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CHAPTERONEINTRODUCTIONTORESERVOIRENGINEERING

Theprimaryobjectiveofthereservoirengineering istopredictfutureperformanceofhydrocarbon(oiland/orgas)reservoirsandfindwaysandmeansofincreasingultimaterecovery.Reservoirengineermustpreparesoundplansfordevelopmentandoperationoftheoilandgasreservoirs.Properapplicationofreservoir engineering not only enable wise invest of the capital, but will also allow for safe,environmentallysound,andefficientoperationofthereservoirleadingtoimprovedrecovery.HYDROCARBONRESERVOIRSA reservoir is a subsurface rock structure where naturally occurring hydrocarbons (petroleum) areaccumulated. For this accumulation of petroleum to be a commercial hydrocarbon reservoir certainconditionsmust bemet. Figure 1 illustrates the key features of a commercial hydrocarbon reservoir.Thesefeaturesincludesourcerock,trap,depth,andreservoirrock.Theyarediscussedbelow.

Figure11:KeyFeaturesofaCommercialReservoir

SourceRockSourcerockisafinegrained,organicrichsedimentaryrockinwhichpetroleumhasbeenoriginated. Thesource rock is formed by deposition of the remains of the plants and animalswith the finegrainedsedimentsinshallowmarineenvironments.Depletionofoxygentakesplaceinquietwaterleadingtoananaerobic condition and preservation of organic matter. These sediments must contain sufficientquantities of the organic matter to form a source bed. As the sediments are buried deeper, thetemperatureandpressurewill increaseand theorganicmaterial isgraduallytransformed intokerogenwhicheventually istransformed intohydrocarbonsasmoreandmoresedimentsaredeposited.At thesametime,thefinegrainedsedimentsarecompactedwithrisingpressureandthewaterisexpelledfromthemtoformclayandshalebedsreferredtoassourcerock.

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The compaction and deeper burial eventually results in the expulsion of the hydrocarbons from thesource rock. Subsequently, the migration of petroleum will take place by a complex interplay ofbuoyancy,capillary forces,andhydrodynamics.Numeroustheorieshavebeenadvanced toexplain thisprocess.Petroleummigratesthousandsoffeetverticallyortensofmileslaterallyfromthesourcebedstotheoverlyingporousandpermeablebeds.Considerabledifferencesofopinionexistas to thedistancewhichpetroleummaycoverinitsmigrationprocess.Ifinitsmigrationpath,thepetroleumencountersasubsurfacerockconditionwhichhaltsfurthermigrationthenaccumulationwilltakeplace.TrapA trap is a subsurface anticlinal shape structurewhich is overlain by a caprock that can prevent theupwardor lateralmovementof thehydrocarbon.Caprock isa finegrainedwater saturated rockwithhighcapillaryforces.Thedensitiesofthehydrocarbonsarelowerthanthatofthewater.Therefore,thepetroleum thatmigrated from the source rock is driven upward by the buoyancy forces through thewatersaturatedrocks. If thebuoyancy forcescannotovercome thecapillary forces in thecaprock, thepetroleumwillaccumulate in the structurebelow thecaprock.The structuremusthave sufficient sizeand closure. The closure is the distance between the crest and the spill point (lowest point of thestructurethatcancontainhydrocarbons).Inmostcases,thestructureisnotfilledwithhydrocarbonstothe spill point. A structurewith a low closuremost likelywill not contain commercial quantities ofhydrocarbons.

Petroleum geologists broadly classify traps into three categories that are based on their geologicalcharacteristics:thestructuraltrap,thestratigraphictrapandcombinationtraps.Examplesofdifferenttrapsare illustrated in Figure 12. Structural traps are formed by themovement of the earths crust. Theearthscrust iscomposedofplatesthatmove(tectonicactivities) inresponsetotheforcesdeepwithinthe earth. As these platesmove, they cause subsurface formations to fold and fault leading to theformation of domes, anticlines, and folds. Structural traps are more easily delineated and moreprospective than their stratigraphic counterparts,with themajorityof theworld'spetroleum reservesbeingfoundinstructuraltraps.

Stratigraphic trapsare formedasa resultof lateralandverticalvariations in thecharacteristicsof therock.Examplesofthistypeoftraparepinchout,unconformity, lens,andreeftraps.Thepinchout isatrapthat isformedduetothe limitedaerialextentoftheformationsuchasandbar.Theunconformityoccurswhen there isan interruption indeposition. Forexample, the lower formationsaredeposited.Then,thereisaperiodoferosionfollowedbyanotherperiodofdeposition.Thatsequenceofeventswillresult in an unconformity.When the unconformity is an eroded reservoirquality rock overlain by animpermeablesediment,oilorgascanaccumulateattheunconformableinterface.

