Abouttheauthor
HuinongZhuang,professorateseniorengineer,graduatedfromPekingUniversityin 1962.Aftergraduationhetookpartinresearchonthedevelopmentprogramofthe DaqingOilFieldinitsearlystages,andafter 1965heservedintheShengliOilField,where hisinterestwasinoil/gaswelltesting.Inthe 1980shetookchargeofandsuccessfullyoperatedinterferencetestsandpulsetestsin carbonatereservoirs;duringthisperiodhe inventedtheinterpretationtypecurvesfor interferencewelltestsindoubleporosityreservoirsandappliedthesetypesofcurvesin thefield.Hemanagedresearchondownhole differentialpressuregaugesandapplied thesegaugesindataacquisitioninthefield, andconsequentlywontheinventionaward fromtheChinaNationalScienceandTechnologyCommitteeandwaspresentatthe FirstInternationalMeetingonPetroleumEngineeringinBeijingin1982;hispaperwas publishedintheJournalofPetroleum
Technology.Since1990hehasservedatthe ResearchInstituteofPetroleumExploration andDevelopment(RIPED)ofPetroChina, hasbeenconcernedwiththeexploration anddevelopmentofseverallargeor mediumscalegasfieldsinChina,andhas carriedoutdynamicperformanceresearch. Hehasdevotedhimselftoperformanceanalysisandwelltestingforabout60years.
YongxinHan isaSeniorReservoirEngineeroftheResearchInstituteofPetroleum ExplorationandDevelopment(RIPED)of PetroChinaandDeputyDirectoroftheDepartmentofGasFieldDevelopment,and amemberoftheSocietyofPetroleum Engineers(SPE).Sincehegraduatedfrom DaqingPetroleumInstitutein1989,hehas workedforRIPED,specializinginpressure transientanalysis,productiondataanalysis, anddynamicgasreservoirdescription. Hehasparticipatedintheexplorationand
developmentofseverallargeormedium scalegasfieldsinChinaandcompleted morethan1000gaswellintervaldynamic performancestudiesoverthepast30years. HeholdsaBSdegreeinreservoirengineeringfromDaqingPetroleumInstituteof China,andMSandPhDdegreesinreservoir engineeringfromChinaUniversityof Geosciences(Beijing).Hehascopublished 6booksandover30papersinpeerreviewed journalsandconferencepresentationson welltestingandgasreservoirevaluation anddevelopment.
HedongSun,PhD,SocietyofPetroleum Engineers(SPE)memberandprofessorate seniorengineer,earnedhisPhDfromXi’an JiaotongUniversityin2004.Since2004, hehasbeenaresearchengineerinthe ResearchInstituteofPetroleumExploration andDevelopment(RIPED)ofPetroChina. Hedonghas23yearsofreservoirengineering
experiencewithafocusonwelltestandproductiondataanalysis.Hehaspublishedover 50papersinpeerreviewedjournalsandSPE conferences.Heisanauthorofthreebooks publishedbyElsevier.
XiaohuaLiu,PhD,isseniorreservoir engineerwiththeResearchInstituteofPetroleumExplorationandDevelopment(RIPED) ofPetroChina.Shehas25yearsofwork experienceinnaturalgasfielddevelopment researchandhasbeeninvolvedinsomeof China’smajorgasfielddevelopmentprogramsandreservoirengineering.Herfocus iscombiningwellshorttermprefracture pressurebuildupwithlongtermperformanceandgeologytoproposeproduction optimization.Shehascoauthored3books andover25papersinpeerreviewedjournals andconferencepresentations. x Abouttheauthor
1.1Thepurposeofthisbook
Themodernwelltesthasbeenaroundsincethebeginningofthe1980s.InChina,during theimplementationofreformandopeninguppolicies,modernwelltestmethods,interpretationsoftware,andadvancedtestinstruments,tools,andequipmentwereintroduced
almostsimultaneously.Lookingbackattheadvancesmadesincetheearly1980s,itisvery excitingtoseethatnewdevelopedknowledgeandtechniqueshavebeenappliedsuccessfullyinthediscovery,developmentpreparation,anddevelopmentoperationofmanymajor gasfieldsinChina.However,itshouldalsobenotedthatapplicationofthemodernwelltest sometimesandinsomeplacesisstillnotgoodenoughandneedstobeimprovedfurther.
Thewelltesttodayisverydifferentfromthatofthreeorfourdecadesago.Justasinall otherfields,duetotheapplicationofcomputersandadvancesinscienceandtechnology, engineerstodayseldommakecalculationsmanually;welltestanalystsandreservoirengineersnolongerfrequentlylookupcomplicatedformulasinwelltestbooksandperform tediouscomputationswithcalculators;theresultscanbeobtainedeasilybysimplyselecting somemenuitemsofsoftware.
Butdoesthismeanthatwelltestworkhasbecomemucheasier?Theanswerisno;onthe contrary,asresearchactivitiesgofurther,thewelltestdoesnotbecomeeasier,butfaces greaterchallenges.
Firstofall,welltestanalysisisrequiredtoprovidenotonlysimpleparameterssuchasreservoirpermeability,butalsomoredetailedinformationaboutreservoirs,suchastheirtypesand boundaryconditions,andultimatelytodelivera“dynamicmodel”ofgaswellsandgasreservoirs—thatis,adynamicmodelreflectingtheconditionsofthegaswellandthegasreservoir trulyandcorrectly,whichcanbeusedingasfieldevaluationandperformanceforecasting.
InChina,therearemanyreservoirtypes,sowelltestanalysisbecomesmuchmoredifficult. Asfarasthereservoirtypeisconcerned,therearesandstoneporousreservoirs,fissuredreservoirsandfracturedvuggyincarbonaterocks,biothermalmassivelimestonereservoirs,and irregularlydistributedblockshapedreservoirsinvolcanicrocks;asfarastheplanarstructure ofareservoirisconcerned,therearewellextended,uniformlydistributedlargeareaformations,faultdissectedreservoirswithcomplicatedboundaries,andbandedlithologicreservoirsformedbyfluvialfaciessedimentation;asfarasthefluidtypeisconcerned,there arecommondrygasreservoirs,condensategasreservoirs,andgascapgasreservoirswith oilringsandedgewaterorbottomwater;andasfarasreservoirpressureisconcerned,there aregasreservoirswithnormalpressurecoefficients,extremelythickgasreservoirswithsuper highpressure,andunderpressuredgasreservoirs.Asindicated,thesereservoirsarerichly varied,whichhasundoubtedlybroughtaboutnewchallengestowelltestanalystsandreservoirengineers.
Moreover,thequalityofpressuredatanowadaysisnolongerasitwasintheearly1980s. Atthattime,pressuredatawereacquiredbymechanicalpressuregaugesandthenumberof pressuredatapointsreadoutfromapressurechartwouldbeabout100orevenfewerthan that.Theresultsinterpretedfromsuchpressuredataarenotonlysimple,butalsowillnot becontroversial.Today,however,thenumberofdatapointsacquiredbyelectronicpressure gaugesisusuallyasmanyas10,000,oreven1,000,000;theyconsistnotonlyofthepressure buildupintervalbutalsothepressure“wholehistory,”includingallflowandshutin intervalsduringthetesting.Evenveryslightdifferences,ifany,betweenthewelltestinterpretationmodelobtainedfromanalysisandtheactualconditions,thatis,thetestedreservoir andthetestedwell,willbeshownatonceintheverificationprocessduringinterpretationso thatnocarelesserrorisallowed.
