SPE煤層氣英文翻譯

畢業(yè)設(shè)計（論文）外文翻譯學(xué)生姓名：田曉學(xué)號：xxx專業(yè)班級：石油工程指導(dǎo)教師：王明教授2012年6月10日原文：tateoftheArtinCoalbedMethaneDrillingFluids現(xiàn)代煤層氣鉆井液工藝水平LenV.Baltoiu,BrentK.Warren,Q'MaxSolutionsInc,ThanosA.Natras,EncanaCorpAbstractTheproductionofmethanefromwetcoalbedsisoftenassociatedwiththeproductionofsignificantamountsofwater.Whileproducingwaterisnecessaryinordertodesorbthemethanefromthecoalitself,thedamagefromthedrillingfluidsusedisdifficulttoassessbecausethegasproductionfollowsweekstomonthsafterthewellisdrilled.Commonlyaskedquestionsinclude:Whataretheimportantparametersfordrillinganorganicreservoirrockthatisboththesourceandthetrapforthemethane?Hasthedrillingfluidaffectedthegasproduction?Arethecleatsplugged?Doesthe“filtercake”haveanimpactontheflowofwaterandgas?Arestimulationtechniquescompatiblewiththedrillingfluidsused?Thispaperdescribesthedevelopmentofauniquedrillingfluidtodrillcoalbedmethanewells,withaspecialemphasisonhorizontalapplications.Thefluiddesignincorporatesproductstomatchthedelicatesurfacechemistryonthecoal,amattingsystemtoprovidebothboreholestabilityandminimizefluidlossestothecleats,andaBREAKERmethodofremovingthemattingsystemoncedrillingiscompleted.Fieldresultsfromthreehorizontalwellswillbediscussed,twoofwhichweredrilledwiththenewdrillingfluidsystem.Thewellshavedemonstratedexceptionalstabilityincoalforlengthsto1000m,controlleddrillingratesandeaseofrunningslottedliners.Methodsfor,andresultsof,placingtheBREAKERinthehorizontalwellsarecoveredindepth.IntroductionProductionofmethanefromcoalhasbecomeoneofthemoreinterestingpracticesinrecentyearstoproducehydrocarbons.1-6IntheUnitedStatesin2005,itisestimatedthat11.7%ofallgasproducedisfromcoalbedmethane(CBM)sources7.Whileinconventionaldrillinginsandstonesandcarbonatesitisoftensimpletotellifadrillingfluidisfullyorpartiallyresponsibleforformationimpairment,thisisoftenmuchmoredifficultinCBMwells.WhenaCBMwelldependsupontheproductionofwatertoreduceformationpressureandthusleadtogasdesorption,theinfluenceofdrillingfluidbecomesmaskedorevenforgotten.AsthefrontiersofCBMwellsarepushedintothehorizontaldrillingrealm,theimportanceofthedrillingfluidismagnified.Thefluidneedstobothstabilizethewellboreduringthedrillingphase,butatthesametimeneedstominimizeanyproductionshortfallsduetodamage.AsimpleN2fracturewhichmaybeusedona5-10meterverticalcoalseamisnotasimplemattertotransfertoa500-1000meterhorizontallydrilledcoalsection.Thispaperdiscusseshowcoalgeologyimpactsdrillingplanning,drillingpractices,thechoiceofdrillingfluidandcompletion/stimulationtechniquesforUpperCretaceousMannvilletypecoalsdrilledwithintheWesternCanadianSedimentaryBasin.AfocusonhorizontalCBMwellsispresented.BasicCoalGeologyIfyouweretohandapieceofcoaltosomeoneyouknewandaskedthemtodescribeit,theywouldprobablysay“it’sblack”.Thefactistheyareright,butthereismoretocoalthanwhatmeetstheeye,especiallyifyoudon’tknowwhatyou’relookingfor.Coalisaverycomplicatedorganicrockmadeupoftinymicroscopicconstituentscalledmacerals,whichareanalogoustothemineralsfoundininorganicrockssuchasquartz.Maceralsaremadeupofvariouslithifiedplantdebrissuchasspores,resins,pollens,waxes,cuticles,andresins.Therearethreemaingroupsinwhichmaceralsareclassified,thevitrinitegroup,inertinitegroup,andtheliptinitegroup,allwhichcanbebrokendownfurtherintoseveralindividualmacerals.Maceralsofsimilarcharactercanbegroupedintowhatarecalledmicrolithotypes,microscopicallydiscernibleunitsanalogoustolaminationsinsedimentaryrockssuchassandstones.Microlithotypesarefurthercombinedtoformmacroscopicallyvisibleunitscalledlithotypes.Lithotypes,whichareanalogoustobedsinothersedimentaryrocks,areclassifiedonthebasisoftheirbrightness.Forexample,alithotypepredominantlymadeupofthevitrinitegroupwouldlookverybright.Incontrast,coalrichininertinitewouldlookverydull.Thedullandbrightbandstellussomethingabouttheheterogeneityofthecoalandhowvariableitsphysicalmakeupisbothverticallyandlaterally.Eachlithotypecomeswithitsownsetofphysicalpropertieswhichcanenhanceorimpedeproductionofcoalbedmethane.Coalrankisanotherimportantphysicalproperty.Coalrankisameasureofthedegreeofchemicalalterationthecoalhasundergone(alsoreferredtoasdiagenesis).Thelongerthecoalificationprocessgoeson,thehigherrankthecoalbecomes.Vitrinitecontentalsochangeswithcoalrank,asdoseveralotherphysicalpropertiesimportanttoCBMpotential8(Figure1).Figure1CoalRankingClassificationbyVitriniteReflectance,VolatileMatter,BedMoistureandCaloricValueCleatingThesinglemostimportantphysicalcharacteristicofanycoalrelativetoCBMproductionispermeability.Permeabilityincoalisadirectfunctionofthecleatand/orfracturenetworkpresent.Ascoalmaturesthroughtheprocessofcoalification,moistureandvolatilegassesareslowlydrivenoffresultingintheshrinkingofthecoalmatrix.Asthecoal‘shrinks’,cleatsbegintoform,similartothecracksthatdevelopwhenmuddriesundertheheatofthesun.Thecleat/fracturesystemincoalisalsoreferredtoasthemacroporesystem.Agoodmetaphorforvisualizationisaloafofslicedbread(Figure2).Thespacesbetweentheslicesofcoalarefracturesthatarereferredtoas“facecleats”.Thespaceswithintheslicesofcoalarereferredtoas“buttcleats”.Theymay,ormaynot,intersectwiththefacecleats.Whenwerefertocoal“permeability”weactuallyrefertothepermeabilityobtainedfromthefracturesnetwork.Facecleatsareveryimportantastheyarethebackboneofcoalpermeability.Buttcleatsmay,contributetosuchpermeabilityiftheyintersectthefacecleatnetwork.Athirdsetoffracturesmaybefoundoncoalsthathavebeenexposedtotectonicstresses.Thesethirdsetsoffracturesarereferredtoas“tectonicfractures”andareveryimportanttoCBMproduction.Tectonicfracturesincreasethepermeabilityofthecoalbytwomechanisms:(a)theirownpresenceand(b)byconnectingtosomeofthebuttcleatspreviouslynotpartofthefracturenetwork.Withthegreatheterogeneitywithinasinglecoalnetwork,therangeoffracturespresencecanvaryfromnofracturestocompletethree-setfracturedevelopment.Figure2Stylizeddepictionofacoalshowingthefacecleats——,buttcleats——andtectonicfractures——Maceraltypeandcoalrankarethetwomostimportantcontrollingfactorsincleatdevelopment.Forexample,avitriniterichhighrankingcoalwillhaveexcellentpotentialforcleatdevelopment.Incontrastaninertiniterich,lowrankcoalwillhaveverylittlepotentialforcleatdevelopment.Asdiscussedbefore,coalscanbeveryheterogeneouswithseverallithotypesoveraverysmallverticaldistance(cmtom).Ifyouhadtwolithotypesverticallyadjacenttooneanother,onerichinvitriniteandtheotherrichininertinite,thevitriniterichlayer,orlithotype,wouldshowbettercleatingeventhoughtheentirecoalisofequalrank9(Figure3).Figure3OutcropoftheHorsehoeCanyonformation,PaintearthMine,Alta.Athicker,poorlycleatedinertiniterichcoaloverlyingathinnerseamofwellcleatedvitriniterichcoal.Depthisanimportantfactortoconsiderwhentalkingaboutcleatsandtheirapertures.Generallyspeaking,theeffectivepermeabilityofcoaldecreaseswithburialduetocompaction.StorageandDesorptionMethanecanbestoredintwoplacesincoal;inthecleats/fractures,ormacroporestructure;andinthematrix,ormicroporestructure.Themacroporesystemiswheretheeffectivepermeabilityismeasuredandaretheconduitsinwhichthemethanetravelsintothewellbore.Thecleats/fracturesarefilledwithmethanemoleculesthathaveattached,oradsorbed,onthesurfaceofthecoal;thisisknownas“free”gas(Figure4).Inthemicroporesystem,thereissignificantlylesspermeability.Imagineaspongewithmillionsoftinycavities,butnoneofthemconnectedtooneanother.Themajorityofthemethanegasisstored(adsorbed)inthesemicropores,ormatrixporosity,andisreferredtoas“bound”or“trapped”methane.Figure4:Methanemoleculesadsorbedontocleatnetworkofcoal.Desorption,theoppositeofadsorption,occurswhenamethanemoleculedetachesfromthecleatfaceandstartstoflowtowardanareaoflowerpressure,inthiscasethewellbore.Asdesorptioncontinuestotakeplace,themethaneadsorbedontothecleatface,or“free”gas,beginstodissipate.Whenthereisnomore“free”gastobedesorbed,theboundortrappedgaswillstarttomakeitswayintothecleatnetworkbytheprocessofdiffusion.10Oncethe“bound”methanemoleculesreachthecleatface,migrationtothewellboreisacceleratedduetothegreaterpermeabilityofthecleatnetwork.Coalvs.Non-coalReservoirsThemaincharacteristicthatdistinguishescoalbedreservoirsfromconventionalreservoirs(i.e.sandstones&carbonates)isthesourceofthehydrocarbonsfoundinthem.Inconventionalreservoirs,thehydrocarbonsaregeneratedinsourcerocksfoundelsewhereinthestratigraphicsectionandovertime,migrateintotheseporousreservoirsandbecometrapped.Coalbedreservoirscanalsobecome‘charged’withhydrocarbonsviamigrationfromorganicrichsourcerocksbut,thedifferenceisthatcoalcantypicallygenerateitsownhydrocarbonfromwithin.Nooutsidesourceisnecessaryascoalitselfisanorganicrichsourcerock.Aseconddifferenceisthesurfaceofthecoalismorechemicallychargedthanthesurfaceofconventionalsandstoneorcarbonaterocks.WhatisevenmoreintriguingisthefactthatitschargehasthetendencytochangewiththechangeofthepHinitsenvironment.Asseenfurtherinthispaper,thesurfacechargeplaysanimportantroleindrilling,formationdamageandproduction.RockMechanicsUnlikemostconventionalreservoirrocks,coalhaslowintegrityandisveryfriable.TheMannvillecoalinthesubsurfaceofAlberta,Canada,canbefoundatdepthsthatvarybetween600mtogreaterthan2000m.Ifweconsideracubeofrockanditsstressdistribution,σv–theverticalstress,σHzmax–themaximumhorizontalstressandσHzmin–theminimumhorizontalstress,incoalfoundatdepth,theσvisthelargest.CoalhasaPoissonsratioasmuchasoneorderofmagnitudehigherthanconventionalrock,andwillhavethetendencytocompressundersufficientload.Thisphenomenoncontributestoboreholeinstabilitymechanismwhendrillingincoal(Figure5).Inconventionalreservoirrocks,theσHzmaxcanbeashighassixtimestheσHzmin.Therefore,thepathofleastimpactwhendrillinghorizontallyisperpendiculartothemaximumhorizontalstress.IncoaltheσHzmaxtypicallyisonly1.6timesasmuchastheσHzmin.However,whencombinedwithcoal’slowstrength,itstandstologicthatthesameprincipleapplies.Fieldapplicationshaveprovedthistobetrueespeciallywhentectonicfractureswerepresentincoal.11Notsurprisingly,withcoalbeingsuchaheterogeneousmaterial,exceptionstotheguidelineabovedooccur.AnarrowcorridorbetweenBarrheadandFortAssiniboine(justnortheastofEdmontonincentralAlberta)existswheredifferentlyorientatedhorizontalwellshavesuccessfullyproducedcoalbedmethane.Figure5:Depictionofstressesandorientations–Verticalstressisshowninblue,themaximumhorizontalstressinred,minimumhorizontalstressingreen.DrillingConsiderationsEarlydrillingofhorizontalwellsintheMannvillecoalinAlbertaencounteredtremendousboreholestabilityproblemsinmostareas.However,innovationandexperimentationhadledtoathreepointsystemthatprovedquitesuccessful.Thissysteminvolvescarefulconsiderationof:(a)welltrajectory/welldesign;(b)drillingpracticesand(c)drillingfluid.WellTrajectorySincecoaldoesnothaveaporosityandpermeabilityintheconventionalsense,thequestionofexpectedenhancedproductionfromahorizontalwell,ascomparedtoaverticalwellisongoing.Whilehigh-densityspacingofverticalwellsfollowedbynitrogenfracturingofthecoalhasprovedtobeeconomicalinshallowcoalssuchastheHorseshoeCanyoninAlberta6,deepercoalhorizonssuchastheMannvillehaveproventobemorechallenging.PoorproductionratesledtoeconomicswhichdonotsupporthighdensityverticalwelldrillingforMannvilleCBMwells.Essentially,theexposuretothedesirablecoalcleatingnetworkisverylimitedinverticalwells.Horizontalwellswerethereforedesignedanddrilledinordertointersectasmuchfracturenetworkaspossible,especiallythefacecleatsinthickerMannvillecoalseams.InAlberta,duetothepresenceoftheRockyMountainsarch,theσHzmaxhasaNE-SWorientation.Interestinglyenough,theorientationoffacecleatsrunsonthesameNE-SWdirection.ThereforethewellsthathaverecordedbettersuccessweredesignedwithtrajectoriesNW-SEorSE-NW,achievingatthesametimeanearperpendicularitytothemaximumhorizontalstressandtothefacecleats(Figure6).Figure6:MapofAlbertashowingstressesandcleating.Thebluelinesrepresentthepreferredorientationofthefacecleats,inaNE-SWdirection.ThisisthesamedirectionastheprinciplehorizontalstressσHzmax.CurvedredlinesindicatethefrontlineoftheRockyMountains.ThemajorityoftheMannvillehorizontalCBMwellshavebeendrilledbasicallyNW/SEdirection.Practicehasshownthatcoalwillnotsupporttightwellradius,doglegsorkeyseating.Thereforelowbuildangles,largeradiuswellswithverysmoothcurvesproducedsignificantbenefitsondrillingstablewellboresincoal.WellDesignTodateinAlberta,Canada,twodistinctwellprofileshavebeenutilized:the“motherbore”designandthe“classic”design.Themotherboredesigninvolvesdrillingadeviatedintermediateholethroughthecoal,settinganintermediatecasing,cuttingawindowanddrillingmultiplehorizontallegsintothecoalseam(Figure7).Aslottedlineristhenrunineachleg.Theperceivedadvantagesofthisdesignrelatetothepossibilityofdrillingmultiplehorizontallegsaswellastheuseofasingledewateringpumpforthoselegs.However,therearemanydisadvantagesassociatedwiththisdesign:(1)Greaterpotentialforboreholeinstabilityissuesonthemainholearoundthebendduetothewelltrajectory,(2)Slottedlinershavetobesetintheopenhole,~4mto6moutsidethewindowandtypicallyinawatersensitiveshale,(3)Inordertoreachthedewateringpump,theformationbrinehastotraveluphillaroundthebendtoreachthewindow,(4)Thedrillingfluidusedtodrillthemainholewillbecomecontaminatedwithcuttingsotherthancoalandthoseareintroducedintothecoalcleats,thereforeincreasingthepotentialforformationdamage.Figure7The“MOTHERBORE”designinwhichmultiplelegsarepossiblefromonecentralcasingstringTheseconddesignistheclassicdesignwhichinvolvesdrillinganintermediateholedeviatedto~900inthecoalandsettinganintermediatecasing.Thendrillinga~1,000mhorizontallegwiththetoeuplifted~20comparedtoheel,suchthatgravitywillhelpwiththedewateringprocess.Allthedisadvantagesfromthepreviousdesignarealleviatedplus,thisdesignallowsformultiplehorizontallegsinthesamegeneraldirection,the“fork”design(Figure8).Figure8:The“CLASSIC”CBMHorizontalwellboredesignwithasinglehorizontalleg.Thislegisoftenslightlyinclinedtoassistindrainageofwaterduringproductionphase.Ineitherdesignitisimperativethatthehorizontallegbemaintainedwithinthecoalhorizon,whichincertainsituationscanbeonly1mthick.Ifthewelltrajectorybouncesinandoutofthecoal,penetratingtheunderandoverlayingbedsoftypicallywatersensitiveshaleandmudstones,twoproblemsmayoccur:(a)greaterpotentialforboreholestabilityissuesand(b)introductionofformationdamagingsolidstocoalreservoirleadingtoareductioninproduction.DrillingPracticesCoalformationshavebeenregardedasnuisanceformationsbythedrillingindustryuntilrecently.Assuch,theconventionalor“oldschool”drillingpracticesborrowedfromdrillingconventionalrockdonotapplyproperlytodrillingincoalreservoirs.Inordertounderstandwhythesepracticesdon’tapply,weneedtounderstandthemechanismsbehindboreholecollapsewhiledrillingcoal.Assumingaportionoftheboreholewalltakenfromtheupperholeinthehorizontalsectionandlookingatitthroughamicroscopewewillnoticefacecleats,buttcleatsandtectonicfractures.Theconventionalwayofthinkingistodrillcoalasfastaspossiblewithinhibitivedrillingfluids,brinesorinhibitivewater,withhighpumpoutputstocleantheholeandcaseitbeforeboreholestabilityissuesdevelop.Inhighlyfracturedcoal,thisdrillingapproachprovedtobeadisasterandoftenresultedinstuckpipeandlostbottomholeassemblies.Duetotheirmicro-permeabilitythatimpedesinitialspurtlossacrosscoal(justthesameasacrossshale),afiltercakecannotbebuiltonthefaceofacoal.Whendrillinghighlyfracturedcoalswithwater/brineorconventionaldrillingfluids,thesefluidsinvadethefracturenetworkandcarrytwothingswiththem:(a)pressure–whichaccordingtoBernoulli’sLawinanincompressiblefluidwilldistributeitselfinstantaneouslyandinalldirectionand(b)particles–coalparticlesandotherfluidcomponentsparticles.Drillingoverbalanceimpliesthatthebottomholepressureisgreaterthantheformationpressure.However,whenfluidpenetratesthemicro-fracturenetwork,theformationpressureintheimmediatevicinityofthewellborebecomesequaltothebottomholepressure,thereforethepressurebelowandaboveacoalchipcontainedwithinthefracturenetworkequalizesandthechipfallsinsidethewellbore.Typically,themorefluidlosttotheformation,thedeeperthepressureequalizationthatoccurs,andthegreaterlikelihoodforsloughingofcoalcavings.Suchcavingsareusuallyblocky,withsquareedgesandcanbequitelarge,upto10-13cmacross.Sloughinghappensinarelativelyshortperiodoftimeandinverylargevolumes.The“oldschool”approachhasbeentoincreasethemuddensityto“holdthewellback”,toincreasethepumpoutputandfluidrheologytocleanthehole,tomixconventionalLCMtoreducethelossesandtoworkthepipeacrossthetightzoneinthewellbore.Oncloseanalysis,noneoftheseapproachesproducetherequiredresults.Whenthemuddensityisincreasedwithbariteorcalciumcarbonate,thebottomholepressure(BHP)vectorisincreasedforashortperiodoftime.However,atthistimethefracturesincoalarepartiallyproppedopenandmorefluidpenetratesinsidethem.InamatterofhourstheBHPdissipatestoequaltheformationpressure(FP),andthesloughingcontinues(Figure9).IncreasingthepumpoutputandfluidrheologywillincreasetheBHPthroughpressurelosses,butatthesametimeincreasingfluidpenetrationandwithgreaterforcesbetweenthefluidandthechipsofcoalintheimmediatevicinityofthewellbore.Infracturedcoal,higherhydraulicprofilesarenotdesired.Whilethepracticeofincreasingthepumpoutputandmudrheologyforholecleaningworksinconventionalrock,coaldoesnotneedsuchanapproach.Duetocoal’slowspecificgravityof1.2to1.5,CBMwellscaneasilybecleanedwithwater-basedfluidsoflowviscosity.Itisnottheweightofthecoalcuttings/cavingsthatposeaholecleaningissuebutthesizeandvolumesofthemandthefastrateatwhichcoalsloughs.MixingconventionalLCMsdoesnotworkeither,especiallywhenrigidparticlesareinvolved.Thehardrigidparticlesactasproppantagentsforthefracturesthusleadingtoenhancedlossestothecleatnetwork.Inaddition,ifbridgesarecreatedwithhardparticles,theymigratedeepinsidethefracturesthereforedrasticallyreducingthepermeability.ConventionalLCM(fibers,cellophane,sawdust,etc.)aretoobigtocreateaneffectivebridge.Figure9:Thepressures–bottomholeandformation–aresoonequalizedincoalthroughthefacecleatandtectonicfracturenetworksOneofthecommon“oldschoolapproaches”thatdoesnotworkincoalishard-workingthepipewhensignsofboreholecollapsestarttoshowatsurface:indicationsincludeincreasedtorqueanddrag,increasedpressure,tightholeand/orreducedflowreturns.Thecollapsehastodowiththepipemovementandthehydraulicscreatedbythatmovement.Theprocedureinvolvesworkingthepipethroughthetroubledzoneatincreasedspeeds,whilepumpingandrotatingtocleanthebridgeformedbythesloughingcoal.Surgeandswabpressuresgeneratedbythepipemovementandthepistoneffectofthebithaveadevastatingeffectonfracturedcoalandoftenresultsinmoresloughing,thenstuckpipewiththelossofexpensivedirectionaldrillingequipment.ThedrillingapproachthathasprovenverysuccessfulinfracturedMannvillecoalsinAlberta,the“newschool”method,couldbedescribedbyoneword–finesse.FollowingaresomeofthenewpracticeswhichaidindrillingCBMwells,andspecificallyhorizontalCBMwellsintheMannville.(1)Rigswithtop-drivesarepreferredduetotheirabilitytocontrolpipespeedsbetterthanconventionalrigs.(2)Controldrillingatratesof<15m/hrhasdemonstratedfarbettersuccessthanthefast,“in-and-out”approach.Whentherate-of-penetration(ROP)iscontrolled,signsofboreholeinstabilityaredetectableandcanbecorrectedbeforetheybecomeaseriousdrillingproblem.(3)Tripspeedshavetobemonitoredatwellbelow0.5m/stoalleviatethepotentialforsurgeandswab.(4)Drillingbitsideallymeetthreecriteria:(a)nonozzles,(b)backreamingcapabilityand(c)generatelargecoalcuttings.NozzlesarenotrequiredbecausecontroldrillingisappliedatrelativelylowROPs.Also,bitsdon’tballincoal.Thebackreamingcapabilitiesarevitaltosuccessfuldrilling.Whenfirstsignsofboreholeinstabilityarenotedatsurface,continuousbackreamingatreducedpumpoutputsisrecommendedthroughthetroubledzone.Thenincrementalreamingatnormalpumpoutputisrecommendedwhileadjustingthemudproperties.Typicallythereisnoneedtoincreasethemuddensityorthemudrheology,whentherightmudisused.Thereasoningforlargecuttingsgenerationhastodowithformationdamage.Finecoalhasaverylargesurfacearea,ischemicallychargedandcanpenetrateinsidethefracturesandposeaseriousformationdamagethreat.Finecoalhastheabilitytobindthecomponentsofthedrillingfluidmakingitlesseffectivetonewwellboreandposingotherdrillingrisks.