Structural traps may have a stratigraphic component or vice versa. Some structural traps arecombinationsofdifferentstructural trapssuchas fold, fault,and fracture.Thedistinctionwhether thetrapbelongs toone systemor another is sometimesquite blurred. Trapswith twoormore trappingelementsarecalledcombinationtraps.

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Figure12:DifferentTypeofTrapsDepthTheamountofhydrocarbon stored ina subsurface structuredependson the subsurfacepressureandtemperature.Thecompressionwillallowsignificantlymorehydrocarbonstobestoredinthesamespace.The pressure and temperature exerted on the fluids in subsurface increases with depth of burial.Therefore, the structuremustbeburied sufficientlydeep to form a commercial reservoir.A reservoirthousandsoffeetundergroundissubjecttoapressurecausedbythecombinedweightoftheformationrockandfluidsknownastheoverburdenpressure.Inthemajorityofsedimentarybasinstheoverburden

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pressureincreaseslinearlywithdepthandtypicallyhasagradientof1psi/ft.Ina sufficientlyconsolidated reservoir, theoverburdenpressure isnot transmitted to the fluids in theporespaces.Thefluidpressureinthereservoirisdictatedbytheprevailingwaterpressureinthevicinityofthereservoir. It iscommonlyassumedthatthere iscontinuityofwaterpressuretothesurfaceeventhoughthewaterbearingsandsareusuallyinterspersedwithimpermeableshales.However,anybreakinthe areal continuity of such apparent seals will lead to the establishment of hydrostatic pressurecontinuity to the surface.Therefore, thisassumption is valid in themajorityof cases.Asa result, thewaterpressureatanydepthcanbecalculatedas:

. ( . )wwater

dpP D 14 7 1 1dD

Where:

.

water

dpdD

14 7

=Hydrostaticpressuregradient,psi/ft.

Atmosphericpressure,psia

The hydrostatic pressure gradient is dependent on the chemical composition (salinity), and for purewaterhasthevalueof0.4335psi/ft. Inclusionoftheatmosphericpressure, inequation1,providesthepressure inabsoluteratherthangaugeunits(psig).Inmany instances inreservoirengineeringthemainconcern iswith pressure differences,which are the samewhether absolute or gauge pressures areemployed,andaredenotedsimplyaspsi.Heat rising from the mantle produces a heat flux which results in a geothermal gradient, Gt. Thegeothermal gradient varies at different areas on the globe depending on the annual mean surfacetemperature and the thermal conductivity of the subsurface formations, but an overall temperaturegradientGtof1.0oF/100feetofdepthhasbeenrecordedaroundtheworld.UsingthisaveragevalueandtheregionsmeanannualsurfacetemperatureTs,anestimateofthesubsurfaceformationtemperaturesTfcanbeobtainedasfollows:

.f s tT T G D 12 WhenthebottomholetemperatureTfofthewellisaccuratelymeasured,thelocalgeothermalgradientGtmaybeobtained from the above equation to estimate the temperatureof the formations at anydepthD.ReservoirRockThe subsurface structurewherepetroleumaccumulatesmust containa threedimensionalnetworkofinterconnectedvoid(pore)spacetostorethefluidsandallowfortheirmovementthroughtherock.Thecapacityofthereservoirrocktocontainliquidorgaseoushydrocarbonsisrepresentedbyarockpropertyreferred to asporosity. Sand grains andparticles thatmake up the reservoir rocks areusuallyhighly

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irregular in shapeandnever fit togetherperfectlyas illustrated inFigure12.Theopen spacecreatedbetweengrainsduringdepositionisreferredtoasporespace.Thevoid,orpore,spaceinhighlyporousreservoirrocksallowsthemtoholdlargevolumeofoilorgas.

Figure12:2DimensionalRepresentationoftheVoidandBulkVolumesinthePorousMediaPorosityisdefinedastheratioofthevoidvolume(Vvoid)tothebulkvolume(Vb):

% ( . )voidb

V 100 13V

Porosityisconventionallyexpressedinpercentageform,ratherthanasafraction,whichisthereasonformultiplyingEquation1by100%.Theabilityofthereservoirrocktotransmit fluids (oil,water,orgas) isrepresentedbyarockpropertyreferred toaspermeability.Permeability isdefinedbasedonanequation,developedbyHenryDarcy,whichdescribesthe flowof fluid throughporous.TodescribetheDarcyequation,considertheporousmediaillustratedFigure13.

