SystematicRock Typingin an IranianOilReservoir

MohammadRezaRasaei1, Shobeir Nabavi2

1:AsistantProfessor,IPE,TehranUniversity

2:M.Sc.,IPE,TehranUniversity

Abstract

Thedegreeof success inmanyproduction activities and secondary recovery processesdependsontheaccuracyofthemodelsusedinthereservoirdescription.Reservoirrocktypingisoneofthemostessentialpartsofproperreservoircharacterization.Thegoalinoptimumrocktypingistoovercometheextremeheterogeneityofthereservoir.Thisistodecreasetheeffectsofheterogeneityfornormalizing/averagingcapillarypressurecurvesand estimation of permeability. Rock typing and hydraulic flow unit identification areelaborated to integrate both geological and petroleum engineering data. Flow unit isdefined as a group of reservoir rocks with similar properties that affects fluid flow.Geological/petrophysical characterization incorporated the analysis of the complexvariations in pore and pore throat geometry that control initial and residual fluiddistribution.An undersaturated oil field reservoir in southwest of the Zagros belt in Iran wasconsidered in this study.Asmari formation in this reservoir compromised of twomainCarbonate and Sandstone bodies. Four lithotypes of Shale, Limestone, Dolomite, andSandstonehavebeendeterminedfromgeologicalandpetrophysicalstudies.In this study, conventional porosity and permeability, mercury injection, capillarypressure, relative permeability and mineralogical data were used to characterize thereservoirporesystems into rocktypes having similar flow andstoragecapacity.WaterSaturation,allofwhichisconsideredimmobile,wasfoundtobedependentonrocktype,withporethroatbeingthedominantcontrolontheflowcharacteristicsofthereservoirs.Also,adifferentflowunitdefinitionofFZI/RQIconceptwasappliedonthisfield.Goodconsistencywasobservedbetweenlithotypesandrocktypestocompromisetheviewsofbothgeologistsandreservoirengineers.

Keywords: Rock type Litho typeHydraulic FlowUnit (HFU) Pore SizeDistribution(PSD)LeverettJFunctionmercuryinjectionRQI/FZI

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Introduction

Thefieldof interest in this studyis situatedinsouthwestof theZagrosbeltandextendsfromnorthwesttosoutheast.Thisoilbearingreservoiris180kmnorthwestoftheAhwazfield,attheedgeofDezfulembayment.Thefieldis28.5kmlongand4.5kmwideatthetopof theAsmari formation.The firstwellonthis structurewasdrilled in1967.Of thetotal 9 wells drilled so far, 2 of them are relatively new and were drilled in 2004.Geological and sedimentological studies showed thatAsmari formation in this reservoircompromisedoftwomainCarbonateandsandstonebodies.Although 30 years from discovery of this field has elapsed, unfortunately no indepthstudyofthisfieldhasbeencarriedout.Asaresult,evenapreliminarydynamicmodelofthisfielddoesnotexist.In the course of developing a proper dynamicmodel, building valid and accurate staticmodel, to the possible extend, is of prime importance. In this phase, propercharacterizationandrocktypingofdifferentgeologicalfeaturescannotbeoverwhelming.ForthedeterminationofthereservoirrocktypeandHFUs,differentfactorsorparameterscan be used. For example, reservoir rocks that exhibit similar or even close propertiessuchaswatersaturations,porositiesorpermeabilitiescanbeusedinthegroupingof therocktypes.Ingeneral,determinationof thereservoirrockscanbecategorizedaccordingto their lithology, petrophysical properties, and/or their geological features. Yet inanother way, they can be classified according to their dynamic physical characteristicsrelatedtopermeability.Thefirstapproachisusuallyelaboratedbygeologists.Thesecondmethod, preferredby reservoir engineers, takes into considerations such factors as theirability to transmit fluids (permeability). Reservoir engineers prefer to use the lattermethodinthecoarsedynamicmodelbuilding.Sincethedynamicbehaviorofrocksistheoutcomeofminutebehaviorof fluids in relation topores, eithermethodsor approachesshouldyieldcloseoutcomes.Inthefollowingsections, thestrategyappliedtouseallavailableinformationtoget toasoundrocktypingispresented.Effortshavebeenmadetousealldataatdifferentscalesandnaturesuchasthinsections, logdata,geologicalandsedimentologicalstudies,coresandroutineandSCALtests.

