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LISTOFACRONYMS

APL Annularpressureloss

BHP Bottomholepressure

CBP Constantbottomholepressure

CCS Continuouscirculationsystem

CWD Casingwhiledrilling

ECD Equivalentcirculatingdensity

FBP Formationbreakdownpressure

FCP Fractureclosurepressure

FIP Fractureinitiationpressure

FIT Formationintegritytest

FMCD Floatingmudcapdrilling

FPP Fracturepropagationpressure

FRP Fracturereopeningpressure

HPHT Highpressure,hightemperature

ISIP Instantaneousshut-inpressure

KGD Khristianovitch Geertsma deKlerk

LCM Lostcirculationmaterial

LOT Leakofftest

LPM Losspreventionmaterial

MD Measureddepth

MPD Managedpressuredrilling

MWD Measurementwhiledrilling

NADF Nonaqueousdrillingﬂuids

OBM Oil-basemud

PDC Polycrystallinediamondcompact

PKN Perkins Kern Nordgren

PMCD Pressurizedmudcapdrilling

PSD Particlesizedistribution

RCD Rotatingcontroldevice

SBM Synthetic-basemud

TVD Trueverticaldepth

UBD Underbalanceddrilling

UCS Unconﬁnedcompressivestrength

WBM Water-basemud

WSM Wellborestrengtheningmaterial

XLOT Extendedleakofftest

BOX1.1LostCirculationinaGeothermalWellinIceland Lostcirculationisacommonproblemingeothermaldrilling,whereitisexacerbatedbyhightemperaturesandhardrocks.Asequenceoflostcirculationevents whiledrillingageothermalwellintheKraﬂaﬁeldinIcelandin2008 09was describedbyPa ´ lssonetal.intheirpaper“DrillingofthewellIDDP-1” (Geothermics,2014,49,23 30).

Thewell,IDDP-1,waspartoftheIcelandDeepDrillingProjectandwas originallydesignedtoreacha4500-mdepth.Severeproblemswereencountered duringdrilling,andthewellhadtobesidetrackedeverytimeitapproached magmaat2100m.Mudlosseswereexperiencedoften,becomingworseasthe depthincreased.Also,thewellbecameincreasinglymoreunstableandwashed outwithdepth,whichaffectedtheholecleaningandreducedtherateof penetrationthatalreadywaslowduetothehardrock.

Minormudlossesoccurredintheﬁrst1000mwhiledrillingfortheintermediate18-5/800 casing.Thelosseswerecuredwithlostcirculationmaterial.

Lossesof20L/swerethenexperiencedat1432m.Theproblemwascuredby cementingthethiefzone.

Lossesinexcessof60L/swereexperiencedat2043m.Thelossescouldnotbe cured.Theweighteddrillingﬂuidwasreplacedwithwater,anddrillingcontinued. At2101m,thebottomholeassemblybroke.Afterunsuccessfulﬁshing,a cementplugwasplaced,andthewellwassidetracked.

Uponthesidetrack,totallossesoccurredat2054m.Acementplugwasplaced inthewell,andthedrillingcontinuedfrom2060m.Lossesresumedat2067m, andtotallossofcirculationoccurredat2076m.

Continuedproblemswithstuckpipeandunsuccessfulﬁshingattemptsat 2103mforcedasecondsidetrack.Mudlossescontinuedafterthesidetrack,and totallossofcirculationoccurredat2071m.Drillingwasterminatedupon reachingmagmaat2100m.Thewellwasthentestedandcompleted.

InanaturallyfracturedcarbonateﬁeldinIran,mudlosseswerereportedin 35%ofdrilledwells [3].InSaudiArabia,32%ofwellsinthenaturally fracturedcarbonateKhuffFormationexperienceballooning,while10% experiencelostcirculation [13]

Thebestwaytodealwithlostcirculationistopreventitfromhappening altogetherinthefirstplace.Inpractice,however,thismaybedifficultto achieve.Nevertheless,technologicalimprovementsinformationcharacterizationanddrillingfluiddesignenablethepreventionoflossesinmanywells. Preventinglostcirculationrequiresthatthemechanicsandphysicsofthis drillingproblemarefullyunderstood.