Figure13:FlowthroughPorousMedia

Iftheflowrateisq,thepressuredifferenceindirectionoftheflowoverthelength,L,isp(P2P1),and

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theareaacrosswhichtheflowoccursis,A.ThenDarcysequationcanbewrittenasfollows:

( . )k pq A 14L

The term, p L , is pressure gradient and is the driving force for the flow. The term,k , is the

conductance(inverseofresistance)ofthesystemwhichisthecombinedconductanceoffluid(inverseofviscosity,)andtheporousmediacapacityforfluidconductanceorpermeability(k).ThenegativesigninEquation1.4isnecessarybecausethepressuredecreasesinthedirectionoftheflow(P2

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geophones.Thesedatathenareprocessedbyhighspeedcomputers inanattempttodetermine ifthegeologic conditions are consistent with those required to produce a trap for the accumulation ofhydrocarbons.The interpretationof the seismic results such theone illustrated in Figure15 requiresspecialistsinthisfieldorgeophysicists.Conventionalseismicisruninlinesandinterpretedlinearly.A3Dseismic survey is carried out on an aerial basis. 4D seismic involves acquisition, processing, andinterpretation of repeated seismic surveys over a producing hydrocarbon field. The objective is todetermine thechangesoccurring in the reservoirasa resultofhydrocarbonproductionor injectionofwater or gas into the reservoir by comparing the repeated datasets. The improvement in seismicinterpretationhascomethroughcomputingability.Thesamedatanowareprocessedbyunimaginablyfastcomputerstorevealevenmoreaboutwhatliesbeneaththesurface.

Figure14TheBasicsofSeismicSurvey

Figure14TheInterpretationoftheSeismicSurvey

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Whileseismicinterpretationcanidentifytheexistenceofatrap,itcannotdeterminewhetherornotthetrap contains hydrocarbons. The onlyway to find out if the trap really exists andwhether or not itcontains oil is to drill awell and test the formations. Oncewells have been drilled and tested, thegeologisthasmoretoolstoworkwith.Thegreateristhenumberofholesdrilled,themoretools.Itthenbecomespossibletoaugmentthemapsofthesubsurfacebasedseismicinterpretationwithactualdata. Oneofthemostbasicmapspreparedbythegeologististhestructuralmap.Astructuralmapissimplyatopographicmapfrom longago. Inotherwords,thegeologistusesthedataavailabletodeterminethelocationofancienthillsthatmightnowserveastrapsfortheaccumulationofhydrocarbons.Figure14illustrateanexampleofastructuremap.

Figure14:AnExampleoftheStructuralMap

There are numerousmaps thatmight be prepared. For example, once a particular horizon has beenidentifiedasbeingofinterest,thegeologistmightprepareanisopachorthicknessmapwhichillustrateswherethe interval isthickerandmightbepotentiallyproductiveandwhere it isthinnerornotpresentandoflessinterest.AtypicalisopachmapisshowninFigure15.

Figure15:AnExampleoftheIsopachMap

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CHARACTERISTICSOFTHERESERVOIRThe characteristics of the reservoir rock and the fluids they contain are necessary to construct anaccuratepictureofthereservoiranditsproductivepotentialinordertomaximizetheeconomicpaybackfrom the production of the hydrocarbons. The volume of hydrocarbons originally in the reservoir (inplace)istheultimatetargetforthedevelopmentofareservoir.Thevolumeofhydrocarbonsoriginallyinplace isproportional to theproductofnet formation thickness,porosity,andhydrocarbon saturation.Theproductionratehasasignificantimpactonthehydrocarbonrecoveryandeconomicperformanceofthe reservoir. The production rate is related to the combination of the net formation thickness, thepermeability,fluidproperties(viscosities),andpressure.Therefore,the informationthataretypicallyofprimaryinterestincludeporosity,permeability,fluidsaturations,netformationthickness,pressure,fluidproperties,andthestructureofthereservoir.Otherformationpropertiessuchasmechanical,electrical,and,acoustic rockproperties,wettability,and relativepermeability canprovideadditional insight intothehydrocarbonvolume,productionrate,andtheoptimizationofthereservoirperformance.Hydrocarbon reservoirs are tappedbywells, and thewells arebasically the sourceof all informationconcerningthereservoir.Formationevaluationdataareobtainedduringthedrillingandcompletionofthewell.Data of this typemust be obtained during particular phases of the drilling and completionoperation.Ifnotobtainedattheappropriatetime,certaintypesofdata(i.e.,coresamples)maybelosttotherecords.Reservoirfluidandproductiondataareobtainedlargelyafterthewellsarecompleted.The reservoircharacteristicsaredeterminedby theanalysisof the informationobtainedbydirectandindirectmeasurements.Directmeasurements inareservoirarepossibleonfluidsamples,todeterminethenatureandcompositionof the fluid (PVTanalysis),andon the reservoir rock samples (cores).Thedesiretoobtainpiecesofreservoirrockhasledtothedevelopmentofcoringtechniqueswhichrelativelylargereservoirrocksamplescanbeobtained,eitherfromthebottomduringdrilling,orthesideoftheboreholewallafterdrilling.Coringisthebackboneforinvestigatingthereservoirrockpropertiesandthereservoir internalanatomy.Coreanalysisprovidesdirectmeasurementsofporosity,permeability,andcompressibility. Additionally, it can provide information regardingwettability, capillary pressure, andrelativepermeabilities.Theresultsofthecoreanalysisreflect localizedreservoirpropertiesanddonotgenerally represent the entire reservoir. Inmost cases, a large number ofwellsmust be drilled andsampled inorder to gain adequate knowledgeof rockproperties.Gatheringofnecessary informationovertheentirereservoirisresourceintensive.Theintensityofsamplecollectionandsubsequentanalysisdependonthedegreeofcomplexityofthereservoir,aswellasonreservoireconomics.