RockTypingBasedonGeologicalConsiderations

As said before, Asmari formation in this reservoir has 7members that compromised oftwomainCarbonate and Sandstonebodies.4 firstmembers ofAs1a,As2a,As1b andAs2barecarbonatesand3 secondmembersofAs3,As4andAs5are sandstones. Indetermining reservoir rock types, based on extensive petrophysical, sedimentology andsequence stratigraphy, four types (lithotypes) have been determined and taken intoaccount: Shale, Limestone, Dolomite, and Sandstone. Anhydrites are another elementexist in the area which is the main constitute of cap rock. This rock has no reservoir

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propertiesanddetectedonlyas small traces inpartsofAs_1aadjacent tocap rock. So,thereisnogainofconsideringanhydritesasindividualrocktypeinreservoirrocktyping.Mean porosity values of these lithologies are 7%, 10% and 24% for Limestone,Dolomite, and Sandstone rocks respectively. This put the limestone as the worst andsandstoneas thebest reservoir rocks in this reservoir in terms of their averageporosityvalues.IrreduciblewatersaturationandporosityRelationRocktypeanalysisbasedontherelationof Swirrandporosityinthetestedcoresisanotherroutinewhichcanbeapplied.Figure1showsdistributionofSwvs.porosityinthreemainrock typesmentioned above. It is clear that sandstones haveminimumwater saturationwhile limestones have the maximum value. This again confirms the rock typing donebasedonlithologyofthecores.

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25 30

Phi(%)

Swirr(%)

Dolomite

Lim

Sand

Figure1Swirrvs.porosityfromSCALtestsfordifferentrocktypes

MercuryInjectionDataforRockTypingIn addition to the abovemethods in the classification of reservoir rock types, mercuryinjectionmethod can also beused. Mercury injectionmethod shows the distribution ofthroat sizes within the porous rock sample. Although the distribution of throat sizespossesses systematic and proven discrepancies, it can form a relative basis upon whichdifferent samples can be compared. In this reservoir, mercury injection test have beencarriedouton58plugstakenfromthecoresofonewellandthedistributionofporosityhavebeenobtained.Basedonthedistributionofbottlenecks,reportedbythelab,averagequantities of throat sizes (BRC) and their apparent radius, after injection 35%mercury(R35%)havebeencalculated.In order to articulate the relation between the permeability of samples and their throatsizeineachcore,theaveragethicknessofthethroatsizesaccordingtothepermeabilityisdrawnon the logarithmic chart.The somewhat large scatter ofpoints indicated lack ofcorrelation between permeability and thickness of throat sizes does not support

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theoretical considerations. Additional considerations in this regard, showed that themajorityofmercury injectiontestsdonothavethe requiredvalidityand theyshouldnotbetakenintoaccount.Theruleof thumbpresentedbyBrunoGrainerwasappliedwhichpostulates that theaveragethicknessof throatsizeof thesampleshouldbeequalor lessthantheapparentthicknessofthroatsizeaftertheinjectionof35%mercury.After the elimination of doubtful lab results, mentioned above, only 24 cores wereselected to examine their throat sizes. Change in throat size vs. permeability is plottedagainandthetrendofchangedemonstratesanacceptablerelationshipbetweenthetwoasshowninfigure2.Tofurtherdeterminethethroatsizeforeachlithotype,inpetrophysicalstudies,thetypeis shownnext to theappropriate throat sizeand thensortedon thatbasis.The resultofthisisshownintable1.Coreswithlessthan12micrometerthroatsizearelimestoneandthose with 1218 micrometer are dolomites. Sandstones show values more than 18micrometer. Thus, there is a good relationship between rock types and throat sizes (atleastinthesamplesof onewell).