Themostobviouswaytopreventlostcirculationistokeepthedownhole pressuresufﬁcientlylow ie,belowtheupperoperationalpressurebound. Inpractice,however,itisnotquiteobvioushowthisupperboundshouldbe chosen.Incompetentintactformations,theupperboundoftheoperational

FIGURE1.1 Casingpointsdeterminedfromthelower(porepressureorboreholestabilitylimit) andupper(lost-circulationpressure)operationalpressureboundsalongthewell.Thestaticbottomholepressureisshownwitha dottedline. Asterisks denotecasingpoints.

aﬁctitiousverticalonshorewellin Fig.1.2.Theresult ie,thelocationsofthe casingpoints is,ofcourse,thesameasin Fig.1.1

Theabilitytopredictthelost-circulationpressureisthereforecrucialfor optimizingthecasingprogram. Figs.1.1and1.2 alsosuggestthatifthe lost-circulationpressurecouldbeincreased,longerintervalscouldbedrilled withthesamemudweight.Itwouldreducethenumberofcasingpointsand increasethewelldiameteratthetargetdepth.Thisisthemotivationfor applyingspecialtreatmentstoincreasethelost-circulationpressureintheopen hole(“wellborestrengthening”),discussedinchapter“PreventingLost Circulation.”

Evenifthebottomholepressurestaysbelowthelost-circulationpressure duringnormaldrilling,pressuresurgesduringtripsandconnectionsmay exceedthisupperboundandcauselostcirculation.Whenaconnectionis made,circulationissuspended.Duringconnection,thedrillingﬂuidstarts developinggelstrength.Whencirculationisresumedafterconnection,the pressureneedstobeincreasedsufﬁcientlytobreakthegel.Thismayleadto

FIGURE1.2 Casingpointsdeterminedfromthegradientproﬁlesalongthewell.Thestaticmud weightisshownwitha dottedline Asterisks denotecasingpoints.

substantialpressurechangesduringconnections.Iftheoperationalpressure windowisnarrow eg,inadeepwaterwell thereductionofpressurebefore connectionmayleadtoaformationﬂuidinﬂux,andthepressuresurgeafter connectionmayleadtolostcirculation.

Tripsmaycauseformationfluidinfluxwhenpullingoutofthehole,and lostcirculationwhenrunninginhole.Preventinginfluxesandlostcirculation duringtripscanbeachieved,forexample,byoptimizingthedrillingfluid rheology.

Drillingthroughdepletedformationsisoftenrequiredinordertoaccess deeperreservoirs.Depletedformationsarepronetomudlosses.Insomewells drilledindepletedformations,lossesontheorderofthousandsofbarrelshave beenreported [14].Theminimumhorizontalstressisusuallyreducedin depletedreservoirs(chapter:StressesinRocks).Thisreductionaffectsthe operationalpressurewindowbyreducingthefracturingpressureandthereby increasingtheriskofmudlosses.

Deviatedorhorizontalwellsarepronetolostcirculation.Theoperational pressurewindowisnarrowinsuchwells.Insomecases,thewindowmayclose altogetherastheinclinationincreases.Inextendedreachwells,theproblemis additionallyaggravatedbyincreasingannularpressurelossesalongthehorizontalsection.Sincethefracturingpressureremainsapproximatelythesame atthesamedepth,thebottomholepressurewilleventuallyexceedit.

Aconsiderableshareofmudlossesoccurwhenrunningcasingorpumping cement.Runningthecasingpipegeneratesanexcessivebottomholepressure thatcanleadtoformationbreakdown.Duringcementing,highdensityand rheologyofcementresultinanelevatedbottomholepressure,likelytobethe highestpressuretheformationiseverexposedtoduringwellconstruction. Thismayleadtolostcirculationduringacementjob.

