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Deformation Mechanisms and Safe Drilling Fluids Density in Extremely Thick Salt Formations

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Deformation Mechanisms and Safe Drilling Fluids Density in Extremely Thick Salt Formations

    Deformation Mechanisms and Safe Drilling

    Fluids Density in Extremely Thick Salt

    Formations

    2007PetroleumScience,,o1.4No.4

    DeformationMechanismsandSafeDrillingFluidsDensityin

    ExtremelyThickSaltFormations

    YangHenglin,ChenMianandZhangGuangqing

    f1.DrillingResearchInstituteCNPC.Beijing100097,China)

    f2.SchoolofPetroleumEngineering,ChinaUniversityofPetroleum,Beijing102249.China)

    Abstract:HydrocarbonsareveryoftenassociatedwithSaltstructures.Theoi1andgasindustryisoftenrequiredtodill1

    alongandthroughlongsaltsectionstoreachandrecoverhydrocarbons.Theuniquephysicalpropertiesofsaltrequirespecial

    techniquestoensureboreholestabilityandadequatecasingdesign.Thispaperassumedthatthemechanicalbehaviorofsalt

    isregulatedbythemagnitudeofmeanstressandoctahedralshearstressandundertheinfluenceofdifferentstressconditions

    thedeformationofrocksaltcanberepresentedbythreedomainsi.e.compressiondomain.volumeunchangeddomain.and

    dilatancydomain,whichareseparatedbyastressdependentboundary.Inthecompressiondomain,thevolumeofsalt

    decreasesunti1al1microcracksareclosed.withonlyelasticdeformationandpurecreep;inthevolumeunchangeddomain

    thedeformationisconsideredsteadyincompressibleflowcontrolledbypurecreep:andinthedilatancydomainthevolume

ofsaltincreasesduringdeformationduetomicro

    cracking.causingdamageandaccelerating"creep"untilfailure.Thispaper presentsahypothesisthattheboreholeisstableonlywhenthemagnitudeofoctahedra1shearst

    ressisbelowthedilatancy

    boundary.Itgivesthedesignmethodfordeterminingdrillingfluidsdensity,andcalculatesthe

    closurerateofboreholewith

    therecommendeddrillingfluidsdensity.Iftheclosurerateoftheboreholeis1essthan0.1%.th

    edrillingfluidsdensity

    windowcanbeusedduringdrillingthroughextremelythicksaltformations. Keywords:Rocksalt,boreholestability,deformationmechanisms,constitutiveequations,e

    quivalentviscosity,drilling

    fluidsdensity

    1.Introduction

    Thepresenceofsaltstructuresinoilandgas

    prospectsis.initself,afactorthatincreasesthe

    probabilityofSuccessduetothefavorableconditions

    forhydrocarbonaccumulationandtrapping.Saltdomes

    aredistributedmainlyintheU.S.GulfCoastBasin.the

    WestAfricaCoastBasin,thePricaspianBasinandthe

    TarimBasinfVolozh.eta1..2003).Withthe

    developmentofpetroleumindustry.theneedtoreach

    hydrocarbonreservesbeneathsaltstructureshas

    increased.Ithasbecomecommonplacetodrillthrough

    saltdomeinordertoreachreservesbeneathsalt

    structures.However,experiencewithdrillingoperations

    throughsaltformationsshowthattherockinthe

    immediatevicinityoftheboreholeiscapableofcreep

    anddilatancy.Manyoperationalproblemssuchas

    reductioninboreholediameterandstickingofbitwere

    reportedwhendrillingthroughthosesaltlayers(Barker,

    eta1.,1992;Willson,eta1.,2002).Thepurposeofthis studyistopresentanexperimentalandtheoreticalstudy ofthedefornlationmechanisms,toprovideguidelines ofdesigningdrillingfluidsdensitythroughextremely thicksaltformations,andtopredictthedegreeofsalt shrinkagewhichmayoccurinagivenwel1.