Figure2 Averaged porethroatsize vs.permeabilityforall50plugs(Left)andfor24validplugs(Right)

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Table1 Averagethroatsizeforvariousrocktypes

Lithology Sample Depth(m) Porosity(%) Perm.(mD) BRC(m) R35(m)Lim S12A 3444.9 17.5 26 7.61 8.95Lim S22A 3471.84 12.8 4.02 8.15 8.34Lim S25A 3474.59 5.6 7.9 11.04 13Lim S11A 3442.39 9.4 17 11.9 12Dol S16A 3458.81 13.1 18 12.3 13.5Dol S33A 3485.66 14.7 74 12.33 14.6Dol S26A 3475.1 14 8.1 12.57 13.4Dol S27A 3476.03 21.3 157 14.25 15Dol S31A 3483.5 15.5 320 15.5 21.6Sand S53A 3546.01 23.7 241 18.36 21.7Sand S45A 3521.9 14.7 35 18.38 20Sand S55A 3548.17 25.8 1383 21.53 26.3Sand S60A 3554.28 25.3 1359 22.29 27.25Sand S46A 3523.2 17.8 2005 26.99 30Sand S47A 3524.23 25.1 2322 28.43 32.5Sand S39A 3513.78 26.4 3941 31.04 36.5Sand S40A 3514.64 26.1 2465 32.11 36Sand S41A 3515.32 28.2 6735 36.07 40.5Sand S36A 3510.52 29 1679 39.71 47.5Sand S37A 3511.79 27.2 3798 43.52 55Sand S35A 3509.72 23.6 7396 45.23 56Sand S42A 3516.68 27.5 7531 53.32 60Sand S43A 3517.56 27.1 7868 58.36 65.3Sand S76B 3621.7 24.9 6592 65.88 83

ConventionalCoreAnalysisTests(CCAL)

Coresofthereservoirweretakeninthreewells.Fromthesethreewellsaltogethersome667plugshavebeenrecovered,tested,andtheirporosityandambientpermeabilityweredetermined.One of the most important usages of results obtained from cores is establishingcorrelations between porosity and permeability in the rock types of reservoir for theconstruction of the geological model. If a reasonable correlation established betweenthesetwoparameters(porosityandpermeability),thiscorrelationandtheresultsobtainedfromwell logscanformasolidbasisforthedistributionofthesetwoparametersinthebuildingof thegeologicmodel.Whenthereareenoughdata foreachwellandtheyarewellseparatedasdifferentcategories,onecandotheanalysis inwellbywellbasesandinvestigatearealvariationofproperties.Hereweexaminetoseehowthedataofdifferentwellsdistributedandtoseeif wecandosuchanalysis.Figure4comparesdataof3wellswhich have conventional core tests. As this figure shows, there is no such differencebetween data of different wells and one get no advantage of considering each wellseparately. Frompermeability versusporositycorrelations indifferentwells, separation

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ofcoreplugsamplesbasedonlithologystilldoesnotassistinbettercategorizingofdataandnomajorchangebetweenthewellsisobservable.

Figure4 Comparisonofcoredatafromdifferentwells(upleft),porosityandpermeabilityRelationshipinSandstoneplugsindifferentwells(upright),in Dolomiteplugs(downleft)&in Limestone (down

right)

Establishment of relationships based on petrophysical properties of variousreservoirrocksThe impact of rock types and their respective lithologies for the establishment of therelationshipbetweenporosityandpermeability shouldbetakenintoaccount.To begin with, cores based on the rock type and lithology taken from each well wasseparated. As discussed before, based on petrophysical studies four rock types(lithotypes) were identified in this reservoir. These rock types are Shale, Limestone,Dolomite, and Sandstone. Shale is not considered a suitable reservoir rock and waseliminated. The remainder was divided into limestone, dolomite, and sandstone. Therelationshipbetweentheirporositiesandpermeabilitieswasthenestablished.The distribution of permeability values vs. porosity shows a wide dispersion. Toeliminate thiswidevariance, theaveragingmethod,asexplainedbelow,wasemployedandwillbeelaboratedupon.