1.1.1Welltest:Akindofsystemengineering
Therefore,wecansaythatthewelltesttodaynolongermerelymeansseveralformulasand simplecalculations,butratherisakindofsystemsengineeringthatincludesseveralpartsas follows:
1. Timelyproposingofappropriatetestprojectsbythosepersonsinchargeofexplorationand development.
2. Creatinganoptimizedwelltestdesign.
3. Acquiringaccuratepressureandflowratedataonsite.
4. Interpretingacquiredpressuredatabywelltestinterpretationsoftwareandintegrating geologicaldataandtesttechnique;performingreservoirparametersevaluation.
5. Providingdynamicdescriptionsofgaswellsandgasreservoirsbyintegratingthepressure andproductionhistorydataacquiredduringproductiontestsofgaswells.
6. Creatingnewwelltestmodelswhennecessaryandaddingthemintowelltest interpretationsoftwareforfutureapplication.
1.1.2Welltest:Multilateralcooperation
Theworklistedpreviouslyshouldbecarriedoutbydifferentdepartments;eachofthemis associatedwithothers,andeachoneaffectsthefinalresults:
1. Onlywhenleadersofthecompetentauthoritieshavethoroughlyrecognizedtheimportant roleofwelltestdataindescribinggasreservoircharacterizationandguidingdevelopment ofthegasfieldcantheyarrangetestprojectsinatimelymannerandprovidefinancial supportforsuchprojectstobeexecuted.
2. Onlybyconductingoptimizeddesignscanwegetbetterresultswithlesseffortandacquire pressuredatathatcanexplainandresolveourproblems.
3. Theacquisitionofpressuredataisusuallydonebyservicecompanies.Thetestcrewofthe servicecompany,althoughworkingpursuanttothecontract,shouldrecognizewhatgood dataareandhowtomeetdesignrequirements.Thewelltestsupervisormustcheckdata beforeacceptanceaccordingtothedesignrequirements,toensurethesuccessofdata acquisition.
4. Dataanalysiswillultimatelydemonstratetheapplicationvalueofthetestresults.Inthis book,suchanalysisissummarizedasa“dynamicreservoirdescription,”whichmeans usingdynamicdataacquiredingaswells,suchaspressureandflowrate,asthemainbasis toevaluatethegasproductionpotentialofgaswells,whileatthesametimeprovidinga descriptionofgeologicalconditionswithinthegasdrainageareathataffectgas deliverabilityanditsstability,includingreservoirstructures,reservoirparameters, boundarydistribution,anddynamicreservescontrolledbythisindividualwell,thereby guidingdeliverabilityplanninganddevelopmentplandesignforthegasfield.Thisis usuallyaccomplishedthroughcollaborationbetweenperformanceanalystsandreservoir engineers.Furthermore,onlywhensuchanalysisresultshavebeenapprovedbythe competentauthoritiescantheyplaytheirdueroles.
Aspartofthestudyofgasreservoirsfromdifferentperspectivesordifferentindividual positions,thepurposeofthisbookistoexplainhowtojointlycomprehendwelltestdata andunderstandgasreservoirsfortheirproperdevelopment.
1.1.3Writingapproachesofthisbook
Theapproachesadoptedinwritingthisbookareasfollows:
1. Theapplicationofwelltestmethodsaimsnotonlyatgaswellsbutalsoatgasreservoirs. Analyzingwelltestdatashouldhavethegasfieldorthegasreservoirinmind:itisinfact thegoalthattheauthorstrivesfor.
2. Establishagraphicalanalysismethod.Thebasisofthegraphicalanalysismethodis utilizingfundamentalflowtheories.Createasetofmodelgraphsofthepressurecurveand establishorganicconnectionsbetweenflowcharacteristicsinreservoirsandwelltestcurve characteristicssothatinterpreterscantakea“quicklook,”thatis,tounderstandreservoir conditionsquicklyandconvenientlyfrommeasuredwelltestcurves.
3. Analysisofmanyfieldexamplesisanotherimportantfeatureofthisbook.Thisbook introducesfieldexamplesofwelltestanalysisapplicationsnotonlytogaswellstudiesbut alsotogasfieldstudies;notonlysuccessfulcasesareexaminedbutalsosomefailedones, fromwhichsomelessonsaredrawn,experiencessummarized,andultimatelysuccesses achievedthroughsuchcontinuousexperiences.
4. Althoughsomebasicformulasareintroducedinonechapter,thisbookwillneitherexplain howtoapplythemincalculationnorderivethem.Thisbookiswrittenforthosewho understandtheseformulasandshowshowtomakeinterpretationsusingwelltest interpretationsoftware.Thisbookwillhelpreadersgraspthecorrectinterpretationand analysismethods,especiallytheresearchmethodsforgasfields.Regardingthederivation andapplicationoftheseformulas,someverygoodmonographsareavailableforreference ( JiangandChen,1985; Liu,2008).
Therefore,thisbookisagoodreferenceforwelltestanalysisapplications.Readersare hereinexpected,withthehelpofthisbook,tocomprehendtheessenceofwelltestanalysis, toacquireandapplywelltestdataproperly,andthentocontributeareliabledescriptionto thedevelopmentofgasfields.Itisthepurposeofthisbooktohelpreadersunderstandthe welltestcomprehensively,makeuseofthewelltestproperly,andestablishandconfirm dynamicmodelsofgasfieldscorrectlywiththepowerfulmeansofthewelltest.
1.2Roleofwelltestingasfieldexplorationanddevelopment
Thewelltestisindispensableintheexplorationanddevelopmentofgasfields.Duringthe entireprocess,startingfromwhenthefirstdiscoverywellinanewgasprovinceisdrilled,to verificationofreservesofthegasfield,andtothewholehistoryofitsdevelopmentandproduction,thewelltestplaysveryimportantrolesinmanyaspects,suchasconfirmingtheexistenceofgaszones,measuringthedeliverabilityofgaswells,calculatingtheparametersof thereservoir,designingthedevelopmentplanofthegasfield,andprovidingperformance analysisduringdevelopment.Infact,noneofthesetasksmentionedcanbedonewithout awelltest. Table1.1 indicatesindetailtherolesofawelltestduringthedifferentexploration anddevelopmentstages.
TABLE1.1 Roleofwelltestinexplorationanddevelopmentofgasfieldsa
Test analysis contents
Implementation items
Find out gas bearing conditions in reservoir
Measure formation pressure of the reservoir
Confirm absolute openflow potential by deliverability test
Gas field exploration phase
Developmental preparation phase
Gas field phasedevelopment
DST during drilling process of exploration wells
Completion gas well test of exploration wells
DST and completion gas well test of detailed prospecting wells
Reserves estimation of gas bearing area
Deliverability test and other transient well tests of development appraisal wells
Stimulation treatments such as acidizing and/or fracturing
Production test and extended test of development appraisal wells
Reserves verification of gas field
Numerical simulation of gas field and making development plan
Dynamic monitoring of gas field
Completion gas well test of adjustment wells
★ Items that must be implemented; ☆ Items that may be implemented; Parameters that must be used; Parameters that may be used.