Anotherexampleofformationwherelossesarecommonissubsaltrubble zones [15].Suchformationsareoftenrepresentedbyrelativelyweakand/or fracturedshale.Ithasbeenarguedthatfracturesintheseshalesarecaused bydeformationintheadjacentsaltthroughoutgeologicalhistory [15].Pore pressureinthesubsaltshalecanbeeitherlowerorhigherthanthepore pressureinthesalt.Theformerscenarioisthecase,forexample,intheHassi Messaoudﬁeld,whereseverelosseswereexperienced [16];thelatterscenario isthecase,forexample,intheGulfofMexico,withporepressurevs.depth schematicallyshownin Fig.1.3.Highporepressureinshalebelowthesaltis

FIGURE1.3 Porepressureproﬁle(solidline)aboveandbelowasaltbodytypicalofsomeGulf ofMexicowells.Thesaltbodyisshownwitha dottedline.Overpressureinthesubsaltshaleis causedbythesalt“trapping”theporepressure. BasedonSweatmanR,FaulR,BallewC.New solutionsforsubsalt-welllostcirculationandoptimizedprimarycementing.SPEpaper56499 presentedatthe1999SPEannualtechnicalconferenceandexhibitionheldinHouston,Texas; 3 6October1999.

causedbythesalt“trapping”theporepressure.Itresultsinanarrowmargin betweentheporepressureandthelost-circulationpressureinshale.Aswith otherfracturedrocks,ﬁllingfracturesinthesubsaltshalewithcementorlost circulationmaterialisnotaneasytask.Accordingtoonereportfrom1999, “thecostofdrillingformationsapproximately1500ftabovethesaltto approximately1500ftbelowthesalthavereachedseveralmilliondollars” [15].Thenonproductivetimeassociatedwithsuchintervalscanbeweeks. Asanexample,morethanhalfofthewellsdrilledintheHassiMessaoud ﬁeldexperiencedtotallossesinsubsaltshale [16].

Lostcirculationisexacerbatedindeepwaterdrilling.Thefracturegradient isoftenquitelowindeepwaterwells [17] (Fig.1.4).Thisresultsinanarrow operationalpressurewindowinsuchwells.Exceedingthefracturegradient canleadtomudlosses.

Lostcirculationisamultidisciplinarychallenge.Combattinglostcirculationrequiresacomplexapproachthatincludesrockmechanicalanalysis, carefulwelltrajectoryplanning,optimizationofdrillingﬂuidrheologyand composition,optimizationoflosspreventionandlostcirculationmaterials, andoptimizationofdrillinghydraulics [18].

Considerableadvancesinpetroleum-relatedrockmechanics,including hydraulicfracturemechanics,overthepastdecadeshaveimprovedour understandingoflostcirculation.Bettermethodsofmudlosspredictionand moreeffectivetreatmentsmakeitpossibletodrillwellsthatwouldbe impossibletodrillafewdecadesago.Atthesametime,increasinglymore difﬁcultdrillingconditionsareencounteredaseverdeeperreservesare involvedinexplorationandproduction.Thisresultsinpersistenceofthe lost-circulationchallengetothisday.

FIGURE1.4 Fracturegradientasafunctionofverticaldepthindeepwaterdrilling:waterdepth incaseAisgreaterthanincaseB.

Chapter2

StressesinRocks

Thestressstateinthereservoirandaroundtheborehole,androckfailure causedbymechanicalorthermalstresses,playcentralrolesinlostcirculation. Theseconceptsarereviewedinthischapter.