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Sincetheporosityvaluescanbeadded,calculationoftheiraveragevaluescaneasilybeobtained. However, permeability data cannot be summed and their average obtained.There does not exist an agreed upon method for the determination of the averagepermeability.However,theproventheoryforthispurposeisasfollows:Thearithmeticmeanforparallellayerscanbecalculatedandisdominatedbythehigherpermeability.Thisaveragingwill,naturally,showthemeanoftheuppermeanvalues.Harmonic averaging for flows perpendicular to the layers is controlled by the lesserpermeabilityandthustheaverageshowslowervalues.Thegeometricaveragingappliesforcrossflowfromregionswhichhavemixeddisorderedvaluesofpermeability.Basedontheabove,theaveragepermeabilityofvariousrangesofporosityiscalculated.Inthelimestonerocks,138coreswereanalyzedandtheresultisshowninfigure5.Thisfiguredepictsthegeneralcorrelationbetweenpermeabilityandporosityinthisrocktypeand displays the relationship based on arithmetic, geometric, and harmonic averaging.The averages show the rangeof values of permeability and enable us to consider bothpessimisticandoptimisticcases.Therelationshipbasedonthegeometricmeancanformthebasecasefortheconstructionof permeability distribution in the dynamicmodel.Once themodel is built and in thephaseof historymatchingonecanmove ineitherdirection (arithmeticorharmonic) toseewhichofthecasesresultsinabettermatch.

Figure 5Generalrelationshipofpermeabilityandporosityforlimestone(left),averagepermeabilitybasedonaverageporosityforlimestone(right)

Inthedolomiterocks208coreswereanalyzed(ordinaryanalysis).Theresultsof theseanalysesareshowninfigure6.ThisFigureshowstherelationshipbetweenpermeabilityandporosity in this reservoir rocktypeand thearithmetic andgeometricmeansof thisrelationship. The relationship based on the geometric mean shows larger R2 than theothersandsocanformthebasecasefortheconstructionofpermeabilitydistribution.

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Figure6GeneralrelationshipofpermeabilityandporosityforDolomite(left),averagepermeabilitybasedonaverageporosityforDolomite(right)

From thesandstone reservoir rocksaltogether110coreswereanalyzed.The resultsareshown in figure7.This figureshowsthegeneral relationshipbetweenpermeabilityandporosity based on arithmetic, geometric, and harmonic averaging. Another time, therelationshipbasedonthegeometricmeanisthebest.

Figure7GeneralrelationshipofpermeabilityandporosityforSandstone(left),averagepermeabilitybasedonaverageporosityforSandstone(right)

6SpecialCoreAnalysis(SCAL)DeterminationofrocktypesbasedonfluidbehavioranddynamicsrequiresSCALonthecore samples of the field. SCALanalysis includes capillary pressure curves and relativepermeabilityofoilandwater.Inonewell,SCALtestshavebeencarriedoutandrelativepermeabilityandcapillarypressurecurvesobtainedfor somecores.Analysis includes40samples of capillary pressure and 11 relative permeability measurements. Capillarypressure curves using centrifugal method for both drainage and imbibition cycles wereobtained.Thecapillarypressurecurves showawide rangeofvariations indicatingdifferentporegeometries and reservoir properties. Since the capillary pressures curves are the directresult of structure and their geometric sizes and throats, for their proper classification,one has to pay special attention to lithology. For this reason, cores were classifiedaccording totheir rock typeandpetrophysicalcharacteristics into: limestone,dolomite,andsandstone.Toconductapropercomparisonofcapillarypressuresofvarioussamples

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withdifferentporositiesandpermeabilities,onemustdrawthesecurvesindimensionlessform andnormalizethewatersaturationasbelow.ThisobjectiveisachievedbyusingthesquarerootofpermeabilitydividedbyporosityofeachsampleandconstructionofthesocalledJfunction,asdisplayedbelow:

( ) /( )

cosc w

w

P S kJ S

j s q

= , 1w w irr

w n o rmw ir r o r

S SS

S S -

= - -

Figure8 shows lithology basedcategorizationofdimensionlessdrainage Jfunction forallavailablecoreplugsamples.Asthisfigureshows,lithologycannicelycategorizethecapillarypressurecurves.