Estimate reservoir permeability by transient well test
Evalute drilling and completion quality by skin factor
Length and flow conductivity of hydraulic fracture
Determine related parameters of double porosity reservoirs
Provide turbulence factor during producing of gas wells
Identify distribution of impermeable boundaries in reservoir
Identify connectivity between wells by interference test
Infer dynamic reserves controlled by individual gas well in gas reservoir
Verify dynamic reserves of gas reservoirs
1.2.1.1Drillstemtestofexplorationwells
Afterdiscoveringapotentialstructureinanewprospectexplorationarea,thefirstexplorationwellsaredrilled.Duringdrilling,theshowofgasandoil(SG&O)maybediscoveredby gasloggingorloggingwhiledrilling.Atthismoment,itisnotcertainwhethertheSG&Oreallymeansthatthosehydrocarbonzonesarethezoneswithcommercialoil/gasflow.Inorder tobecertain,adrillstemtest(DST)needstoberun.Ifthezoneshavequitehighproductivity duringtheDST,afurthertestformeasuringtheirpressuresandflowratesandatransienttest forestimatingtheirpermeabilityandskinfactorshouldbedone.
Highgasproductivityofanexplorationwellforetellsthebirthofanewgasfield,andflow rateandpressuredataacquiredinaDSTarethedirectevidenceofthebirth(Table1.1).
1.2.1.2Explorationwellcompletingtest
Furtherverificationofthescaleandgasdeliverabilityofthegasfieldisgenerallycarried outbywellcompletingtests.Thesetestsareusuallyrunzonebyzonewhenanexploration wellhaspenetratedthetargetbedsandwellcompletionwithcasingorothermodeshasbeen done.Atthismoment,theboreholewallofthewellissolid,thetestconditionsarefairlymature,andthereisenoughtimefortestingsothatvariousparametersofthereservoircanbe estimatedmoreaccurately.Differentflowratescanbeselectedforthedeliverabilitytestso thattheinitialabsoluteopenflowpotential(AOFP) qAOF ofthewellcanbecalculated.
Forsomelowpermeabilityreservoirs,suchasgasreservoirsinCarboniferousandPermian systemsintheOrdosBasin,acommercialflowrateisnotobtainedbyjusttheordinalperforationcompletion,anditisnecessarytorecompletethewellbyundertakingstrongstimulationtreatmentssuchasacidizingand/orfracturing.Inthissituation,reestimatingtheskin factorandfracturingindexisveryimportant(Table1.1).
Sometimestheexpectedgasproductionrateofatestedwelloratestedreservoircannotbe obtainedduringthetestafterperforation;thismaymeanthatthegassaturationisverylowor thereisnogasatallinthereservoir,butitisalsopossiblethatthepermeabilityofthereservoir issolowand/orthatthereservoirnearthewellborewasdamagedsoseriouslythatthegas cannotflowfromthereservoirtothewellbore.Distinguishingtherealreasonsforlowproductionratesisextremelyimportantforevaluationofthereservoir.
Skinfactorisanimportantparameterindicatingifagasproducingwellhasbeendamaged. Importanceshouldbeattachedandmuchattentionmustbepaidtoeverytestedzonein whichatransientpressuretestcanberun,especiallytothosezoneswithhighpermeability andlowpressure,penetratedwithdensedrillingfluidandalongsoaktime,becausethose zonesareprobablydamagedsoseriouslythattheirproductivitiesarereducedtoomuch.In thiscase,acidizingshouldbedonetoremoveorreducethedamage;ifthepermeabilityofthe testedzoneisknownfromthewelltesttobeverylow,saylessthan0.1md,fracturingmaybe necessarytoimproveitsproductivity.
Whetherthetestedzoneneedstobestimulated,andtheeffectsofthestimulationtreatment,arebothidentifiedbythewelltest.
1.2.1.3Reservesestimation
Oncedataonproductionrate,reservoirpressure,andpermeabilityofanexplorationwell haveconfirmedthebirthofagasfield,estimationofreservesofthefieldshouldbecommenced.
Severalissuesworthnotinginreservesestimation
Volumetricmethodsarecommonlyusednowforcalculatingreservesbasedonstaticdata providedbygeophysicalprospecting,logging,andcoreanalysis.Thentheanalogymethodis appliedtoestimatetherecoverablereservesbyusingagivenrecoveryfactor.
However,ithasbeendiscoveredfrompracticeinrecentyearsthatthereisaseriousriskin estimatingreservesdependingonlyonstaticdata;thefollowingproblems,attheleast,need tobenoted:
1. Reservescalculatedbythevolumetricmethodareerroneousforfissuredreservoirswith groupand/orseriesdistributedfissures.
Fissuredreservoirswithgroupand/orseriesdistributedfractureshereinmeantheancientburiedhilltypedfissuredreservoirswithheterogeneouslydistributedfractures;their specialcharacteristhattheoilorgasisstoredinthefissuresystemwitharealandgroup and/orseriesdistributedfractures.Also,somelocalregionsofthesystemhaveveryhigh permeability,andthematrixrockisverytight:thatis,inwelltestterms,adoublemedia withveryhighstorativityratio ω,whosevaluecanbeashighas0.3–0.5.
Thiskindofreservoircanbebestidentifiedbytheshapeofthewelltestcurves:
• Theshapeofpressurebuildupcurves,especiallyofapressurederivativecurve,isoften verystrangeorunusual:itoftenhasnoobviousradialflowportion;itgoesupanddown steeplyandsoshowssharpfluctuations,andthenapproachesatrendofabruptupdipat alatertime.
• Thepressuredrawdowncurvedeclinesrapidly,andthebottomholepressurecannot builduptoitsoriginalvalueaftershuttingin.
• Inmostofthiskindofgaswell,thewatercontentratiorisesquicklyafterwater breakthrough;thepressurebuildupcurvewillbecomemorecomplicatedearlyonif thereiscondensateoilinthereservoir.
2. Estimationofrecoverablereservesinlithologicgasreservoirsformedbyfluvialfacies deposition.
Inthe1990s,manystudiesintheworldshowedthattherecoveryefficiencyofsome lowpermeabilitygasreservoirsformedbyfluv ialfaciesdepositionisquitelow.Further studiesdiscoveredthattheexistenceoflithologicboundarieshinderstheimprovement oftherecoveryefficiencyundertheconditionsofanormalwellpattern(Junkinetal., 1995 ).Itispossibletoimprovetherecoveryeff iciencyofthiskindofreservoirbydrilling infillwells.
3. Integralreservoircharacteristicsshownbypressuredistribution.
Ifallgaswellsinagasfieldarelocatedinanintegralconnectedreservoir,whenmeasuringtheirinitialreservoirpressuresandconvertingthemfromthemeasureddepthsinto correspondingelevationdepths,therelationoftheinitialpressuresofthesegaswellswith thedepthswillbeconsistentwiththepressuregradientmeasuredinanysinglegaswellin thereservoir.Theoverallcharacteristicsofthegasfieldcanbedeterminedbysuchasimple principle.
Ifagasfieldisanintegralreservoir,thecalculationofreservesistypicallyfairlysimple; ifnot,thecausesthereinmustbefoundandanalyzed,combinedcarefullywithitsgeologic characteristicsandreflectedinreservesestimation.Sometimes,thepooraccuracyoftested pressuredatabringsdifficultytoanalysisandidentification,orevenmakestheresearch insignificant.Therefore,payingmoreattentiontothisstudyandacquiringrawtestdata
properlyareundoubtedlythebasisforallevaluationwork.However,iftheformationsin thesamehorizondrilledbyanexplorationwellareindeednotinthesamepressuresystem, reservesestimationmustbeevaluatedfurther.