(5)Moreslidingandlessrotatingisrecommendedwhiledrillinghorizontally.(6)LowerRPMsofthebitarerecommendedforthesamereasons.DrillingFluidsTodate,anewdrillingfluidhasbeenusedtosuccessfullydrilloverthirtyCBMMannvillehorizontalwellsbynineoperatorsinAlberta,Canada,withhorizontallegsbetween750–1200m,allofthemreachingtheplannedtotaldepth.Thesuccessbehindthisnewdrillingfluidresidesinitsinnovativeapproach.Ifafiltercakecannotbeachievedacrossthecoal,thenthenextbestthingisaseriesofintimateverylowpermeabilitysurfacebridges,ormats,acrosstheintersectedfractures,facecleatsandtectonicfractures.Thefluidachievesthisbyexploitingthestrongsurfacechemicalchargeofthecoal.Therearetwoversionsofthenewlydevelopeddrillingfluid.Oneisacombinationofnaturalpolymers,bentoniteandotherproprietaryproductsanditisintendedforcasedholecompletions.Theotheroneisformedofacombinationofnaturalpolymersandotherproprietaryproductsandisintendedforopenholecompletions.ThissecondversionofthefluidcanbedestroyedwithaBREAKER.Whendensityisrequired,thefluidisbuiltonabrinebackbonesuchthatdensityisachievedwithoutintroducinghardparticlestothesystem.Asdrillingprogressesinfracturedcoal,thepolymerswithinthefluidattachthemselvestotheendsofthefracturescreating“bridgeheads”.Then,incombinationwiththeotherproductsinthefluidtheyformsurfacebridgesacrossfractures,sometimesreferredtointheindustryas“buttons”.Thesearemulti-layerflexiblematsastheyareformedofflexiblematerialsonly.Fieldapplicationhasdemonstratedtheflexibilityofthebridges.Whenhydraulicsvaluesareincreased,itispossibletohave3to4m3offluidlosttothehole,onlytoberegainedonconnections.Bycreatingtheseverylowpermeabilitybridgesacrossthefractures,thedrillingfluidmaintainsaconstantpositivepressure(ΔP=BHP–FP)onthewallsofthewellbore.Ifthepressureequalizationstops,thentheboreholeinstabilityisalleviated(Figure10).Figure10:Flexiblebridge/matformedacrossthefacecleatandtectonicfractures.TheBHPismaintainedhigherthantheFPwithresultingboreholestabilityanddecreasedformationdamage.Atthispointintimeithastobeemphasizedthatifthedrillingpracticesareincorrectorifthewelltrajectoryisnotcarefullyplannedtomitigateintersectingthefacecleatswiththelocalizedrockstressdistribution,thefluidalonewillnotalleviatethedrillingproblems.Completion/StimulationAssuggestedbefore,twotypesofwellcompletionwereperformedtodateonwellsdrilledwiththisinnovativedrillingfluid:openholecompletionandcasedholecompletion.Inthecaseofcasedholecompletion,atwelltotaldepthalinerisrunandcementedinthehorizontalsection.Theholeisthenperforatedandanitrogenfracoperationisconducted.Inthecaseofopenholecompletion,atwelltotaldepthaslottedlinerisrun.ThehorizontalsectionisdisplacedtoaBREAKERthatcompletelydestroystherheologyofthefluid.ThechallengeistoobtainmaximumdisplacementoftheBREAKERintotheannulus,andafewmethodshavebeentriedwithdifferentdegreesofsuccess.Withallmethods,the