The indirectmeasurementscanbeobtainedfromthewireline logsandthewelltests.Wireline logsarerecordingsofphysicalparametersobtainedby running instruments into theboreholeat theendof awire.Thewellloggingtechniquesweredevelopedtodeterminetherockpropertiesatinsituconditionsinordertoavoidanyalterationcausedbyremovaloftherocksample(core)and itspreparation inthelaboratory. Several types of logging equipment have been developed in the industry to collectinformationregardinglithology,rockporosity,thickness,andfluidsaturation.Loggingtools,whenplaceddownhole,cangatherdatafromafew inchestoafewfeet intothereservoir.Comparedtocoringandcoreanalysis,well logging is lessresource intensiveandprovidescontinuousmeasurementsoftherock

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propertiesagainstdepth.The informationobtained fromwell loggingmethodsshouldbeconsideredacomplement tocoreanalysis resultsandnota replacement.Studiesarecommonplacewhere logdataandcoredataarecorrelatedandintegratedtobuildadetailedviewofthereservoir.Asuccessfulloggingprogram, along with core analysis, can supply data for subsurface structural mapping, define thelithology, identify theproductive zones and accuratelydescribe theirdepth and thickness,distinguishbetween oil and gas, and permit a valid quantitative and qualitative interpretation of reservoircharacteristics,suchasfluidsaturation,porosity,andpermeability.

Well or pressure transient testing provides information on the reservoir behavior under dynamicconditions. In a well test, the pressure response of the reservoir to a change in the rate of fluidproductionisrecordedasafunctionoftime.Thispressureresponseisinfluencedbyfluid(viscosityandcompressibility)androckproperties(permeabilityandnetthickness)inthereservoir.ThetestresultsarecombinedwithfluidpropertiesobtainedfromPVTanalysisandrockpropertiesobtainedfromthecoreanalysisandwell log interpretationtoestimateformationpermeability.The informationobtainedfromtransienttestscoversarelatively largerarea,typicallyhundredsoffeet intothereservoir.Theanalysisresults canalsobeused todiagnose thewell conditions inorder todetermine theneed for remedialtreatmenttooptimizethewellperformance.

Intheearlystagesoffielddevelopment,veryfewwellshavebeendrilled,andinformationregardingvitalrock characteristics can only be obtained from core and well log data. However when sufficientproductionandpressuredatafromthereservoirhavebecomeavailable,productiondataanalysiscanbeperformedtodetermineoilorgasinplace,theproductionmechanism,andthepotentialrecoveryfactor.Common techniques forproductiondataanalysis includematerialbalance,decline trendanalysis,andreservoirsimulation.

Reservoir engineermust successfully blend geology, geophysics, petrophysics, petroleum engineering,andeconomicsknowledgetoobtainanaccurateunderstandingofthepast,present,andfuturebehaviorofoilandgasreservoirsandmustforecasttheeconomicrecovery.Areservoirmodel isusedtopredictthephysicalbehaviorof the reservoir, in termsofproduction rate,operating scenarios,developmentpatterns, primary production and secondary or tertiary recovery processes.Models are valuable forstudying realworldoperationsandprovide themeans for reducing complexproblems tomanageablesize. Whereas the field can be produced only once and at considerable expense amodel can beproduced or "run" many times at low expense over a short period of time. Observation of modelperformanceunderdifferentproducingconditions,then,aids inselectinganoptimumsetofproducingconditions for the reservoir. The reservoir model is built using available information from geology,geophysics, andmeasurements performed inwells. The reservoirmodel isnever definitive, butmustcontinuously be adjusted as new information becomes available when additional wells are drilled.Computermodelsareoftenused tomodelpastperformanceofreservoirsand to forecast their futureperformance. Practice of reservoir engineering has changed from application of the basic theory andmathematical equations to placing numbers into the computer models. It is easy to run computerprograms. However, finding accurate values for each input value is much more difficult. Sound

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engineeringdemandsthatthebestpossibleormostaccurateandrepresentativenumericalvaluesforalltheinputbeused.Computermodeloutputmustalsobeanalyzedindetailtobecertainthatresultsarerealisticwhen comparedwith past experience, results observed in adjacent or comparable reservoirshaving similar characteristics, and general concepts regarding reservoir behavior and productionpractices.Adashofcommonsensecanbemostvaluable,andthecomputershouldnotbeusedblindly.