    2.Deformationmechanismsofrocksalt

    Inthestudyofthemechanicalbehaviorofrocksalt intheimmediatevicinityofaborehole,itisimportant tohaveagoodunderstandingofthecompression, dilatancy,anddeformationpropertiesofrocksalt.The elasticbehavior,damage,andfailureofrocksaltare bestdescribedbyacompressionanddilatancytheory reportedinrecentyears(Chan,eta1.,1992;Cristescu andHunsche,1992;1993).Giventhattheprincipal stressactingonanelementare0-1,0-2,and0-3,itis possibletodefinetwoquantities,i.e.themeanstress0-m andtheoctahedralshearstressesr0ct.Theycanbe givenbythefollowingequations:

    1

    o-=?,(1)

    f0c1=

    whereI1isthefirstinvariantofthestresstensor:and isthesecondinvariantofthestressdeviatortensor. Thesequantitiesmaybeexpressedasfollows: I1=(7i,

    1

    J2s

    

    su

Vo1.4No.4DeformationMechanismsandSafeDrillingFluidsDensityinExtremelyThickS

    altFormations57

    Themeanstressisameasureofthetensionor compressionthattheelementexperiences,andthe octahedralshearstressisameasureofthedeformation stresstowhichtheelementissubjected.Fig.1isa graphicalrepresentationofthisstatement. Fig.1Meanandoctahedralstresses

    Theknowledgeofdilatancybehavioraswellasthe domainsofvolumeunchangedandcompressionofrock saltisessentialforboreholestability.Shortand longtermtestshaveshowntheexistenceofthe boundarybetweendilatancyandcompressiondomains inthestressspaceforrocksalt.Fromtruetriaxialtests. Hunschef1998)obtainedtheequationofcompression boundaryasfollows:

    r=C12+C2

    whereTo

    istheoctahedralshearstressatthe

    compressionboundary;C1andC2areconstants, C1=0.1697andC2=0.8996.Wu(2003)reported analogousresultsinhisPhDthesis.

    Mcanstress,MPa

    Fig.2Thedilatancy,volumeunchangedandcompression behaviorofrocksalt

    Inthecompressiondomain,therateofirreversible volumetricstrainispositive,herethecompressionstrain consideredistakenaspositive,whichmeansthatthe absolutevalueofirreversiblevolumeisreduced.The

    decreaseinirreversiblevolumetricstrainmayclose microcracksandporeswhichmaybepresentin

    geomaterials.Besidesthehealingeffect,oneonlyobserves purecreepproducedpredominantlybythemovementof dislocationsand,ofcourse,elasticdeformation. Whenthemeanstressismorethan25MPa.the

    boundarybet,veencompressiondomainanddilatancy domainisratherabandthanalinesincetheminimum inthecurveofthevolumetricstrainisquitewide; withinthisbandthevolumeispracticallyunchanged withtime.Alkana,eta1.(2007)presentedan experimentalinvestigationoftherocksaltdilatancy boundarybasedonacousticemission.Thedilatancy boundarycanbeestimatedfromtheintersectionofthe cumulativeAEnumberlinesbeforeandafterthe dilatancyboundary.Afterameanstressvalueof approximately50MPa.nofurtherincreaseofthe dilatancyboundaryisexpectedwithincreasing deviatoricstress.Sotheboundarylineisassumedtobe ahorizontallineapproximatelytangentialtothe compressionboundary:

    r.d=11.92MPa

    where0distheoctahedralshearstressatdilatancy boundary.

    Inthedilatancydomain.therateofirreversible volumetricstrainisnegative.whichmeansthatthe absolutevalueofirreversiblevolumeisincreasing.The increaseinirreversiblevolumetricdeformationis relatedtotheopeningofmicrocracksandpores,which

    alsocanbecalledcreep.andeven"stationarycreep".It

    causesdamageandincreasespermeabilitysoasto accelerating"creep"andcreeprupture.Thefollowing equationcanbeusedtoapproximatethefailure boundary(Hunsche,1998).

    Tof=C3C4expC)

    whereofistheoetahedralshearstressatfailure;C3 =

    38.0MPa.C4=34.9MPaandC5=0.04.

    3.Constitutivecharacteristicsofrocksalt Itisveryimportanttodeterminethedeformation behaviorwithadequateaccuracyfortheradialclosure ratemustbelimitedtoaspecificregionduringdrilling throughsaltformations.Constitutiveequations characterizethematerialanditsresponsetoloadings conditions.

    3.1Elasticrelation

    Assumingthattherocksaltishomogeneousand isotropicfortheelasticresponseofrocksalt,the ,?2高兰二毒p.0

58PetroleumScience2007

    instantaneousresponsecanbeexpressedbytheelastic relation:

    ge=

    2G

    whereg:isthedeviatoricstraintensor;

    deviatoricstresstensor:andGistheelastic modulus.

    Theelasticdeviatoricstrainrateisgivenby

    .

=

    2G

    with

    ..