Roleofwelltestmethodinreservesestimation
Duringtheexplorationstage,welltestdatacannotbeuseddirectlyinreservescalculation, butcansupplementorcorrectittoacertainextent,including:
1. Providingdeliverabilityasabasisofreservescalculation.
Theevaluatedoriginalgasinplaceofgasreservoirsmeansreserves,undertheconditionthattheflowratesofthegaswellsmeet thecommercialflowstandard.Whetherthe wellsmeetthisstandardornotmustbeeva luatedbyawelltest.Sometimesthezones neartheboreholehavebeendamagedseri ouslyduringdrillingand/orcompletion; therefore,thevalueofskinfactor S ofthistypeofwellisveryhighandtheflowrate isratherlow,evenverylow.However,itmetthecommercialgasflowstandardafter stimulationtreatmentforeliminatingthedamage.Also,whetheragaswellhasbeen damagedandhowmuchitsabsoluteopenflowpotentialisafterstimulationmustboth bedeterminedbythewelltest.
2. Providingcharacteristiccoefficientofstabilizedproductionfordoubleporosityreservoirs.
Geologicstudiesveryoftenregardallcarbonatereservoirscontainingfracturesas“doubleporosity”butdonotdistinguishsuchadoubleporosityreservoirfromhomogeneous sandstonesinreservesanalysis.Thisspecialtermofdoubleporositywassuggestedby Barenblattetal.(1960) whenhewasstudyingthemathematicalmodelofwelltestsforfissuredreservoirs,andaflowmodelgraphwasalsogivenbyhim.Barenblattproposedtwo parameters:storativityratio ω andinterporosityflowcoefficient λ,todescribeflowcharacteristicsofthiskindofreservoir.Thestorativityratio ω meanstheratioofhydrocarbon storedinfissurestothatstoredinthewholereservoir,thatis,inbothfissuresandthematrixofit.Thegreaterthe ω,themorehydrocarbonstoredinfissures.Becausethehydrocarboninfissurescanflowveryeasilyintothewellandbeproduced,itisthereforethe fissuresthatbringahighflowrateatthebeginningofproduction.However,ifthe ω value ishigh,astimeelapsesalittlefurther,duetolittlehydrocarbonbeingsupplementedfrom thematrix,thedeliverabilitywilldropsharply;however,ifthe ω valueisverylow,forexample, ω ¼ 0.01orevenlower,whichmeansmorehydrocarbonisstoredinthematrix,the deliverabilityofthereservoirswillbeverystabilized.
Anotherparameter,theinterporosityflowcoefficient λ,isalsoveryimportant.Itmeans theflowconductivityofhydrocarbonfromthematrixtothefissures.Ifthe λ valueisfairly high,whenthepressureinfissuresdecreasesduetothefluidthatflowsintothewell,the fluidinthematrixwillbesupplementedintothefissurespromptlysothatthewellwill maintainstableproduction.However,ifthe λ valueisverylow,evenifquitealotofhydrocarbondoesexistinthematrix,thematrixstillcannotfeedthefissuressufficientlyfora verylongtime,evenaslongasseveralyearsafteranextremelysharpdropoffissurepressure.Forthisreason,suchreserveshavenocommercialvalueatall.
Therefore,forreservoirswithdoubleporositycharacteristics,theparameters ω and λ calculatedfromthewelltestarereallyveryimportantindicesfordiagnosisofthestabilized productioncharacteristicsofthereserves; ω and λ canbedeterminedonlybythewelltest.
Moreover,determinationofthesetwoparametersimposesverystringentrequirementson welltestingconditions,asdiscussedfurtherin Chapter5.
TherearealargenumberoffissuredcarbonatereservoirsinChina.Someoil/gaswells haveveryhighdeliverabilityintheverybeginning.Encouragedbythisphenomenon,field managementpersonnelmaythinkthattheyhavefounda“goldmine.”However,theymay failtoanalyzetherolesofparameters ω and λ properly.Forexample,somewellsstart flowingatarateof100,000m3 ofnaturalgasperday,buttheylastonlyafewdaysand thenaredepleted.Thisisindeedabitterlessontobelearned.
3. Providinginformationaboutplanardistributionofthereservoirforreservesestimation. Ifreservoirsofagasfieldextendcontinuouslyonahorizontalplane,onlytheouter boundarymustbedemarcatedinreservesestimation,resultinginmoreroomformaneuveringinplacingdevelopmentwells.Whenthewellspacingisquitelargeintheearlyexplorationstage,aneffectivethicknessdistributionmapcanonlybedrawnbythemethodof interpolationwithafewthicknessvaluesofdrilledwells,butthismapcannotreflectthe truedistributioncharacteristicsofthereservoir.However,welltestdata,especiallylongtermwelltestdata,canauthenticallyreflectthechangeofextensionofthereservoir.For example,theareaandshapeofablockoil/gasfieldinwhichthetestedwellislocatedcan beconfirmedbywelltestanalysis;thedistanceofthegaswatercontacttothetestedwell locatedinagasfieldwithedgewatercanalsobeestimatedbywelltestanalysis.TaketheJB gasfieldasanexample:theconclusionthatitsOrdovicianreservoirsarewidespreadbut extremelyheterogeneouslywasobtainedfromtheanalysisofpressurebuildupsandinterferencetestsbetweenwellsrunduringshorttermproductiontests;theseresultsprovided powerfulevidenceoftheplanardistributioncharacteristicsofthereservoirs,thusfreeing themanagers’mindsofapprehensionsaboutthereservesultimatelypassingexamination andapprovalbytheNationalReserveCommitteeofChina.Thisexampleisdiscussedin detaillateroninthisbook.
4. Providingoriginalreservoirpressuredataforreservesestimation.
Inadditiontobeingrelatedtostaticparametersofreservoirssuchasarea,thickness, porosity,andgassaturation,thereservesofagasreservoirarealsoproportionaltoitsoriginalreservoirpressure;foroverpressuredgasreservoirs,theinfluenceoftheoriginalreservoirpressureisevenmoreprominent.Therefore,theoriginalreservoirpressuremustbe determinedaccuratelybeforebeginningreservesestimationofagasreservoir.
Itwasrequiredsomewhereandsometimethatthecontrolledreservesofanindividual wellmustbecalculatedusingdatafromeverywelltest.Sucharequirementisimproper, forithastoomuchoversimplifiedreservescalculationfromwelltestanalysisortoomany overestimatedwelltestmethods,andsonousefulconclusionscanusuallybedrawn.
Whenenteringintoapseudosteadyflowperiodatamediumlatestageofdevelopment ofagasfield,manymethodscanbeusedtocheckthereserves.Thisisdiscussedfurtherin moredetaillateroninthisbook.
1.2.2Roleofwelltestinpredevelopment
Thedependenceonwelltestdataatthisstageisdefinitelymoreserious. Aforeigncompany,forexample,decidedtodevelopagasfieldincooperationwitha Chinesepartner.Thereservesofthisgasfieldhadbeenexaminedandverified.Thecompany
insistedonspendingayearoftimeandmuchmanpowerandmoneytoconductdynamictests andanalysisonmorethan10wells.Initially,thenecessityofdoingsowassuspected,butlaterit wasprovedtobeeffective.Itisjustthisdynamicperformanceresearchthatresultsinwhathas becomethedecisivebasisformakingdevelopmentplans.
