9,8thiswell were the CompensatedNeutron-CompensatedDensityendDua% Inductionlogs. The LowerMorrowsend in the A. B. No. 1 calcu-Conventionallog analysisindic?tedthat latedto have an averageporosi~yof 6.3 percencommercialquantitiesof gas mightbe present end water saturationof 40.3 percent. Thein severalof the Morrowsand sections, There-fore,pipewas cementedin placeand testing pr~posedequationsestimatedgas saturationtobe 50.1percentand watersaturationto be 49.8commenced. percent. Sinceboth methodsindicateda reason,

Conventionallog analysisfor the Upper ablyuniformgas saturation,it was decidedtotestthe LowerMorrow. Upon perforating?MorrowSend yieldedvaluesfor averageporosity acidizing,end h c?raulicracturing,the wellandwater saturationof 8.9 percentand33.8 1lowed1.3MMcf D while liftingsmallquantitiepercent,respectively.The valuesmeasuredby of water.the CompensatedNeutron- CompensatedDensityand Dual Inductionlogs for en UpperMorrowsand Denver- Julesbur~Basin,Colo.interval(Fig.2) were:One greatproblemwith interpretationofAssum~dformationwater resistivity

(%;),0- open-holelogs for the Muddy J sandsof this.08 ~ea is the l~ge v~iation ~ formationwaterApparentdensityporosity(@D), resistivity.As a result,the onlyprocedurepercent 11.1 thatinsuresthat productiveintervalswillnotApperentneutronporosity(ON), be bypassedis to assumethatthe format~.onpercent~e~rosity (@T) waterresistivityis the lowestvaluemeasured::; b the area.Water saturationbyconventional Of course,thisusuallyresultsi


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A METHODFOR =TIMATING FLUIDSATURATIONSFROM WELL I&W sPlr53$I

optimisticsaturationvaluesfor hydrocarbons. saturations,but they do oftenyieldbetterTo confirmproductiw:ty,most operatorswill estimates,conducta drillstemtest. Thisprocedureresultsin evaluationof a reservoir,but does NOMENCIATUR~so at significantadditionalexpenses B. = oL: formationvolumefactor,res bbl@fB

The D. J. No. 1 was drilledas a Muddy J ~ = water formationvolumefactor,res bbl/STEsandtestin Adams County,Colo. The log values ~ =bulk densityof reservotirock, gin/ccof a J sandinterval(Fig.3) for thiswell Df = averageformationfluiddensityat reser-were: mir conditions,gin/ccDg = densityof gas at reservoirconditions~Assumedformationwaterresistivity(~), ohm-m gin/cc0.12 D% = graindensity,gin/ccApparentdensityporosity(D)? = densityof oil at reservoirconditions~percent 15.6Apparentneutronporosity(ON),

gin/ccDw = densityof formationwater at reservoirpercent 10*2

hWe pOIOSity (@T), pWCE!nt 13.4 conditions,gin/ccWatersaturationb~ conventional % = specificgravityof formationgas. formationpressure,psiamethods(Sw),percent 39*$ ~ = gas saturation,fractionsoWhen formationwaterresistivitywas assumedto = oil saturation?fraction~ water saturation,fractionbe 0.12ohm-m,watersaturationcalculatedto Tf = formationtemperature,ORbe less than 40 percentby conventionalmethods.Applicationof the proposedequationsresulted ~: . gas compressibilityactorin watersaturationvaluesthat approached10Q $N = porositymeasuredby densitylog,fractiol= porositymeasuredby neutronlog, fractiolpercentand averaged$7.5percent. At the time *T . true porosity, fractionthiswell was loggedthe proposedequationswere~tried and untested. Therefore,the credit- ACKNOWUDGMENTSabilityof the resultswere somewhatquestion-able. Shortlyafterlogging,thiswell wasdrillstemtested. No flowto the surfacewas I expressmy gratitudeto JohnM. Campbellfor his guidanceand criticismwhilepursuingobtainedanddrillpiperecoveryconsistedof 30ft of heavilymud cut water and 3$5 ft of salt this study. Appreciationis also extendedtoHavingobtainedthe testdata and con- the U. of OklahomaPetroleumEngineeringstaffwater. end,in particular,A. W, McCray. I especiallysideringthe resultsof the proposedequation thankhne Sk ProducingCo. for supplyingtheinterpretation,hiswell was immediately datafor this study.pluggedand abandonedas a dry hole. If theoperatorhad possessedconfidencein the satura R~~~T~tion equations,a drillstemtestwouldhavebeen unnecessaryand a significantsumof moneywould 1.have been saved, Alger,R. P., Locke,S.,Nagel,W. A. andSherman,H,: I?The~al SpacingNeutronCONCLUSIONS Iog,J. Pet. Tech. (Sept.1972)~, 107:..2. Tittman,J., Sherman,H., Nagel,W. A. endAlger,R. P.: TheSidewalll@ithermalThe proposedsaturationequationsare NeutronPorosityLog,?J. Pet. Tech. (Oct.reliablefor most cleanend limeysandswhere 1966)XVTII,1351.porosityis greaterthan6 perceflt.Further- 3. Wahl,~, Tittman,J., Johnstone,C. W.more,theseequationsbecomeless sensitiveas and Alger,R. P,: ItTheDual SpacingFormaporosityincreases. Uufortunatelytthe validity tionDensitybg, !!J. pet. Tech, (Dec.is stillquestionablein limestoneand dolomitebecauseof the difficultyinobtsining good 1964)XVI, 1411.4. Archie~. E.: TheElectricalResistivitporosityvaluesdue to the vugulark.aturendthe largevariationti graindensity. Log as an Aid in DeterminingSomeResermLCheracteristicst. Pet.Tech. (Jsn.1942IT,No. 1.Evenwith the weshesses of thisnew con- 5. Von Gonten,W. D. andoroba, J. S.: Acept,actualapplicationsof the proposedequa- Methodof PredictingSaturationExponentstionsby the authorhave resultedin very In Logging,paperpresentedat SP&AIMEsignificant~*esults.Theseresultsindicate AnnualFallMeeting,1969.that conclusionsdrawn from conventionalmethods 6. Rodermund,C. G., Alger,R. P. snd Tittmanare ofteninferiorto thosedrawnfrom these J.: LoggingFmptyHoles? Oil and Gas J,equationswhen viewedon a relativebasis. This (June12, 1961).is not to say thatthe valuesc~lculatedfrom 7. Schlumberger:LogInterpretationCharts?the saturationequationsdepic.true reservoir IW2 Ea.