Figure8Lithologybasedcategorizingofavailabledimensionlesscapillarypressurecurves

Forabettercomparison,classificationofcurves basedon their lithologywasdoneandeach groupwas averaged so as to construct a representative Jfunction curve for eachlithological group.As can be seen in figure 9, the Jfunction for limestone shows thesmallestandsandstonethelargestvaluesindicatingtheworstandthebestreservoirrockcharacteristics respectively. Dolomites show a relatively better reservoir rock qualitycomparedtolimestone.Asimilarapproachwascarriedoutonthecapillarypressureofcores.AverageCapillaryPressureCurves indifferentreservoirrocksamplesareshowninthisfigure.Basedonthese, limestonethathas thelargest irreduciblewatersaturation57%exhibitstheworstreservoirrocktype.Sandstones,withirreduciblewatersaturationaround 18%, are the best reservoir rock type. Dolomites with irreducible watersaturations around 36% are consideredmedium quality reservoir rocks. The reliabilityand the relevance of each group of curves will be examined in the dynamic modelbuilding. As explained, the measurements of relative permeability were carried on 11cores.Ofthese, in7casescapillarypressuretestsoncoreswerecarriedoutevenlythusenablingavalidcomparison.Capillarypressurecurvesincomparisonwithpermeabilitycurvesshouldshowsmallerendpointsaturationsandsovalidityofcurveswerecheckedwiththisruleandallthemwerecorrect.

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Figure 9AveragedJfunction (left)andAveragedPccurves(Right) forthreereservoirrocktypes

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100Sw(percent)

Kr(fraction)

Krdolomite

Krlimestone

Krsandstone

Figure10 Averagedrelativepermeabilityforallreservoirrocks

Similartoclassificationofcapillarypressurecurveslithologicalconsiderationswerealsotaken into account for the construction of relative permeability curves. Attemptsweremade to construct the averaged curves for each rock type (limestone, dolomite, andsandstone). The results show that 1 core falls into limestone rocks, 4 cores in thedolomites, and 6 cores in the sandstones. The combination of all three relativepermeabilitycurvesforlimestone,dolomite,andsandstonesisshowninfigure10.These curves are normalized and the normal saturation for water phase (Sw,norm) andrelative permeability of normal phase (Krp,norm) are calculated with the followingequations:

,,max

rprp norm

rp

KK

K =&, 1

w wirrw norm

wirr or

S SS

S S -

= - -

Unfortunately inthe limestonegroup,onlyonecoreexistsandtherelativepermeabilitycurveforthisrocktypecannotbeconsideredrepresentativeofthepopulation(limestone

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rocks of the reservoir). Nevertheless, it was temporarily used until such time that thedynamic model is constructed and in the history matching process the requiredadjustment could be applied. Based on the end points of saturation and relativepermeability values of maximum phases, limestone, dolomite and sandstone show themaximumtominimumirreduciblewatersaturationandmaximumtominimumeffectivepermeability of the water phase (the ability to flow water). Thus, the quality of thereservoirrockcanbeclassifiedaspoor,intermediate,andgood.Acriticallookatfigure10revealsthatthenormalizedrelativepermeabilitycurvesforthethree rock types (limestone, dolomite, and sandstone), especially in the oil phase, arecomparable and thus can be exhibited as onegeneral curve.However, to facilitate thehistory matching process,we recommend the use of separate relative permeability foreachrocktype.