ManyuncompartmentalizedgasfieldshavebeendiscoveredinChinainrecentyears—the numberofthemismorethanthenumberofthoseeverdiscoveredbefore.Performanceresearchduringthepredevelopmentstageisalsograduallybeingputontheagenda.Itisthereforeespeciallyimportanttofocusonperformanceresearchbasedonpreviousexperiences andlessonslearned.
1.2.2.1Deliverabilitytestofdevelopmentappraisalwells
Deliverabilityvaluesofindividualwellsaretakenastheprimarybasisformakingdevelopmentplans.TheAOFPisusuallyusedtoindicatethedeliverabilitylevel.Theinflowperformancerelationship(IPR)curveisfurtherrequiredtobeplottedfromtheinitial deliverabilityanalysis.
Justasisdiscussedin Chapter3 ofthisbook,severaldeliverabilitytestmethodsareused onsitetodeterminetheAOFP.Thedeliverabilitytestmethodsappliedinexplorationand predevelopmentstagesaredifferent:intheexplorationstage,somesimplemethods,forexample,thesinglepointtestmethod,canbeusedonlyforidentifyingwhetherthedeliverabilityofthegaswellhasmetthecommercialgasflowstandard,andforsettingupthelowerlimit ofitforreservesestimationinthepredevelopmentstage,however,thedeliverabilitytestis notonlyforaccuratecalculationofdeliverabilityindicesandtheplanardistributionofthe reservoirsinthegasfield,butalsoforfindingoutthelong-termstabilitycharacteristicsof thedeliverability.
Itwillbeintroducedin Chapter3 ofthisbookthat,forsomelowpermeabilitylithologicgas reservoirsformedbyfluvialfaciessedimentation,becausetheeffectivedrainageareacontrolledbyanindividualwellislimitedandtheflowabilityofreservoirsispoor,thetransient absoluteopenflowpotentialevaluatedduringtheearlystageofexplorationwouldbevery differentfromthecommonlyreferreddeliverabilityunderstableproductionconditions. Sometimes,suchadifferencecouldbe10timesorevenlarger.SomeChineseandoverseas researchresultssuggestthatifthereservoirsareconfirmedtobelikethis,anewdevelopment strategyshouldbeadopted.Inaddition,thosefissuredreservoirswithgroup-and/orseriesdistributedfracturesinburiedhillgasfieldsobviouslycannotbeputintoproductionwitha conventionallydesignedstableproductionrate.
Therefore,duringthepredevelopmentstage,asystematicandrigorousdeliverabilitytest ofdevelopmentappraisalwellsisessentialforagasfield,especiallyforalargeuncompartmentalizedgasfield.Testanalysisandcalculationsmustnotonlygiveconventionalinitial AOFP,butalsoevaluateandprovidedynamicdeliverabilityindicesduringtheproduction process,andevenprovideaproper“productionratearrangementoveritswholelife”bywell testanalysisanddeliverabilitypredictionconductedbysoftwarewhennecessary(Mattar etal.,1993).
1.2.2.2Transientwelltestofdevelopmentappraisalwells
DevelopmentappraisalwellsinlargeuncompartmentalizedgasfieldsinChinaareusually studiedbyshorttermproductionteststoday.Duringtheshorttermproductiontests,highprecisionelectronicpressuregaugesareusedtomeasureormonitorthebottomholepressure
(flowingpressureandshutinpressure)throughouttheentireprocess.Suchtestsnotonlycan determinethedeliverabilityofgaswells,butalsocanprovideshutinpressurebuildupcurves andtheentirepressurehistory.Justlikewhatisshownin Table1.1,muchimportantinformationaboutagasreservoircanbeobtainedfromthetests,suchas:
1. Informationaboutdistributionofgasbearingareasandgasbearingformationsinthegas fields.
2. Initialreservoirpressure pi.
3. Initialabsoluteopenflowpotentialanddynamicabsoluteopenflowpotentialofmaingas zones,aswellasplanarandverticaldistributionofthedeliverability.
4. Effectivepermeabilityofgaszonesandtherelationshipbetweeneffectivepermeability (fromwelltestanalysis)andpermeabilityfromlogginganalysis.
5. Informationaboutdamageofgaswells,whetheracidizingand/orfracturingstimulation treatmentisneeded,andtheskinfactorafterstimulation.
6. Forfracturedwells,estimationoftheeffectofthefracturingtreatmentandcalculationof thelength,permeabilitythickness,andskinfactorofthegeneratedfracture.
7. Fordoubleporosityreservoirs,whensignificantdoubleporositycharacteristiccurves appear,storativityratio ω andinterporosityflowcoefficient λ valuesareanalyzed, andspecialpropertiesofthereservesandstabilizedproductioncharacteristicsare evaluated.
8. NonDarcyflowcoefficientduringtheproductionofgaswellsisprovided.Inthedesign ofgasfielddevelopmentplan,nonDarcyflowcoefficient D mustbeusedwhenever selectingparametersrelatedtotherelationshipbetweenflowrateandpressure drawdown.NonDarcyflowisformedduetoturbulentflownearthebottomhole,andthe skinduetononDarcyflowisamajorpartofthepseudoskin.Thereasonsresultingin turbulentflowareverycomplicated.NonDarcyflowcoefficient D canonlybe determinedbythewelltest,aserrorsarealwaysgeneratedwhenitisestimatedby theoreticalmethods.
9. Informationaboutreservoirboundariescanbeobtainedifthepressurebuilduptestlasts longenough.Also,ifinformationaboutboundariesisobtainedsoonafterbeginningthe test,itindicatestheboundariesareneartothetestedwell.
Welltestinterpretationsoftwaretodayusuallycontainswelltestmodelscomprising differenttypesofboundarycombinations.Furthermore,numericalwelltestsoftwareis able,byconsideringthespecificgeologicalcharacteristicsofthegaszones,toassemble thereservoirmodelwithpropershapedboundariesandformationparameterdistributionsandtoproviderelatedtheoreticalwelltestcurves.Vividdescriptionsofaspecific testedobjectcanbeobtainedbymatchingthetheoreticalwelltestcurveswithmeasured ones.Itisespeciallyworthytonotethatsuchadescriptioncomesfromthevividexhibitionofgaszonesintheprocessofproductionandsoreflectsthefeaturesmuchcloserto thereality.
10. Ifconditionsallow,theplanarandverticalconnectivityoflayersinthereservoircanbe studiedthroughaninterferencetestbetweenwellsoraverticalinterferencetest.
Aninterferencetestbetweenwellsisverydifficulttoruningasfields.Thisissimply becausethecompressibilityofnaturalgasismuchgreaterthanthatofoilorwater.Moreover,thepermeabilityofgaszonesisusuallyverylowandthewellspacingislarge,so thatasuccessfultestoftentakesalongtime.IntheJBgasfield,forinstance,the
interferencetestbetweenwellL5andotherwellslasted10months.Thistestdelivered extremelyvaluableknowledge:itverifiedtheinterwellcommunicationwithinthegasbearingareaandalsorevealedobviousheterogeneitycharacteristics.