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. SPE5395 ROBERTA. CAMPBELL 4

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8. Jennings,HarleyY. and Timur,A.:SignificantContributionsin FormationEvaluationandWell Testing,J. pet.Tech.(Dec.1%3).~, 1432.9. Poupon,A.t Hoyle,W. R. and Schmidt,A.w Uog AnalysisIn FormationsWith:Complexfithologies,J. Pet.Tech. (Aug.1971)XXIII,995.10. Savre,~.: Determinationof A MoreAccuratePorosityand MineralCom~sitionin ComplexLithologieswith the Use of theSonic,Neutronend DensitySurveys,L*~et.Tech. (Sept.1963)~, 945.

11. Savre,W. C. end BurkelJ. A.: Determha.tionof TYue Porosityend MineralComposi-tionin ComplexLithologiesWith theUseof the Sonic,Neutron,endDensitySurveys,ReprintSeries.No. 1~ Societyof Petrolwm R@neers of AIME,Dallas12 (1971)06.

q Schlumberger: LogInterpretation/Principles,1972Ed.13. Frick,T.C.: PetroleumProductionHand-book,VolumeII, ReservoirEr@neering,McGraw-HillBook Co., Inc.~New York(1962).

TABLE 1 - SATURATIONEQUATIONSFOR THREE PHASE SYSTEMS

Solvin~or Gas Saturation When Wa~er Saturation Is Known

(Dg-Do)sg= {(D~+$TDgr- Dgr) /$T} Dwsw - Do(l - Sw)Solving For Oil Saturation When Water Saturation Is Known

TABLE 2 - SATURATIONEQUATILN FOR TWO PHASE SYSTEMStDo-Dg)so= {(DB + ~TDgr - Dgr) /41T}- Dwsw - Dg(l - Sw)

Solving For Water Saturation When Gas Saturation Is Known Solving For Water Saturation In A Gas-Water System

(Dw - Dg)Sw= {(DB + $TDgr- Dgr) / $T) - Dg(DW-DO)SW = {(DB+ $TDgr - Dgr) /$JT}- DgSg - Do(l - Sg)

Solving For Oil Saturation When Gas Saturation Is Known Solving For Gas Saturation In A Gas-Water System

(DO-DW)SO - {(DB+ $TDgr- Dgr) /OT} - Dgsg - Dw(l - Sg) (Dg - Dw)Sg = {(DB + $TDgr - Dgr) / $T}- Dw

Solving For Water Saturation In A 011-!ater System

(Dw - L)O)SW= {(DB + $TDgr - Egr) / I$T}- DO

Solving For Oil Saturation In A Oil-Water System

(l).- DW)SO = {(DB+ $TDgr - Dgr) J $T?- Dw


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100

Flushedk Zone B

-ion =-=DSaturationo Distance From Wellbore

Fig. 1 - Schematic diagram of filtrate in/asiono

Fig.2 - Open holelogs,A. B. No. 1, AnardarkaBasin, Oklahoma.

Fig. 3 - Open hole logs, D. J. No. 1, AdemsCotitY,Coloraclo.

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A Method for Estimating Fluid Saturations From Well Logs