2.1STRESS,STRENGTH,ANDFAILURE

Whenmechanicalloadsareappliedtotherock,orwhentherockisheatedor cooled,stressesareinduced. Stress isdeﬁnedastheforceactingonasurfaceof unitarea.Ifwecutoutanimaginarycubeinsidetherockwiththefacesnormal tothecoordinateaxes,thestressoneachfacecanbedecomposedintothe normalstressandtheshearstress.Thelattercanbedecomposedonceagain intotwocomponentsactingalongtwocoordinatedirectionsparalleltotheface asshownin Fig.2.1.Normalstressesaredenotedby s,andshearstresses aredenotedby s,in Fig.2.1.Theﬁrstindexintheshearstress eg,“x”in

FIGURE2.1 Stresscomponents.

http://dx.doi.org/10.1016/B978-0-12-803916-8.00002-9

sxy denotesthefaceonwhichthestressisacting.Thesecondindexdenotes thedirectionofthestress.Thedirectionsofnormalstressesin Fig.2.1 correspond tocompressivestresses.Normalstressesinducelongitudinaldeformations. Compressivenormalstressesinducecontraction,whereastensilenormalstresses induceextension.Asignconventionisneededfornormalstresses.Inrockmechanics,compressivestressesareusuallyassumedpositive,andtensilestresses negative.

Equilibriumconditionsrequirethattheshearstresseswiththesamebut permutedindexesbeequal,forinstance sxy ¼ syx,andsimilarlyforothershear stresscomponents.

Theninestresscomponentsmakeupasecond-order stresstensor:

Duetothesymmetrypropertiesoftheshearstresses,thestresstensoris symmetricandthushassixindependentcomponents.

Rotatingthecubein Fig.2.1 would,ingeneral,changethevaluesof normalandshearstresses(exceptinthecaseofauniform,so-called hydrostatic compressionortensioninwhichcasetherearenoshearstresses,andall normalstresseshavethesamemagnitude).Thereexistsonespecialorientation ofthecubesuchthattherearenoshearstressesonitsfaces(Fig.2.2).Normal stressesactingonthecubefacesinthiscasearecalled principalstresses.They aredenotedby s1, s2,and s3,with s1 beingthegreatestofthethree(themost compressiveone)and s3 beingthesmallestofthethree(thetensileorleast compressiveone).Thedirectionsofthesestressesarecalled principalaxes

FIGURE2.2 Principalstresses.

Inthe s0 s coordinates,theMohr’shypothesiscanberepresentedusing thefailureenvelopeshownasthesolidlinein Fig.2.4.Thestressstateslocated betweenthefailureenvelopeandthe s0 -axisareadmissible.Thestressstates locatedaboveortotheleftofthefailureenvelopecannotbeattainedwithout breakingtherock.Therockfailswhenapointinthestressspacereachesthe failureenvelopeduringloading.Itisconvenienttoillustratethisbymeansofa Mohr’scircle whichisshownasadashedlinein Fig.2.4.TheMohr’scirclein Fig.2.4 issupportedbytheminimumandmaximumeffectivestressesand showsthestressstateonplaneswithdifferentorientations.Whilethefailure envelopedescribesthepropertiesoftherockitself,theMohr’scircledescribes theactualstateofstressinducedintherockbyloading.Inthesituationshown in Fig.2.4,theMohr’scircledoesnotcrossthefailureenvelope.Thus,no failureoccurs(notethattheminimumprincipalstress, s0 3 ,iscompressivein Fig.2.4,andthusnotensilefailureoccursatthatpoint,either).If,asaresultof loading,theMohr’scircleisexpandedbyincreasingthemaximumeffective stressordecreasingtheminimumeffectivestress,theMohr’scirclemay eventuallytouchthefailureenvelope,resultinginshearfailure.Shearfailure canbeinducedalsobymovingtheMohr’scircleasawholeleftwardby increasingtheporepressureandthusreducingtheeffectivestresses.Eventually,thecirclewilltouchtheenvelopeinthiscase,againresultinginfailure.

Inpracticalrockmechanicsanalysis,thecurvedfailureenvelopeof Fig.2.4 isoftenapproximatedwithastraightline,asshownin Fig.2.5.The resultingfailurecriterionisknownasthe Mohr Coulombfailurecriterion. Theinterceptofthefailureenvelopewiththe s-axisisknownasthe cohesion, S0.Theslopeofthefailureenvelopeischaracterizedbythe coefﬁcientof internalfriction, m.Thus,theMohr Coulombcriterionhasthreeparameters: S0, m,and T0.