    6-kk6l

    3

    (8)

    s

    f,

    1s

    shear

    3.2Power-lawcreep(Steadystatecreep)

    Theknowledgeofcreepbehaviorisveryimportant f0rreliablemodelcalculationsespeciallyinsoftrocks. Itisthatpartofgeomechanicalbehaviorwhichhasthe greatestinfluenceonthetimedependentstrainand stressevolutioninasystem.Therefore.itisvery importanttodeterminethecreepbehaviorwith adequateaccuracy.ConstantstressfHansenandCarter. 1984)andconstantstrainrate(HorsemanandHandin, 1990)experimentsathightemperaturesandpressures werecarriedouttoobtainaconstitutiverelationforthe steady.stateflowofrocksalt.Atsteady.state,bothsets ofexperimentsfittedapower.lawcreeprelationofthe form:

    

    (-

    whereAisthepreexponentialcoefficient,MPa-n.s;Q

    istheapparentactivationenergyforflow.J/mol;is thestressexponent;A,Q,andareconstants

    determinedexperimentally;Ristheuniversalgas constant,R=8.32J/(mol'K);Tistheabsolute temperature,K;o-eqiStheequivalentstressandeq istheequivalentstrainrate,andcanbeexpressedas follows:

    e.

    =

    ==(13)

    whereD2isthesecondinvariantofthestrainratetensor andgivenby

    D:=(14)

    Therearemanycreepparametersforthesteadystate creeplaw,awidelyusedoneisBGRa(Hunscheand Hampel,1999):

    2.p

    (-

    Sincethecreepdeviatoricstrainrateiscoaxialwith

    thedeviatoricstresstensor,andtheviscousdeviatoric strainrateisgivenby

    Inthecompressionregion,thetotaldeviatoric strainrateisgivenby:

    .

    3s

    +

    3.3Equivalentviscosity

    Inthevolumeunchangeddomain,creepis

    consideredsteadyincompressibleflow,whichcanbe solvedfromfluiddynamicsequations.Thepower.1aw creepcanbeexpressedintermsofequivalentstressand strainasshowninEq.(11),soanexpressionforan

    equivalentviscositykteqcanbeobtainedinthevolume unchangeddomaih;thegeneralconstitutiveequation relatesthedeviatoricstresstothestrainratetensorsas: s=2q

    ItisusefultoexpressEq.(18)intermsofeffective orequivalentquantitiessothatanexpressionfor equivalentviscositycanbederived.Squaringeachside ofEq.(18)andcombiningwithEqs.(12)and(13), leadstotheexpressionforanequivalentviscosity: e0

    3e0

    Substitutingthegeneralpowerlawcreeprelation

    (Eq.(11))andequivalentstrainrate(Eq.(13))intoEq. (19),givesanexpressionforthenonlinearviscosityfor thepowerlaw:

    1"

    .q=0?D2"(20)

    with

    0

    I

    

    -

    

    n

    2

    JrQp

    Jl

Vo1.4No.4DeformationMechanismsandSafeDrillingFluidsDensityinExtremelyThickS

    altFormations59

    4.Safedrillingfluidsdensitywindowforrock saltformations

    4.1Designofdrillingfluidsdensity

    Intheneighborhoodoftheboreholeawayfromthe bit.consideringaninfinitelylongcylinderinaninfinite medium.thestressdistributionintheimmediatevicinity oftheboreholecanbeobtainedbythefollowing expressionfInfanteandChenevert,1989).

    .gz.P)

    +.c22

    19-Pogz

    wherepqisrocksaltdensity;rwisthewellboreradius; andpwiswellborehydrostaticpressure.

    Forthesestresses,themeanstressandoctahedral ompogZ(23)

    o.8?7()(24)

    Themaximumoctahedralshearstressoccursatthe wellborewall,i.e.,where

    f.=0.817(p.gzP)(25)

    Ifthehydrostaticpressureofthedrillingfluidused indrillingawellislessthanthesaltstress.saltwill creepintothewellboreduringdrillingthroughsalt formations.Theclosurerateofsaltincreaseswith increasingtemperatureandincreasingdifferential pressurebetweensaltstressanddrillingfluidcolumn hydrostaticpressure.Aconstantvolumeoraslowly increasingsaltvolumecanbeobservedaslongasthe octahedralshearstressisbelowthedilatancyboundary, andarapidlyincreasingvolumeabovethatboundary. Soitissafeonlyifthereisnodilatancyinthe

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