CLASSIFICATIONOFTHERESERVOIRSThereservoirscanbeclassifiedinseveraldifferentwaysaslistedbelow:

a.Initialstateofthefluidsinthereservoirb.Production/drivemechanismc.Theporesystemd.Recovery/productiontechnology

ClassificationoftheReservoirsbytheInitialStateoftheFluids

Thepetroleumreservoirscanbeclassifiedaccordingtothephysicalstateoffluidsinthereservoiratthediscoveryconditions.Therearethreebroadcategoriesincludingoil,gascondensate,andgasreservoirs.Thephysicalstateof fluids inthereservoirdependsonthepressure,temperature,andcompositionofthepetroleum.Petroleum isacomplexmixturecontaining thousandsofdifferentcompounds,mostofwhicharecomposedexclusivelyofhydrogenandcarbon(hydrocarbons).Phasediagramsareoftenusedto illustrate theeffectsofpressureand temperatureon thephysicalstateofhydrocarbons.Thephaserelationshipsofhydrocarbonshavebeenextensively studied, andmuchhasbeen learnedabout theirbehavior.ApressuretemperaturephasediagramforapuresubstanceisshowninFigure16.Thephasediagramismerelyalinewhichseparatestheliquidregionfromthevapor(gas)region.Thelocationofthesolidstatewillnotbeshownbecauseitisoflittleinterestinstudyingofapetroleumdeposit.ExaminationofFigure16showsthatthelinerepresentspressuresandtemperaturesatwhichboth liquidandvaporcanexistinequilibrium.PointCistheCriticalPoint,whichisdefinedasthetemperatureandpressureabovewhichtwophasescannolongerexistinequilibrium.

Figure16:PhaseDiagramforaPureSubstance

Twodifferentpurecomponentsxandy,mighthavethephasediagramsshownbysolidlinesinFigure17. If these two materials are mixed in equal proportions, the phase diagram for the resulting

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mixturewillhavetheshapeshownbythedotted line inFigure17.Thedotted lineseparatesthetwophase region from thesinglephase region.Thephasediagram for themixture liesbetween thephaselinesforthetwopurecomponents.Thereasonforthisisthatalongtheupperportionofthedottedlinethehighermolecularweightmaterialy,whichisintheliquidphase,willholdthelowermolecularweightmaterial x in the liquid state for a longer period. Conversely, along the lower portion of the phasediagram,thexmoleculeswillpreventtheymoleculesfrombeingliquefiedforalongerperiodoftime.

Figure17:PhaseDiagramforaBinaryMixtureAphasediagramfortypicalhydrocarbonreservoirfluidisshowninFigure18.Aphasediagramconsistsof thebubblepoint and thedewpoint curveswhich form thephase envelop. The twophase region isenclosedbythephaseenvelopandthesinglephaseregionliesoutsideofthephaseenvelop.Thepointwherethebubblepointandthedewpointcurvesconvergeisreferredtoasthecriticalpoint(C).Thetwophasesare indistinguishableat thecriticalpoint.Thehighest temperature (T)atwhich the twophasescouldcoexistinequilibriumisreferredtoasthecricondentherm.Whilethehighestpressureatthetwophasescouldcoexist inequilibrium isreferredtoasthecricondenbar.Thecompositionofthereservoirfluiddetermines the shapeof twophaseenvelopeand itspositionon thePTdiagram.Each reservoirfluidhasauniquephasediagram.Figure16isaphasediagramtypicalofareservoirfluid.Thepetroleumcanexitsassinglephase liquid,singlephasegas,orbothatthe initialpressureandtemperatureofthereservoir.IftheinitialPTconditionsofthereservoiraretotheleftofthecriticalpointCandoutsideofthephaseenvelope,point1ionFigure18 then thereservoir fluid is initiallysinglephase liquid.Thisreservoir isreferred to as an oil reservoir. Points 1i and 1a on Figure 18 depict the initial and abandonmentconditions, respectively,ofanoil reservoir.At the initialconditions thehydrocarbonsare100percentliquid.As theproductionoccurs, thepressurewilldecline isothermally since it isknown that reservoirtemperaturedoesnotchangeas fluidsareproduced from the reservoir.Theconditionsof fluid in thereservoiraredepictedbyadashedverticallineconnectingpoints1iand1aonFigure18.Thestateoftheoildoesnotchangeuntilpoint"S"on thedashed line is reached.At thispressuresomeof the lighterhydrocarbons,principallymethane,willbeevolvedfromtheoil,andwillexistasfreegasinthereservoir.Thepressureatwhichthegasisfirstreleasedfromsolutionisreferredtoasthesaturationpressure,orbubblepointpressure.Continuedreductioninpressurewillresultinmoreandmoregasbeingreleased

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fromtheoiluntiltheabandonmentconditionsarereached.Oilreservoirscanbefurtherclassified intothreebroadclassesinorderofincreasingmolecularweight;theyarevolatileoil,blackoil,andheavyoil.

