DRILLBITSELECTION

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

Code:522R

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-other

Secondcodedesignationsarelongtaper/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,heavy

Example

Code:M582

M=codefor"polycrystallinediamondcutters/matrixbody“ 5=codefor"mediumtaper/mediumcone“ 8=codefor"fixedports/openface“ 2=codefor"mediumcuttersize/lightcutterdensity"

DrillBitCharacteristics

TeethlengthandsizeNumberofteeth(indexing)ConeoffsetConebearingtypeHeelrowteethconfigurationGaugeconfigurationJournalshaftangle(howflatorroundthebottomholepattern)TheteethlengthandsizeandconeoffsetaffecttheROPThenextthreeitemsonthelisthavealargeaffectonthebitlife.Item7affectsholestraightness.

Rollerconedrillbitcharacteristicsofimportanceinregardtoselectionare:ROCKFAILUREMODELSTworockfailuremodelsarerecognized:thetensionandshearIthasbeenarguedthatthesheartypeactionismoreefficientthanthetensionmodel.DRILLBITSELECTIONCRITERIA

Apopularequationforcostcomparisonsisthetotalcostperfootequation.Ifthebitwereoperatedtoitsoptimallifeatitsoptimumweightandspeed,thenthecostwouldbethelowestcostperfootforthebit.IsthebitlifeifthebitisterminatedbyBearingfailure,Totaltoothwear,orThereductionofitsdrillingratetoanuneconomicalone.p.s.Formationchangesoftenmaskthelatteroptimallifeandcaremustbeexercised.

OPTIMALLIFEExampleFollowingperformancedataisgiven:

ITEM

STEELTOOTH INSERTTOOTH

BitCost $277.88 $1521.32 Trip 6:18hrs/min 6:32hrs/min RotationTime 10:01hrs/min 58:42hrs/min DownTime None None BitFootage 180ft 880.5ft RigCostRate 375$/hr Same FractionToothWear 1/2 NotApplicable FractionBearingWear 6/8 8/8 AverageDrillingRate 18ft/hr 12.5ft/hr BitSize 7-7/8in. 7-7/8in. BitWeight 30,000lbs 40,000lbs RotarySpeed 85rpm 60rpm

Theplotshowsthathadthesteeltoothbitbeenpulledatitsoptimallife(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&RPMThepurposeofselectingbitweightandrotaryspeedsareto

1. producethelowestdrillingcostperfoot2. controlthedirectionofthedrillhole3. findoverpressuredzones4. satisfyequipmentlimitations(solidsremoval,BHA,etc.)5. developasatisfactorycuttingsdescriptionlog

CONTOURMETHOD

Themethodconsistsofconstructingthepreviousfigureinatwodimensionalpresentation.ExampleSixinsertbitshavebeenrunandtheirperformanceandtotalcostperfootdataarelistedinthetable.

BIT# ROT.TIMEFOOTAGETCF Hr:Min $/FT1 58:42 734 35.282 39:05 360 51.513 101:21 855 48.994 62:07 614 44.245 47:16 517 41.276 56:33 658 38.14

TheoptimalWOBandRPMare40,000lband50rpm($35.00/ft)ANALYTICALMETHODThesameprocedurecouldbeusedfordeterminingoptimalWOB,andRPMforthesteeltooth,rollerbearingbit;however,anGalleandWoods’analyticalmethodisavailableforthesebits.DEFINITIONOFTERMS

BS =

Bitdiameter;inchW' =

Actualweightonbit;1000poundsW =

Equivalentweightona7-7/8inchbit;1000poundsTT =

Triptime;hrsR =

Costrateofthedrillingoperation;$/hrsT =

Time;hrsG =

BitCost+TripCost;$TC =

Tripcost;R*TT;$D =

Fractionoftoothheightwornaway;8thsB =

Fractionofbearinglifeconsumed;8ths

DT =

Downtime;hrsRT =

Rotationtime;hrsF =Bitfootage;ft

GalleandWoodspublishedthefollowingrelationshipsfortherateoftoothwear.

TOOTHWEARA= Correlationcoefficientfortoothwearandiscalledtheformationabrasivenessfactor

IntegrationofthetoothwearequationgivesbitrotationtimeToothwearEquationRotarySpeed,N=80BEARINGWEAR

S= correlationcoefficientcalledthedrillingfluidfactor

Otherspublishedthefollowingrelationshipfortherateofbearingwear.RotarySpeed,N=80RATEOFPENETRATIONC= correlationcoefficientcalledtheformationdrillabilityfactor

Thevaluesofandaredeterminedfromdrill-offtestandrangebetween .6<

<1.0 .4<

<1.0

p= 1.0ifflattoothwear(non-sharpening)p= 0.5ifselfsharpening

Bitfootage(F)isequaltoRateofPenetrationxRotationTimeEquationofROPSofar,RTisdeterminedeitherbysettingD=1(completebittoothwear)orB=1(completebearingwear)Whichevercomesfirst.DRTFDRTFDRTFDRTFDRTFDRTFDRTFRotarySpeed,N=80ThetotalcostperfootequationisDRTFTCFDRTFTCFDRTFTCFDRTFTCFDRTFTCFDRTFTCFRotarySpeed,N=80TheparametersA,C&Sareusuallycomputedwithdatagatheredfromothersimilardrillbitswhichwereruninsimilarformations,atsimilardepths,withsimilarbottomholeassemblies,with