Ifacylindricalrockspecimenissubjectedtocompressiveloadingalong thecylinder’saxiswhileitssidesurfaceremainsstress-free,failureoccurs

FIGURE2.4 FailureenvelopeandMohr’scircle. StressesinRocks Chapter j 2 17

Apartfromtectonicforces,elevatedhorizontalstressescanbecausedby geologicalhistory,inparticulariftheformationunderwentupliftcausedby erosionoftheoverburden.Thehorizontalstressesactingbeforetheuplift remainlockedintherockunlesstheyaregraduallydissipatedbystress relaxationcausedbyviscoelasticbehavioroftherock.

Therefore,inreality,thecoefﬁcientinfrontof s0 v in Eq.[2.9] isnot necessarilyequalto nd =ð1 nd Þ,andtherelationshipbetweenthehorizontal andtheverticaleffectivestresscouldmoreaccuratelybewrittenasfollows:

Thecoefﬁcient K0 typicallyvariesbetween1and10atshallowdepth,and between0.2and1.5atgreatdepth [1]

Crudeestimatesofhorizontalstressescanbeobtainedbyusingempirical relations,suchastheBreckels vanEekelenequationsforthetotalminimum horizontalstressattheUSGulfCoast [1]: sh ¼ 0:0053D1 145 þ 0:46ðPp Ppn Þ at

where D isthedepth; Pp istheporepressure;and Ppn isthe“normalpore pressure”correspondingtotheporepressuregradientof10.5MPa/km.

Contrarytothesimpliﬁedmodelgivenby Eq.[2.11],thehorizontal stressesareoftennotequal: sH s sh.

Sofar,wehaveassumedthatoneoftheinsituprincipalstressesisvertical. Eventhoughthisistrueinmanycases,thedirectionsofprincipalstressesmay begravelyaffectedbysurfacetopography.Inparticular,nearaslope,the verticaldirectionmightnotcoincidewithanyoftheprincipalstressdirection (eg,seeRef. [3]).Inthevalley,thehorizontalstresscanbelargerthanthe verticalstressevenintheabsenceoftectonicforces.

Fig.2.6 illustratestheimpactofstructuralfeaturesontheinsitustresses onshore.Indeepwaterdrilling,horizontalstressanomaliessimilartothose

FIGURE2.6 Effectofsurfacetopographyonprincipalstressdirectionsandmagnitudes.

Chapter j 2 21

shownin Fig.2.6 areobservednearsurfacesofescarpments,asopposedto intrabasinareas.Nottakingthereducedhorizontalstressesintoaccountwhen designingawellmayresultinlostcirculationincidentswhendrillingthe top-holesection [4].Directionsofinsitustressesinthereservoircanbe affectedbynearbyfaults,domes,andothergeologicalfeatures.

Toillustratecomplexitiesofinsitustressstates,letusconsiderstress distributionsaroundsaltbodies.Drillingthroughsaltisoftenaccompanied withboreholeinstabilitiesandonexitingthesalt,lostcirculation.Anumerical geomechanicalanalysisperformedinRef. [5] providesavaluableinsightinto theproblemsexperiencedwhiledrillingsalt.Saltcannotsustainshearstresses overlongperiods.Stressrelaxationleadstogradualreductionofshearstresses insalt,sothatthestateofstressgraduallyapproacheshydrostaticcompression,with sv z sH z sh.Theinabilitytosustainshearstressesforlong periodsmakessaltsimilartoaveryviscousfluidwhich,asanyfluid,would eventuallyapproachhydrostaticequilibrium.Theinsitustressesinsurroundingrocks(eg,shale)mayontheotherhandbefarfromhydrostatic.In theGulfofMexico,thetectonicregimeisnormalfaulting,andthefar-field stressesinrocksaroundthesaltaresuchthat sv > sH z sh.Thediscrepancybetweenthestressfieldsinsidethesaltbodyandoutsideitleadsto significantcomplexityofthestressfieldaroundtheboundaryofthesalt. Aroundasaltbodythatisclosetosphericalinshape,thestressesarealtered comparedwiththeirfar-fieldvalues,asshownin Fig.2.7.Horizontalstresses arereducedaboveandbelowthesalt,whichexplainsthereducedfracturing pressureexperiencedonexitingthesaltwhiledrilling.Inthesideburden ie, intherockslaterallyadjacenttothesalt thestresschangesaremore