Figure18:TypicalReservoirFluidPhaseDiagram

IftheinitialPTconditionsofthereservoiraretotherightofthecricondentherm,point2ionFigure18,thenthereservoir fluidwill initiallybesinglephasegas.Thisreservoir isreferredtoasagasreservoir.Points2iand2adepicttheinitialandabandonmentconditions,respectively,ofagasreservoir.Itcanbenoted thatnopointon theverticaldashed line,depicting the isothermaldepletion,crosses thephaseenvelope.Therefore,thefluidinthereservoirisalwaysinagaseousstate.Gasreservoirscanbefurtherclassifiedintotwoclassesbasedontheircomposition;theyarewetgasanddrygas.Ifthegasproducedfromthereservoirfallsinsidethephaseenvelopeatthesurfacepressureandtemperatureconditions,itis called a wet gas. The surface separation facilities are often installed to recover the condensiblehydrocarbonsor condensate from thewetgas. If thegas consistsofprimarilymethaneanddoesnotcontainanycondensiblehydrocarbonsatthesurfaceconditions,itiscalledadrygas.If the initialPT conditionsof the reservoirare to the rightofpointCand to the leftofpointT,andoutsideof thephaseenvelope, then the reservoir is referred toasgascondensate reservoir.Points3iand3aonFigure18depictrespectivelytheinitialandabandonmentconditionsofthistypeofreservoir.Initially,thestateoffluidinthereservoirissinglephasegas.Asproductionbeginsfromthereservoirandpressuredeclines,nochangeinthestateofthereservoirfluidoccursuntilpoint"D"isreached.Point"D"is called the dew point pressure because the dew point line has been crossed. Further reduction inpressurewillcauseliquidtocondensefromthegas.Thisisnotconsideredtobeanormalsituation,sinceformosthydrocarbonfluidsareductioninpressurewilltendtoincreasetheamountofgas.Thisbehavioris referred toas retrogradecondensation.Fora reservoir fluid toexhibit the retrogradebehavior, theinitialconditionsofpressureandtemperaturemustexistoutsidethephaseenvelopetotherightofthepointCandtotheleftofpointT,orwithinthephaseenvelopeintheregionmarkedbythedashedcurve.Theproducedgasfromthecondensatereservoiriswetgasandthecondensatecanberecoveredatthe

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surface separation facilities. However, the condensate recovery at surface declines as the reservoirpressuredeclinebelowthedewpointpressureasthecondensablehydrocarbonsareformedandtrappedinthereservoir.IftheinitialPTconditionsofthereservoirfallinsidethetwophaseenvelop,initiallybothoilandgaswillbepresentunderequilibriumconditions in the reservoir.Sincegas is lighter thanoil, itwill rise to thehigher parts of the structurewhile the oilwill occupy the lower parts of the structure. This type ofreservoirisreferredtoasanoilreservoirwiththegascap.IftheinitialPTconditionsofthereservoirfallinside the retrograde region, theareawithin the twophaseenvelopemarkedby thedashed curve inFigure18,thegascapwillexhibittheretrogradebehavior.Eachof these fluid types,blackoil,volatileoil,gascondensate,wetgas,anddrygas requiredifferentapproacheswhenanalyzingthereservoir,soitisimportanttoidentifythecorrectfluidtypeearlyinthelifeofthereservoir.Laboratoryanalysisistheprimarymethodfordeterminingandquantifyingfluidtype,butproduction informationsuchas initialproductiongasoil ratio,gravityof thestocktank liquid,andthecolorofthestocktankliquidarealsousefulindicators.