11. Inprinciple,thedynamicreservesofgaswellsandthegasbearingareacanbepredicted onthebasisofthesesuccessfulwelltests.
The“inprinciple”herereferstothefactthatthedynamicreservespredictedbythe resultsofdynamictestsareonlythereservesintheareathathavebeeninfluencedby thedynamictests,butdonotcontainthereservesoutsidethisarea.
Ifagaswellislocatedwithinaclosedornearlyclosedlithologicblock,thereserves withintheblockaffectedbythiswellcanbeestimatedbyanalysisofdynamiccharacteristics.However,datafromthiswellmeannothingforjudgmentofanotherveryclosely adjacentregionpartitionedbytheboundary.
Ifthewellislocatedwithinpartofacontinuouslydistributedreservoir,dynamic datacannotcuttheboundaryoftheregioncontrolledbyanyadjacentwells,and thereforedynamicdatacanonlyprovideinformationaboutthemutualconnectionof thesewells.
1.2.2.3Welltestofpilotproductiontestwells
Ifthepilotproductiontestwellshavealreadybeenconnectedtothepipelinenetworkand socanproducecontinuouslyforseveralmonths,theycanprovidemuchricherinformation thatcanbeusedinthedesignofdevelopmentplans.Inparticular,thedynamicmodelsofthe gaswellscanbeimprovedthroughpressurehistoryverification.
1. Duringalongtermproductiontest,theinfluenceofboundariesaroundthegaswellwillbe graduallyreflectedinthedecreaseofbottomholeflowingpressure.Thedynamicmodelof gaswellscanbeimprovedbyverifyingpressurehistory,addingand/ormodifying boundaryinfluences,adjustingthelocationanddistanceoftheboundarytothewell,and soon.
2. Aperfectmodifieddynamicmodelnotonlyverifiesandconfirmstheformation parametersnearthewellbutalsodeterminestheareaanddynamicreservesofthearea controlledbythewellandsocanbeusedforperformanceprediction.
3. Aperfectmodifieddynamicmodelofagaswellinaconstantvolumeblockcanbeusedto calculatetheaveragereservoirpressureduringproductionandthevariationofdynamic deliverabilityindices.
1.2.2.4Selectionandevaluationofstimulationtreatment
Selectingthestimulationtreatmentmeasureisaverycriticalelementinthedevelopment plan.However,evaluatingwhetheragaswellneedsstimulationtreatmentandtheeffectivenessofsuchstimulationtreatmentcanonlybedonebywelltestanalysis.Insomeforeign countries,thefieldownermust,whenengagingaservicecompanytoimplementgaswell stimulationtreatment,firstprovidetheparametersofitsgeologyandcompletionandthose fromwelltestevaluationofthewell,sothatthestimulationmeasurecanbedesigned;after stimulationtreatment,inordertoevaluatetheeffectivenessofthetreatment,theownermust alsorequestthirdparties,suchasawelltestservicecompanyorrelevantconsultingcompany,toappraisetheresultsofthetreatmentbywelltestanalysis.
1.2.2.5Verifyingreservesandcreatingthedevelopmentplan
Onlyaftercompletingtheperformanceanalysisandresearchmentionedearlierdoesthe timeforcreatingtheformaldevelopmentplanreallycome.
1. Thereserveshavebeenverifiedbyperformanceresearch,inwhichparametersprovided byatransientwelltestwereused.
2. Reservoirparametershavebeencorrected.Permeability k,forexample,isnotthe permeabilityfromlogginginterpretationbuttheeffectivepermeability;skinfactor S,nonDarcyflowcoefficient D,doubleporosityparameters ω and λ,andsoonarealsothe parametersactuallyacquiredfromtheformation.Inaddition,thedescriptionofreservoir boundariesisaparticularlyverycriticalconditionfornumericalsimulation.
3. Theproductiontesthistorycanbeusedtomatchandcorrecttheparametersusedfor numericalmodeling.
Whentherequirementsmentionedhereareallmet,anumericalsimulationstudycanbe carriedoutandapracticalandfeasibledevelopmentplancanbemade.
1.2.3Roleofwelltestindevelopment
Conventionalwelltestmethodscanbeusedalmostthroughouttheentiredevelopment processofagasfieldtoprovidedynamicmonitoring,withoutanydifficultiesbroughtfrom swabbingandsoon,suchasisthecaseinoilfields.
Foranormallyproducinggaswell,however,unlesspermanentbottomholepressure gaugesareused,itisobviouslyinappropriatetoperformawelltestbyoperationsthatrun pressuregaugesintheholeandputthemoutofthehole,whilefrequentlyopeningandshut inthewell.Infact,becausetheformationconditionshavealreadybeenknownthoroughly throughearlyresearch,retestingthewellisrequiredonlyinthecaseofanomalousevents happeningduringproductionofthegaswell.
However,thefollowingtestsareabsolutelynecessary:
1. Regularmonitoringofdownholeflowingpressureandstaticpressureforinferring dynamicdeliverabilityindicesofthegaswell.
2. Fornewlydrilledadjustmentwells,thebasicformationparametersmustbeobtainedfrom welltestanalysisandtheirinitialdeliverabilityequationmustbeestablishedbefore puttingintoproduction(see Table1.1).
Itisnotedthroughtheaforementionedanalysisthatitemslistedatthetoprightcorner in Table1.1 areblank,meaningthattheseitemsarenotfeasible.Moredeepeningorintensiveunderstandingofthereservoircanonlybeobtainedthroughwelltests,asthegasfield researchisbeingdeepenedcontinuously.Itisnotpracticaltoexpectthatalltheseparameterscanbedeterminedsimplythroughwelltestsduringtheearlystageofexploration. Forexample,itisimpossibletodeterminetheexactinitialAOFPofanexplorationwell simplythroughshorttermDST;itisalsoimpossibletodoanoverallanalysisofboundariesortodeterminethedoubleporosityparametersofreservoirssimplythroughvery shorttermwelltestsintheexplorationwells.Evenifwelltestanalystsdogivethoseparametersmentionedpreviously,suchparame tersaremerelyspeculativeandcannotsufficeasabasisforfurtheranalysis.However,asmoregaswellsareputintoproduction testsorproduction,andastheflowingofthesewellsgoesonandtheradiusofinfluence
increases,andpressurebuilduptestinglastingquitealongtimeiscarriedout,theresearch workwillcontinuetointensify.Someparameters,whichcouldnotbeobtainedpreviously, canandshouldbedeterminedthroughwelltes tanalysisatthistime;suchparametersincludeinitialanddynamicdelivera bilityindices;boundarydistance Lb andshape;block sizes A;doubleporosityparameters ω, λ;doublepermeabilityformationparameters κ ; compositeformationparameters Mc and ωc ;nonDarcyflowcoefficient D ;reservoirconnectivityparameters ε and η;andthedynamicreservesofblock.Withthisknowledge,the gasreservoirdynamicmodelcanbeestablishedandusedeffectivelyfortheperformance analysisofgaszonesandgasreservoirs.T hesearejustthephasedandcomprehensive characteristicsofawelltest.
1.3Keysofwelltestanalysis
Welltestresearchstartedinthe1930s.Bythe1970–80,itevolvedintothe“modernwell test.”Throughadvancesintheoreticalresearchonflowmechanicsandcontinuousimprovementsinwelltestsoftware,theroleofthewelltestingasfieldexplorationanddevelopment expandsanddeepenscontinuously.