FIGURE2.7 Alterationofinsitustressesaroundanidealized,spheroidalsaltbody(shownin gray). BasedontheresultsofFredrichJT,CoblentzD,FossumAF,ThorneBJ.Stressperturbations adjacenttosaltbodiesinthedeepwaterGulfofMexico.SPEpaper84554presentedattheSPE AnnualTechnicalConferenceandExhibitionheldinDenver,Colorado,USA,October5 8,2003.

complex:horizontalstressesinthoseareasbecomeanisotropic,evenifthe far-ﬁeldhorizontalstressesareisotropic.Inaddition,principalstressesmaybe rotatedinthoseareas,withtheverticaldirectionnotbeingaprincipaldirection anymore [5].Thesestresschangeshaveadetrimentaleffectonborehole stability.

Reducedverticalstressesabovesaltbodiesindicatethatusingdensity proﬁlestoevaluatetheverticalinsitustress, Eq.[2.8],wouldoverestimatethe insituverticalstressintheseareas.Thereductionintheverticalstressabove thesaltundernaturalconditionsisanexampleofthe archingeffect,similarto theoneobservedduringdepletion(Section2.4).Note,however,thatthe archingeffectin Fig.2.7 iscausedsolelybynaturalstressredistributiondueto stressrelaxationbeforeanyreservoirdepletion.Inparticular,sincesaltis unabletosustainshearstresses,theverticalstressneedstoberedistributedso thatsomepartoftheverticalloadthatwouldotherwisebebornebythesaltis carriedbythesurroundingrock.

Thesphericalgeometryofthesaltbodyin Fig.2.7 isastrongidealization. Numericalanalyseswithmorerealisticgeometriesconﬁrmthatreducedhorizontalinsitustressespersistrightaboveandrightbelowthesalt [6].Thisis consistentwithlostcirculationincidentsexperiencedwhenenteringorexiting thesalt.Areductionofthefracturegradientby3ppg(0.36g/cm3)underneath thesaltwasreportedforawellintheGulfofMexico [6].

Iftheshapeofthesaltbodyisnotspheroidal(eg,inthecaseofasalt sheet),stressperturbationsaroundthesaltarelesssevere.Accordingtothe numericalresultsofFredrichetal. [5],thehydrostaticstressmagnitudeina spheroidalsaltbodyliesbetweenthefar-ﬁeldvaluesof sv and sH z sh.Ina saltbodywithlateralextensionlargerthanitsheightbyafactorof4ormore, thehydrostaticstressinthesaltisclosetothefar-ﬁeldverticalstress.

Inadditiontoreducedhorizontalstresses,therockbelowthesaltisoften damagedorbroken.These rubblezones furthercontributetomudlosseson exitingthesalt.

Theuncertaintyaboutthevaluesof sH, sh,andtheirdirectionscallfor experimentalmethodsthatwouldenablemeasurementoftheseparametersin theﬁeld.Someofthemethodscurrentlyusedintheindustrywillbediscussed in Section2.7.

Numericalmodelingisanotherwaytoobtainestimatesofbothverticaland horizontalstressesinandaroundthereservoir.Theuseofnumericalmodeling requiresthattherocks(reservoir,overburden,andsideburden)areproperly characterizedintermsofrockmechanicalproperties,oratleastthevaluesof theseparametersareconstrainedtowithinsufficientlynarrowmargins.Dueto scarcityofcorematerial,particularlyfromthecaprock,suchconstrainingis oftendifficult.Inspiteofthisdifficulty,examplesofnumericalevaluationof rockstressesareavailable.Forinstance,finite-elementstressanalysiswas usedtoevaluatestressesinandaroundasaltdiapirinthedeepwaterGulfof Mexico [6]