ClassificationoftheReservoirsbytheProduction/DriveMechanism

Whilethereservoirclassificationbythe initialstateofthefluid ishelpful, it isalsousefultoclassifythereservoirbasedonthedrivemechanism.Thistypeofclassification ismoresignificant inunderstandingthe reservoirbehaviorand the implementationof theappropriate technology tooptimizeoilandgasrecovery. Thedrivemechanism is thenatural energy source that results in expansionof fluids in thereservoir.The increase in the fluidvolumes isequivalent to theproduction.Theenergysourceswhichmaybeavailabletomoveoilandgastothewellbore(production)includeexpansionofthedissolvedandfreegas,expansionoftheoil;water;andtherock,aswellasthegravity.Mostoftheseforcesareactiveduring theproductive lifeof a reservoir and a singledrivemechanism isnot sufficient to explain theperformanceof the reservoir throughout itsentireproductive life. Inmany cases,however, reservoirscanbesingledoutashavingpredominantlyonemaintypeofdrivemechanismincomparisontowhichallothermechanismshaveanegligibleeffect.Themajordrivemechanisms include solutiongas,gascap,andwaterdrive.Inaddition,gravitydrainagecanactasasupplementaldrivemechanism.Forsimplicity,eachmechanismandtheassociatedreservoirbehaviorarediscussedhereinthecontextofasingledrivereservoir.

SolutiongasDrive

Theprincipaldrivemechanisminasolutiongasdrivereservoiristheliberationandexpansionofthedissolvedgasintheoil,asthepressuredeclinesduringproduction.Initiallyinthereservoir,thereisnofreegasandthewaterflowintothereservoirfromthecontiguouswaterzonesisinsignificant.Solutiongasdriveisalsoknownasdepletiondrive,dissolvedgasdrive,orinternalgasdrive.Twoperiodsofproductionmayexist:(a)whenthereservoirpressureisabovethebubblepointpressure,thesolutiongasremaininsolutionandtheproductionmechanismistheexpansionoftheoil;water;

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andthereservoirrock.Thistypeofreservoir,oftenreferredtoasundersaturatedoilreservoir,ischaracterizedbyaconstantproducinggas/oilratioandlowoilrecovery;(b)whenthepressureisbelowthebubblepointpressure,gaswillbeliberatedfromthesaturatedoilandafreegassaturationwilldevelopinthereservoir.Astheproductioncontinuesandpressuredeclines,gassaturationwillincreaseresultinginrapidincreaseinthegas/oilratiowhichreflectstheincreasinggasflowrateandthediminishingoilproductionrate.Theoilrecoveryfromasolutiongasdrivereservoiristypicallyintherangeof1015%.Theoilleftbehindinthereservoiruponcompletionofprimaryproductioncanberecoveredbywaterfloodingwhichisacommonsecondaryrecoverytechnology.Ifthereservoirisinitiallyundersaturated,anadditional13%recoverymaybeachievedbeforebubblepointpressureisreached.

GascapDriveAgascap reservoir initiallycontainsa freegaszoneabove theoilzone.Theoil isat saturationorbubblepoint pressure at theoilgas contact, and thewater influx isnegligible. Theprincipaldrivemechanism isthecombinationofthefreegasexpansion,whichpermeatesanddiffuses intotheoilzonefromthegascap,andthesolutiongasexpansion.Becauseofthegascapexpansion,thedeclineinoilproductionrateandthepressure isnotasrapidas insolutiongasdrivereservoirs.Generally,theoilrecoveryfromagascapdrivereservoirisgreaterthansolutiongasdrivereservoir,typicallyintherangeof2030%,dependentonthesizeofthegascap.Theoilrecoveryismoresensitivetorateatwhichthereservoirisproducedthanforsolutiongasdrivepools.

WaterDriveAreservoirwhichisincontactwithanextensiveaquifercanbesubjecttowaterinfluxprovidedithassufficient permeability. The reduction in the reservoir pressure with oil production results in apressure difference at the reservoiraquifer boundary. Consequently, the water in the aquiferexpandsand flows intothereservoir.Thedegreetowhichtheproducedfluidsarereplacedbythewater determines the efficiency of the waterdrive mechanism. The production rates greatly inexcess of the rate of water influx can lead to performance similar to that of solutiongasdrivereservoirs. Generally, the production capacity and the produced gas/oil ratio are substantiallyconstant until water begins to be produced. Recovery factor depends on the reservoir rockcharacteristics,mobility ratioand reservoirgeometry.Recovery fromwaterdrive reservoirscanbeveryhigh, inexcessof50%,but the residualoilwillnowbe trappedbehind the advancingwaterwhichcanonlyberecoveredbyresortingtomoreadvancedrecoverymethods.