Whatarethekeyelementsofwelltestresearch?Whathasbeendrivingtheadvancesand developmentofwelltestresearch?Howdoesthewelltestservegasfieldstudies?Allthese questionsareroughlyansweredin Fig.1.1.
1.3.1Directandinverseproblemsinwelltestresearch
Welltestresearchroughlyresolvestwotypesofproblems:directandinverse.
Direct problem
Formations
Geologic classification
Percolation mechanics classification
Establish well test model mathematical model physical model
Analytic solution numerical solution
Pressure/time plot
Cartesian plot
Semilog plot
Log-log plot
Reverse problem
Parameters and results
FIG1.1 Illustrationofwelltestresearchcontents.
Identify reservoir type by pressure history match
Well test interpretation
Model analysis parameter estimation
Plot curves
Cartesian plot
Semilog plot Log-log plot
Examine test data delete anomaly data
Acquire pressure and rate data in gas field
Repeat interpretation
Directandinverseproblemsaredefinedfromtheviewpointofinformationtheory.A directproblemmeansdescribingtheperformanceofaknownformationintermsofitsgas productionrateandreservoirpressureonthebasisofflowmechanicstheory,whereasresolvinganinverseproblemmeans,ifthevariationsofgasproductionratesandbottomholepressuresofoneorseveralwellsinagasreservoirduringtheirflowingandshutinprocesshave beenmeasured,findingoutinverselythestaticconditionsofthegaszones,includingthe valuesofformationparameters,thestructureofpermeableareasinthereservoir,theplanar distributionofgaszones,andsoon.
Thisbookexplainstheprocedureofresolvingtheseproblemsbywelltestresearchwiththe hopethatreadersofthisbook,especiallythoseinterestedinparticipatinginwelltestresearch, cancorrectly“locate”thejobstheyareparticipatinginorareinterestedinandstraightenout therelationshipbetweenwelltestresearchandgeologicresearch.
Earlywelltestresearchfailedtodistinguishdifferenttypesofformationsorbelievedthat allformationswere“homogeneousmedia”identically.Thesemilogstraightlineanalysis methods[Miller-Dyes-Hutchison(MDH)methodandHornermethod]inventedinthe 1950sfoundthatflowwillentertheradialflowstagethatreflectsreservoirconditionswhen wellborestoragedisappears;inthisstage,pressurevariationshowsastraightlineonsemilog paper:thatis,thecoordinatesofpressurevslogarithmoftime,andaninverseproportion relationshipexistsbetweentheslopeofthestraightline m andtheformationpermeability k:
Thisissimplythebasisofthe“conventionalwelltestinterpretationmethod”thatusesthe welltestmethodtodetermineformationparametersinversely(Milleretal.,1950; Horner,1951).
However,measuredcurvesarefarmorecomplicated,especiallyinthecaseofcarbonate formations,multilayerformations,orformationswithcomplexboundaries;inthesecasesitis oftenverydifficulttofindoutproperstraightlineportions.Furthermore,straightlinesectionsalonecanhardlydescribeothercharacteristicparametersofthereservoir.Therefore, inthe1970s,thetypecurvematchmethodwascreated(Agarwaletal.,1970; Gringarten etal.,1979; BourdetandGringarten,1980; Earlougher,1977).
Intheearly1980s,Bourdetinventedpressurederivativetypecurves(Bourdetetal.,1983). Onthistypeofcurve,eachkindofflowintheformationcorrespondstoaspecialcharacteristicpattern,whileeachkindofflowisdeterminedbythespecialgeologicalconditionsofthe specificformation.Therefore,anorganicconnectionisestablishedbetweenthegeologiccharacteristicsandthegraphicalcharacteristics.
Sofar,thecombinationoflog-loganalysis(i.e.,pressureanditsderivativetypecurve matchanalysis)andsemiloganalysis(i.e.,conventionalanalysismethod)hasformedthe dominanttheoreticalfoundationofmodernwelltestinterpretationandhasbecomethedominantanalysismethodofwelltestinterpretationsoftware.Awidevarietyofcalculationformulasandanalysisplotswereusedbefore—providedtheycanbeintegratedintothemodern welltestinterpretationmodel,theycanbeaddedintotheinterpretationsoftwareandwidely used.However,asanalystsbecomemoredependentonwelltestanalysissoftware,some othermethods,suchastheY-functionmethodforjudgingthepresenceoffaults,theMasket methodforcalculatingformationparameters,andvariousuniquepointmethodsfor conductinginterferencetestanalysis,areincreasinglylosingtheirchanceofbeingused.
1.3.2Howtounderstanddirectproblems
Theprocessofestablishingtherelationshipbetweencharacteristicsoftheformationand thoseofwelltestplotsstartsfromsolvingthedirectproblem.Theresearchtasksinresolving thedirectproblemcanbesummarizedinseveralparts,asdescribedinthefollowingsections.
1.3.2.1Analyzingtheformationwheretheoil/gaswelllocatesandclassifyingit geologically
ThegeologicbodiesacrossChinawherethegasfieldslocateareverycomplicated;their roughclassificationisgivenin Table1.2.Foreasycomparison, Table1.2 alsoliststypicalexamplesofgasfieldsinChina.Infact,therearefarmoretypesofgasreservoirsthanthese,and evenmanydifferenttypesmayexistsimultaneouslyinonegasfield(Wang,1992).
1.3.2.2Classifying,simulating,andreproducingformationfromtheviewpointofflow mechanics
Itisseenthatthegeneratingconditionsofvariousreservoirsareverydifferent;ifdescribed byflowmechanicsequations,theymustbesimplifiedandclassifiedintosomemajorcategories,andthedescriptionmustbeusedonlywithinacertainscope.Sandstonereservoirs,for example,areusuallysimplifiedintoamodelofaninfinitelyhomogeneousporousmedium.
TABLE1.2 Typesandexamplesofgasproducingzones.
TypeofgasreservoirTypicalexamplesofgasfieldinChina
LargehomogeneoussandstonesTN,SB,andYA13-1gasfield
FaultdissectedlocalhomogeneoussandstonesFaultblockgasfieldsinSL,LHandZY oilfields,HTBgasfield
RockssomepartsofwhicharehomogeneoussandstonesP5gasfield
CarbonaterockssomepartsofwhicharelikehomogeneousSomeareasofsouthregionofJBgasfield
CarbonaterocksshowingsignificantheterogeneityMostareasofJBgasfield
Fissuredcarbonaterocksshowingsignificantdoubleporosity characteristics L5AreaincenterregionofJBgasfield
ExtremelythickfluvialfaciessedimentarysandstonesKL2,DN2,andDBgasfield
Thinlayersandstoneshavinglithologicboundariesformedby fluvialfaciessedimentation
Carboniferous/permiangasfieldsinOrdos basin
CondensategasfieldsinfaultdissectedsandstoneYHandYTKgasfield
GascapsinsandstonewithoilringsandedgeorbottomwaterQLandXLTgasfield
Carbonaterockswithgroupor/andseriesdistributedfracturesQMQ,SQ,andCNYACgasfield
BiohermallimestonebodiesLJZ,PFW,andSLgasfield
VolcanicmassifsoferuptivefaciesXSgasfieldinDQoilfield
Offshoreshoal,sandbarDFgasfield
Strictlyspeaking,theexistenceofthistypeofreservoirinnatureisimpossible.However,the welltestingdurationislimitedandsotherangeofpressureinfluenceisalsolimited;therefore,withinsuchlimitedscopesoftimeandspace,thetargetbeingstudiedcanbeconsidered roughlyconsistentwithaninfinitelyhomogeneousformation.
