Chapter 5-Formation Evaluation

CHAPTER5–FORMATIONEVALUATION518551955205521552255235524552555265527552855295530553155325533553455355536553755385539554055415-100-50050100SPCONTENTS1.MudLogging2.Coring3.Open-holeLogging4.LoggingWhileDrilling5.FormationTesting6.CasedHoleLoggingFormationEvaluationWhatisFormationEvaluation?FormationEvaluation(FE)istheprocessofinterpretingacombinationofmeasurementstakeninsideawellboretodetectandquantifyoilandgasreservesintherockadjacenttothewell.FEdatacanbegatheredwithwirelinelogginginstrumentsorlogging-while-drillingtools.Studyofthephysicalpropertiesofrocksandthefluidscontainedwithinthem.Dataareorganizedandinterpretedbydepthandrepresentedonagraphcalledalog(arecordofinformationabouttheformationsthroughwhichawellhasbeendrilled).FormationEvaluationWhyFormationEvaluation?Toevaluatehydrocarbonsreservoirsandpredictoilrecovery.Toprovidethereservoirengineerswiththeformation’sgeologicalandphysicalparametersnecessaryfortheconstructionofafluid-flowmodelofthereservoir.Measurementofinsituformationfluidpressureandacquisitionofformationfluidsamples.Inpetroleumexplorationanddevelopment,formationevaluationisusedtodeterminetheabilityofaboreholetoproducepetroleum.MudLoggingMudlogging(orWellsiteGeology)isawellloggingprocessinwhichdrillingmudanddrillbitcuttingsfromtheformationareevaluatedduringdrillingandtheirpropertiesrecordedonastripchartasavisualanalyticaltoolandstratigraphiccrosssectionalrepresentationofthewell.Providecontinuousrecordofpenetrationrate,lithologyandhydrocarbonshows.Theseinformationsupportswirelinelogdata.Fromthecuttings,anoilstainsorodorofoilmaybedetected,becomeanexcellentqualitativeindicator.Thefluorescentlampisalsoagreathelpindetectingoilshows.MudLoggingThegasrecordandlithologicalsampleareplottedalongwithsurfaceparameterssuchasrateofpenetration(ROP),WeightOnBit(WOB),rotationperminuteetc.onthemudlogwhichserveasatoolforthedrillingengineersandmudengineers.Someproblem:adiscrepancybetweenthetimetherockwasdrilledandthetimeitreachedthesurface–particularlyfordeepwells,whereittaketwoormovehourstoreachthesurface.CoringOnewaytogetmoredetailedsamplesofaformationisbycoring,whereformationsampleisdrilledoutbymeansofspecialbit.Thissamplecanprovide:Detailedlithologicaldecscription.Porosity,permeability,fluidsaturationandgraindensity.Theseparametersaremeasuredinthelaboratoryandserveasabasisforcalibratingtheresponseoftheporosityloggingtoolsandtoestablishaporosity/permeabilityrelationship.CoringTwotechniquescommonlyusedatpresent.Thefirstisthewholecore,acylinderofrock,usuallyabout3to4indiameterandupto50feet(15m)to60feet(18m)long.Itiscutwithacorebarrel,ahollowpipetippedwitharing-shapeddiamondchip-studdedbitthatcancutaplugandbringittothesurface.Takingafullcoreisanexpensiveoperationthatusuallystopsorslowsdrillingoperation,andcanbedoneonlybeforethedrillinghasbeendone.CoringTool&CoreBarrelCoringTheother,cheaper,techniqueforobtainingsamplesoftheformationisSidewallCoring.Inthismethod,asteelcylinder—acoringgun—hashollow-pointsteelbulletsmountedalongitssidesandmooredtothegunbyshortsteelcables.Thecoringgunisloweredtothebottomoftheintervalofinterestandthebulletsarefiredindividuallyandthecorewillberetrieved.Advantagesofthistechniquearelowcostandtheabilitytosampletheformationafterithasbeendrilled.CorePreservationOncethecoreisretrievetosurfacethenitisimportantthatitshouldremainasunchangedaspossible.Thecoreshouldbepreventedfromdryingout,comingintocontactwithoxygenorbeingmechanicallydamaged.Corebarrelisfilledwithresintopreventthecorefrommovingandtominimizetheexposedsurfacearea.Freezingthecoreinfreezercontainers.Coresampleiswrappedinaplasticfilm,aluminiumfoilandthendippedinmoltenwax.CoreAnalysisCanbedividedintotwocategories:ConventionalCoreAnalysis.SpecialCoreAnalysis.ConventionalCoreAnalysis.Thecoreisusuallyslabbed,cutlengthwisetomakethestructurevisible.Providesinformationonlithology,residualfluidsaturation,ambientporosity,ambientgaspermeabilityandgraindensity.GasPermeameterLiquidPermeameterPorosimeterCoreAnalysisSpecialCoreAnalysis:Providesthefollowinginformation:Porosityandpermeabilityatelevatedconfiningstress.Electricalpropertiessuchasformationfactorandresistivityindex.Capillarypressure.Wettabilityandrelativepermeability.Mechanicalrockpropertiessuchascompressibility.Waterfloodsensitivityforinjectivityandwellperformance.Open-holeLoggingOpen-holelogging,alsoknownaswellloggingisthepracticeofmakingadetailedrecord(awelllog)ofthegeologicformationspenetratedbyaborehole.OpenholelogsarerunbeforetheoilorgaswellislinedwithpipeorcasedPrincipalofWellLoggingAwelllogisarecordofcertainformationdataversusdepth.Theappropriatedownholeloggingtoolsinstrumentcalled‘sonde’,about3.5inchesindiameterisloweredintomud-filledholeonloggingcable.Thistoolswillmeasuretheelectrical,acoustic,andradioactivepropertiesoftheformation.Theresultwillbeanalyzedtodeterminewhichofthelayersareporousandpermeable,andlikelytocontainhidrocarbon.Adepthcalibrationwheelrecordsthelengthofcableinthehole.PrincipalofWellLoggingSurveyisnormallydone