GravityDrainageGravitydrainageisconsideredasupplementaldrivemechanismsincerarelyanoilreservoirproducesstrictlyundergravitydrainage.Inoilreservoirssubjecttogravitydrainage,thesegregationoftheoiland gas results inmigrationofoil to the lowerpartsof the structurewhile the free gas forms asecondarygascapinthehigherpartsofthestructure.Ifaprimarygascapexists,thenitwillexpandasa resultof thisprocess.This segregationprocessmaintainsahighoil saturation in theoil zonewhich leads to increased oil recovery. Themagnitude of the gravity drainage depends on the oil

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gravity,permeabilityof thezone,and formationdip.Gravitationaleffectsexist inall thereservoirsbutdueto lowverticalpermeability,thesegregationofoilandgas isaveryslowprocessanddoesnothaveasignificant impactontheproductionperformanceoroilrecovery.However,thesteeplydippingbedsaccentuate thedownstructureoilmovementbecause thepermeability is thehighestparalleltothebedding.Lateinthelifeofthesolutiongasdrivereservoirswhentheproductionrateisverylowduetosignificantpressuredepletion,gravitydrainageoftenbecomesthemainproductionmechanism.

CombinationDriveA sequenceofdrivemechanisms in reservoirs, such as solutiongasdrive, gravitydrainage,waterdrive,may develop during primary production in some reservoirs. Also two different drivesmaycompete simultaneously such as waterdrive reservoir and gascap. The performance of thecombinationdrive reservoirs is affected by the method of operation and by individual well andreservoirrates.

DriveMechanismsinGasReservoirsThediscussionabovewas limited tooil reservoirsbutasimilarclassificationcanbeapplied togasreservoirs. However, since gas reservoir contain singlephase gas, only three drive mechanismsincluding gas expansion,water drive, and rock andwater expansion could be responsible for gasrecovery. Typically, volumetric gas reservoirswhichproduce onlyby gas expansionhavehigh gasrecovery,intherangeof9095%.Therecoveryforthewaterdrivereservoirisgenerallylower,ininthe range of 5060%, due to entrapment of gas by the advancing water. The rock and waterexpansion are only significant in geopressured reservoirs. The reservoir fluid pressure ingeopressuredreservoirssignificantlyexceedshydrostaticpressure.Geopressuredaccumulationsarefrequentlyassociatedwithsubstantialfoldingandfaultingandhavebeenobservedinmanyareasoftheworld.

ClassificationofReservoirsbyTypeofPoreSystemThe geologic formations thatmakeup thepetroleum reservoirs are formedovermillionsof yearsbysedimentdepositionwhichstrongly influencesthearchitectureoftherockand inturntheporosityandthe permeability.However, the postdeposition or diagenetic changes can alter the rocks.Diageneticchanges can createordestroy theoriginalporosity andpermeability,or createbarriers to fluid flow.Porosity,usuallycausedbyfracturingand/ordissolutionoftheoriginalrockmatrix,isoftenreferredtoassecondaryporosity.Insomecases,thesecondaryporosityconsiderablyincreasestheporosityoftherockmatrix and is the primarymechanism for fluid storage and flow. A three category scheme has beenproposed forclassificationof theporesystems.They include intergranularintercrystalline inwhich thepore space in the reservoirs ismainlyconsistedof theprimaryporosity formedduring thedeposition,vugularsolution inwhichtheporespace inthereservoirs ismainlyconsistedofthesecondaryporosityformed during diagenesis, and mixed porosity systems in which the pore space in the reservoirs isconsistedof twocoupledpore systems.Theperformanceof this type (heterogeneous)of reservoirs ismarkedlydifferentfromthatthoseconsistingofasingleporetype(homogeneous)reservoirs.

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ClassificationofReservoirsBasedonRecovery/ProductionTechnologyThe reservoirs are often classified as conventional or unconventional based on the technologicalrequirementsfordevelopmentandproduction.Conventionalreservoirscanproduceeconomicvolumesofoilandgas througha verticalwellswithoutany special recoveryprocess.They tend tobe small involume but easy to develop because they have high to medium permeability. Only one third ofworldwide oil and gas reserves are associatedwith conventional reservoirs. As easy to develop andinexpensive to produce conventional reservoirs have been discovered and depleted, the industry hasturned to unconventional reservoirs that cannot be produced economically with conventionaldevelopmentandproduction technology.Unconventional reservoirsare typicallycontinuousoilorgasaccumulationsthatpersistovera largegeographicalarea.Themainchallenge intheseaccumulations isnot in finding them,but in commerciallyextracting thehydrocarbons. Theunconventional reservoirsgenerally require stimulation treatmentsor special recoveryprocessesand technologies toproduceateconomicrates.Advancedtechnologyapplication isthekeytothedevelopmentoftheunconventionalreservoirs. Technical and engineering advances over the past decade have transformed theunconventional reservoirs from the emerging resources tomain targetof exploration andproductionglobally.Advanced fracturing techniques combinedwith horizontal drilling have been instrumental ineconomicallyexploitingmanyunconventional reservoirs.The termunconventional reservoirs includeawidevarietyofthehydrocarbonbearingformationandreservoirtypes.Theycommonlyincludetightgassandstones,coalbedmethane(CBM),shalegasandoil,heavyoil/tarsands,andgashydrates.