Basedontheknowledgementionedpreviously,reservoirscanbefurthersimplifiedand classifiedfromtheviewpointofflowmechanics,asshowninthefollowinglists.
Basicmediumtypes
• Homogeneousmedium,includingsandstones,fissuredcarbonaterocksshowing homogeneousbehavior,etc.
• Doubleporositymedium,includingsandstonesandcarbonaterockscomprisingnatural fissures
• Doublepermeabilitymedium,mainlymeanslayeredsandstones Thesemediaareusuallyassumedtohavelaminartwodimensionaldistribution.
Bottomholeboundaryconditions(i.e.,innerboundaryconditions)
• Generalcompletionconditionofwellborestorageandskin
• Completionconditionofhavinghydraulicfractureconnectingthewellhole
• Partiallyperforatedcompletionconditions
• Completionconditionsofhorizontalwellsordeviatingwells Outerboundaryconditions
• Infinitelyouterboundary
• Impermeableouterboundariesofsinglestraightlineorofsomepatternsformedby severalimpermeableboundaries
• Closedouterboundary:closedsmallfaultedblocksorlithologictraps
• Heterogeneousboundariesformedbyvariationoflithologyorfluidproperties
• Semipermeableboundaries,congruentboundariesofriverchannelsformedbyfluvial faciessedimentationindifferentperiods
• Constantpressureboundaries(inoilreservoirsonly) Assumptionoffluidproperties
• Oil,gas,water,orcondensategas
• Anycombinationofoil,gas,andwater
Anyassemblageofanyfourelements,eachoneofthemhavingbeenselectedfromone ofthefouraforementionedconditions,constructsaphysicalsimulationforacertaingas reservoirandreproducesthebehaviorofaspecificgasfieldduringtheresearchprocess.
1.3.2.3Constructingthewelltestinterpretationmodelandresolvingtherelated problem
Thesocalledwelltestinterpretationmodelshouldcontainbothaphysicalmodelanda mathematicalmodel.
Thelistsgivenin Section1.3.2.2 arejustthedescriptionsofphysicalmodels.Atthesame time,thesephysicalmodelscanalsobeexpressedinmathematicalforms.Forexample,the flowindifferenttypesofmediacanbeexpressedbydifferentdifferentialequations;different boundaryconditionscanalsobeexpressedbydifferentmathematicalexpressions.Theseare thesocalledmathematicalmodels.
Inthe1960s,thephysicalmodelsmentio nedearlierwerematerializedduringthe studyofwelltestproblems.Manmadesandstonebodieswerebuiltandusedinthelaboratoryasareducedphysicalmicrominiatureformationormodel.Themodelwassaturatedwithoilorwater,andtheflowratechangewasimplementedbydrillingholesin themodel.Thepressurechangeatindividu alpointsonthemodelwasmeasured.Sucha practice,however,notonlywasverydifficultinconstructingthemodelandverycostly, butalsocouldhardlysimulatetheelastictr ansientprocess.Therefore,itwasabandoned longago.
Establishingthemathematicalequationscorrectlyisonlythebeginningoftheprocessof resolvingdirectproblems,whilesolvingtheseequationsisreallymoreimportant.
Inthepast,basicallytheanalyticmethodwasappliedforresolvingtheequations.Because theseequations,underthehypothesisofDarcy’slaw,aremostlypartialdifferentialequations,mathematicalmanipulationmethodssuchastheLaplacetransformmusttherefore beusedtoconvertthemintoordinarydifferentialequationsintheLaplacespaceforresolving themandthentoinvertthesolutionbackintotherealspace.Moreover,thesolutionprocedurewasusuallyaimedonlyatsomerelativelysimpleboundaryconditions,suchascircular andsquareshapes.Todaytheseequationscanstillberesolveddirectlybynumerical methods,butittakesafairlylongtimeandsometimesmodeladjustmentcannotbecompleted immediately.
Theoreticalresearchonthewelltesthasmostlytargetedconstructionofthewelltestmodel andresolvedtherelatedmathematicalproblemasmanyfamousresearcherssuchasvan Everdingen(1949),Agarwal(1970), Earlougher(1977), Gringarten(1979),and Bourdet (1989) did.Itcanbesummarizedas:
Identifytypicalgeologicmodel ! Constructwelltestmodel ðformulateits mathematicalequationsÞ! Resolvetheequationsbyanalyticornumerical method ! Drawpressurevariationcurve,thatis,maketypecurves
Thisisthewholeprocessofresolvingdirectproblems.
1.3.2.4Expressionformsofresolvingdirectproblemsinwelltest
Theultimateexpressionformoftheresearchresultsofresolvingdirectproblemsinthe welltestistypecurves:thatis,theplotoftherelationbetweenpressureandpressurederivativeandtime,usedforwelltestanalysis.Forinstance,thetypecurveforhomogeneousreservoirsisasshownin Fig.1.2.
Differenttypecurvesfordifferentformationshavebeenobtained.Boththetheoretical modelcorrespondingtotheactualformationandtheparametersoftheformationcanbe obtainedthroughthetypecurvematch.Thisisthetheoreticalbasisofunderstandingtheformationsusingthewelltestmethod.
1.3.3Describinggasreservoirswithwelltestanalysis:Resolvinginverseproblem
Thegreatmajorityofengineersinvolvedinwelltestingaretryingtoresolvetheinverse problemsratherthanresolvingthedirectproblemsdiscussedearlier.Whatresolvinginverse problemsmeansis:
FIG.1.2 Welltestinterpretationtype curveforhomogeneousreservoirs.
Makewelltestingarrangementandwelltestdesignbasedontherequirements ofgasfieldexplorationanddevelopment ! Acquirepressureandflowrate dataonsite ! Analyzeorinterpretacquireddata ! Analyzeorinterpretgas reservoircharacteristicsandcalculatereservoirparameters ! Constructthe dynamicgasreservoirmodeltobeusedforgasreservoirdescription
Thisprocesswillbeexplainedherestepbystep.
1.3.3.1Welltestdesign
Justasacompletesetofdesigndrawingsofabuildingmustbeavailablebefore constructing,carefuldesignmustbedonebeforeperformingwelltestresearchofagasfield. Welltestdesignincludesthefollowingsteps:
1. Definetheproblemtoberesolvedbythewelltestaccordingtotherequirementsofgasfield research.Forexample,makeclearthedeliverabilityofthegaswell,calculategasreservoir parameters,understandboundarysituationsorconnectivitybetweenwells,andsoon;see Table1.1.
2. Properlyarrangejobsoftestingandroughtimeschedulesforexistinggaswellsandthose gaswellsexpectedtobecompleted.
3. Maketestdesignsforeachspecifictestedwell.
• Collectgeologicandloggingdata
• Collectdrillingandcompletiondata
• Simulatethetestwithwelltestinterpretationsoftwaretogetwelltestcurves
• Makewelltestoperationplan,includingtimeschedule,typesofinstrumentstobeused, methodanddepthofrunninggaugesinhole,requirementsofdataacquisition,andsoon
• Wellflowingandshutinduration,flowratesarrangement,measuringrequirements, andsoon.
