DRILLBITSELECTION

Oneofthemoreconfusingaspectsofoilwelldrillingtotheyoungandoldalikeisthenumeroustypesofdrillbits.Thelargestmanufacturerofdrillbitsmakes34typesand33sizes(diameters)andmanymoreonrequest.IADCorganizationhasasystemofassigningadesignationcodetoeachbittype.ROLLERCONESeriesTypeFeatureFeature(2)ROLLERCONEExample

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5=codefor"insertbits“2=codefor"softtomediumformation“2=codefor"rollerbearingaircooled“R=codefor"reinforcedwelds"

FIXEDCUTTERSCuttertypeandbodymaterial,Cross-sectionprofile,Hydraulicdesign,andCuttersizeanddensityFIXEDCUTTERSFirstcodedesignationsareD-naturaldiamond/matrixbodyM-polycrystallinediamondcutters/matrixbodyS-polycrystallinediamondcutters/steelbodyT-thermallystablepolycrystallinediamondcutters/matrixbodyO-otherSecondcodedesignationsarelongtaper/deepconelongtaper/mediumconelongtaper/shallowornocone(parabolic)mediumtaper/deepconemediumtaper/mediumconemediumtaper/shallowornocone(rounded)shorttaper/deepcone(inverted)shorttaper/mediumconeshorttaper/shallowornocone(flat)Thirdcodedesignationsarebladed/changeablejetsbladed/fixedportsbladed/openthroatribbed/changeablejetsribbed/fixedportsribbed/openthroatopenface/changeablejetsopenface/fixedportsopenface/openthroatalternativecodesradialflowcrossflowotherFourthcodedesignationsarecuttersize;large,medium,andsmallcutterdensity;light,medium,heavyExampleCode:M582M=codefor"polycrystallinediamondcutters/matrixbody““5=codefor"mediumtaper/mediumcone“8=codefor"fixedports/openface“2=codefor"mediumcuttersize/lightcutterdensity"DrillBitCharacteristicsTeethlengthandsizeNumberofteeth(indexing)ConeoffsetConebearingtypeHeelrowteethconfigurationGaugeconfigurationJournalshaftangle(howflatorroundthebottomholepattern)TheteethlengthandsizeandconeoffsetaffecttheROPThenextthreeitemsonthelisthavealargeaffectonthebitlife.Item7affectsholestraightness.Rollerconedrillbitcharacteristicsofimportanceinregardtoselectionare:ROCKFAILUREMODELSTworockfailuremodelsarerecognized:thetensionandshearIthasbeenarguedthatthesheartypeactionismoreefficientthanthetensionmodel.DRILLBITSELECTIONCRITERIAApopularequationforcostcomparisonsisthetotalcostperfootequation.Ifthebitwereoperatedtoitsoptimallifeatitsoptimumweightandspeed,thenthecostwouldbethelowestcostperfootforthebit.IsthebitlifeifthebitisterminatedbyBearingfailure,Totaltoothwear,orThereductionofitsdrillingratetoanuneconomicalone.p.s.Formationchangesoftenmaskthelatteroptimallifeandcaremustbeexercised.OPTIMALLIFEExampleFollowingperformancedataisgiven:ITEMSTEELTOOTHINSERTTOOTHBitCost$277.88$1521.32Trip6:18hrs/min6:32hrs/minRotationTime10:01hrs/min58:42hrs/minDownTimeNoneNoneBitFootage180ft880.5ftRigCostRate375$/hrSameFractionToothWear1/2NotApplicableFractionBearingWear6/88/8AverageDrillingRate18ft/hr12.5ft/hrBitSize7-7/8in.7-7/8in.BitWeight30,000lbs40,000lbsRotarySpeed85rpm60rpmTheplotshowsthathadthesteeltoothbitbeenpulledatitsoptimallife(6hoursand34minutes)itwouldhavehadalowercostperfoot:$34.05/ftversus$35.47/ft.TriptimesmaybetakenfromtheIADCtoursheetormaybetakenfromtheGeolographunitorsomeotherdevice.Achartversusdepthmustbeconstructedforeachrigforultimateanalyses.TRIPTIMEExampleAbitisrunatadepthof8,000feetandpulledatadepthof8,500feet.Howmanyhoursisthetriptimeforthisbit?TripT=(5.2+5.7)/2=5.45hrFromthechartaroundtripsat8,000feetand8,500feetrequire5.2hoursand5.7hours,respectively.OPTIMALWOB&RPMTducethelowestdrillingcostperfoot2.controlthedirectionofthedrillhole3.findoverpressuredzones4.satisfyequipmentlimitations(solidsremoval,BHA,etc.)5.developasatisfactorycuttingsdescriptionlogCONTOURMETHODThemethodconsistsofconstructingthepreviousfigureinatwodimensionalpresentation.ExampleSixinsertbitshavebeenrunandtheirperformanceandtotalcostperfootdataarelistedinthetable.BIT#ROT.TIMEFOOTAGETCFHr:Min$/FT158:4273435.28239:0536051.513101:2185548.99462:0761444.24547:1651741.27656:3365838.14TheoptimalWOBandRPMare40,000lband50rpm($35.00/ft)ANALYTICALMETHODThesameprocedurecouldbeusedfordeterminingoptimalWOB,andRPMforthesteeltooth,rollerbearingbit;however,anGalleandWoods’’analyticalmethodisavailableforthesebits.DEFINITIONOFTERMSBS=Bitdiameter;inchW'=Actualweightonbit;1000poundsW=Equivalentweightona7-7/8inchbit;1000poundsTT=Triptime;hrsR=Costrateofthedrillingoperation;$/hrsT=Time;hrsG=BitCost+TripCost;$TC=Tripcost;R*TT;$D=Fractionoftoothheightwornaway;8thsB=Fractionofbearinglifeconsumed;8thsDT=Downtime;hrsRT=Rotationtime;hrsF=Bitfootage;ftGalleandWoodspublishedthefollowingrelationshipsfortherateoftoothwear.TOOTHWEARA= CorrelationcoefficientfortoothwearandiscalledtheformationabrasivenessfactorIntegrationofthetoothwearequationgivesbitrotationtimeToothwearEquationRotarySpeed,N=80BEARINGWEARS= correlationcoefficientcalledthedrillingfluidfactorOtherspublishedthefollowingrelationshipfortherateofbearingwear.RotarySpeed,N=80RATEOFPENETRATIONC= correlationcoefficientcalledtheformationdrillabilityfactorThevaluesofandaredeterminedfromdrill-offtestandrangebetween.6<<1.0.4<<1.0p= 1.0ifflattoothwear(non-sharpening)p= 0.5ifselfsharpeningBitfootage(F)isequaltoRateofPenetrationxRotationTimeEquationofROPSofar,RTisdeterminedeitherbysettingD=1(completebittoothwear)orB=1(completebearingwear)Whichevercomesfirst.DRTFDRTFDRTFDRTFDRTFDRTFDRTFRotarySpeed,N=80ThetotalcostperfootequationisDRTFTCFDRTFTCFDRTFTCFDRTFTCFDRTFTCFDRTFTCFRotarySpeed,N=80TheparametersA,C&Sareusuallycomputedwithdatagatheredfromothersimilardrillbitswhichwereruninsimilarformations,atsimilardepths,withsimilarbottomholeassemblies,withsimilarhydraulics,withsimilarmuds,andsimilarrigs.Nodirect