RockTypingBasedonRQIFZIMethodAmaefuleetal.introducedtheconceptofReservoirQualityIndex(RQI)consideringtheporethroat, pore and grain distribution, and other macroscopic parameters and theconceptof flowzoneindicator asdefinedfrom thebasicequation ofbelow:

gvse

e

e SF

k t f

f f

1

10314.0

-

= ,e

kRQI

f0314.0 = ,

gvs SFFZI

t1

= ,

-

=e

en f

f f

1

ThusthisEquationcanbewrittenas:LogRQI= log n f +logFZIThisequationyieldsastraightlineonaloglogplotofRQIversusnwithaunitslope.Theinterceptofthisstraightlineatn=1istheflowzoneindicator(FZI).SampleswithdifferentFZIvalueswill lieonotherparallellines.Samplesthatlieonthesamestraightlinehavesimilarporethroatcharacteristicsand,therefore,constituteaflowunit.Thismethodwasappliedfor590plugsofallthewellsthatroutineanalysesonthemwerecarried out. These samples had the lithology of Sandstone, Limestone and Dolomite.Index of RockQuality andNormalizedporosityare plottedon loglog scale as can beseeninfigure11.

0.001

0.010

0.100

1.000

10.000

0.0010 0.0100 0.1000 1.0000

RQI

Phin

Dolomite

Limestone

Sandstone

Figure11 RQIFZIfor3lithologygroupsofreservoirrocks

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Ascanbeseeninfigure11,onecannotseparatethe3groupsofdifferentrocktypeinthegraph and all data shrink in one small area. So this method isnt proper for HFUclassificationofthereservoirzones.

ConclusionAnintegratedapproachispresentedforproperrocktypingtohonorbothgeologicalandflowbehaviorcharacteristicsof thecores.It isrecommendedtostartrocktypingbasedongeologicalandpetrophysicalpropertiesofthecoresandthendothenexttuningbasedon mercury injection, routine and SCAL testes. This can results to a proper initialgroupingofrocktypesandthenobtaintherepresentativedynamicfunctionsforeachoftheconfirmedgroups.Throatsizesofmercury injectionandJfunctionsaregoodcheckpointsforthisconfirmation. AlthoughthemethodofRQI/FZIisoneofthemethodsforFlow unit classification but it is not applicable for this reservoir and no consistencybetweenthismethodandtheothersseen.

AcknowledgementThisresearchwaskindlysupportedbyICOFC.WewouldliketogivespecialthankstoMr.ZiyaeiandMr.Jahankhahfortheirsupportandcooperation.

10References:1BrunoGranier,Anewapproachinrocktyping,documentedbyacasestudyoflayercakereservoirsinfieldA,offshoreAbuDhabi(U.A.E),NotebooksonGeology,Article2003/04(CG2003_A04_BG).

2(G.V. Chilingarian, S.J. Mazzullo, H.H. Rieke, Carbonate reservoir characterization: a geologicengineeringanalysis,partI,Elsevier,1992).

3Renard, Ph., Calculating of equivalent permeability: a review, Advances in water resources, Vol 20,1997.

4WenXH.,andGomezHernandezJ.J.,Upscalinghydraulicconductivities inheterogeneousmedia:anoverview,Journalofhydrology183(1996).

5VaravurS.,SheblH.,SalmanS.M.,ReservoirRockTypinginaGiantCarbonate,SPE93477

6Pittman, E.D. 1992, Relationship of Porosity and Permeability to Various Parameters Derived fromMercury InjectionCapillary Pressure Curves for Sandstone. Bull.American Association of PetroleumGeologists,76,191198.

7StephanieLafage,AnAlternativeToTheWinlandR35MethodFor DeterminingCarbonateReservoirQuality,August2008

8Amaefule, J.O, Altunbay, M., Tiab, D, Kersey, D.G., and Keelan, D.K.: "Enhanced ReservoirDescription: Using core and log data to identify Hydraulic (Flow) Units and predict permeability inuncoredintervals/wells",SPE26436,presentedat68thAnn.Tech.Conf. AndExhibit.,Houston,Tx,1993.

9Pestov,V.V.,DoroginitskayaL.M.: "Applicationand integrationofcore, logandwell testingdata forreservoir description", JSC "TomskNIPIneft", final project report # 243.5384 prepared for Yukos EP,Tomsk,2002.

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