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Gas kick due to hydrates in the drillingfor offshore natural gas and oil

! ! !Leif Rune Helgeland

Andreas Andersen Kinn

Ole Flokketveit Kvalheim

Anders Wenaas

Department of Petroleum Engineering and Applied Geophysics

NTNU, Trondheim, November 2012

!

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Abstract

Dissociationofgashydratesareinvestigatedasapossiblereasonforgaskicksduringdrilling.

Gashydratesaresolidice-likestructurescontaininggasmolecules.Thebasicprinciplesofgas

hydratesandkicksareexplained,andmethodsfordiscoveringgashydratezonesbefore

drillingcommencesarereviewed.AnappraisalwellontheShenziBieldinthenorthernGulfof

Mexicothatmayhavehadahydraterelatedkickisusedasanexample.Gashydratesarealso

investigatedasapossiblecontributiontothe2010DeepwaterHorizonaccident.

Themainfocusisonavoidinggashydrateproblemswhendrilling,describingvariousdrilling

methodsandmuddesignsfoundmainlyinSPEliterature.ManagedPressureDrillingand

drillingwithcasingaredescribedasdrillingmethods.Mudweight,compositionand

temperatureofthemudarefoundtobeimportantfactors.

EarlyidentiBicationofhydratezonesbyacquiringseismicdatawasfoundtobeakeyfactorin

ordertotakepreventiveactionsbeforehydrateproblemsoccur.Itwasfoundthatafurther

understandingofhydratesisneededtoeffectivelysolvethechallengescurrentlyfaced.

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Table of contents

1. Introduction! 12. Gas hydrates

!2

2.1.Formation of gas hydrate zones! 42.2.Detection of gas hydrates! 5

3. Gas kick! 74. Drilling through hydrates! 9

4.1.Possible hydrate-related kick in the deepwater Gulf of Mexico! 104.2.Deepwater Horizon did hydrates cause the blowout?! 12

5. Avoiding gas hydrate problems ! 145.1.Drilling techniques to help prevent problems with hydrates [24]! 155.2.Drilling fluid design [25]! 165.3.Cement [27]! 17

6. Discussion! 187. Conclusion! 198. Abbreviations ! 209. References ! 21

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1. Introduction

Gashydrateshavealwaysprovidedchallengesinoffshoreproductionofnaturalgasandoil.

Themostcommonproblemispluggingofpipesandsubseaequipment,asubjectthathas

beenthoroughlyinvestigatedanddocumented.Thispaperwillfocusonproblemsregarding

hydratedissociationleadingtokicksduringdrillingoperations.Kicksasaresultofhydrate

dissociationhasnotbeengivenmuchattention,butithaspreviouslybeendocumentedboth

kicksandblowoutscausedbydrillingthroughhydratebearingsediments.

Sincepresenceofhydratescanresultinbothkicksandblowoutsitisimportanttounderstand

thebehaviorofhydrates,wheretheyarelocatedandhowwecanavoidtheproblemsweare

facingtodayregardinghydrates.Currentlythemostprominentdrillinghazardrelatedto

hydratesisunawarenessofitspresenceintheformation.Inordertodrillthroughhydratesin

asafemanner,earlydetectionofitspresenceisakeyfactor.Collectingseismicdataisone

waytomapthepresenceandlocationofhydrates.Ifthedatagivesindicationofhydratesin

theformation,actioncanbetakentosecureasafedrillingoperation.

GashydratesisalsobelievedtohavebeenacontributingfactorintheDeepwaterHorizon

accidentintheGulfofMexico.Formationofgashydrateshamperedthecontainmentofthe

blowoutafterthecatastrophewasafact.

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2. Gas hydrates

Gashydratesaresolidice-likestructures

containinggasmolecules.Theybelongtoa

groupofsubstancescalledclathrates,whichisa

chemicalsubstanceconsistingofacrystal

structureofonetypeofmoleculetrappingand

containingasecondtypeofmolecule[1].Inthe

caseofgashydrates,watercreatesthecrystal

structureandnaturalgasmoleculesare

trappedinside,asshowninFigure2.1.

Threedifferentstructuresofhydrateshave

beenidentiBied:I,IIandH.Theseareillustrated

inFigure2.2.Gashydratesusuallyforminthe

twoBirststructures.StructureIconsistsof46watermolecules,formingsmallandlargecages.

Thesmallcageisshapedlikeapentagonaldodecahedron(ageometricalshapewithtwelve

BlatfaceseachhavingBiveangels,written512),andthelargecageisshapedlikeahexagonal

truncatedtrapezohedron(14Blatfaces,twelveofthemwithBiveanglesandtwowithsix

angles,written51262).StructureIIconsistsof136watermolecules,alsoformingsmalland

largecages.ThesmallcageisthesameasintypeI,butthelargeoneisshapedlikea

hexadecahedron(16Blatfaces,twelveofthemwithBiveanglesandfourofthemwithsix

angles,written51264)[2].Thesetwostructuresarestabilizedwhengasmoleculesaretrapped

insidethecages[3].

2

Figure 2.1: Gas molecules trapped inside a crystal water

structure [1]


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Gasesthatarelikelytoformhydratesalongwithwaterare[4]:

Lightalkanes(methaneandiso-butane)

Carbondioxide

HydrogensulBide

Nitrogen

Oxygen

Argon

Animportantfeatureofgashydratesisthatonecubicmeterofsolidhydratecancontainupto

180standardcubicmetersofgas,dependingonthegas.Methaneisthepredominantgasin

naturaloccurringhydrates,makingup>99%ofhydrateintheoceanBloor[5].Formethane,

thevalueisabout164sm3[6].

Gashydratesformwhenwaterandgassesaremixedatrelativelyhighpressuresand

relativelylowtemperatures.Whilewaterformicecrystalsattemperaturesof0Candbelow,

gashydratescanformattemperaturesabovethisassoonasthepressureishigherthanafew

tensofbars,asshowninFigure2.3.Typicalconditionsforformationofhydratesare

pressuresabove30barsandtemperaturesbelow20C[3].Shallowdepthsoffshorecanhave

3

Figure 2.2: Structures of hydrates [3].


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theseproperties,usuallybetween100mand500mbelowtheoceanBloor[4].Thedepth

wheregashydrateswillformiscalledtheGasHydrateStabilityZone(GHSZ).

2.1. Formation of gas hydrate zones

Themethaneingashydratesisformedbybacteriainaprocesscalledanaerobicbacterial

methanogenesis.acteriahavebeenfoundatdepthsofupto800mbelowtheseaBloorin

marinesediments,andcanbeactiveattemperaturesupto113Candpressuresabove1000

bar[8].Thismeansthatthemethanemaybeformedwithinthegashydratestabilityzone

(GHSZ),oritcanmigratefromdeepersedimentsuntilitreachestheGHSZ.Atthisdepth,it

willformgashydratesifwaterispresent.

Gashydratesinoffshoresedimentscanexistinmanydifferentforms.InBinesilt/clayitoften

appearsasnodulesandveins,whileincoarse-grainedsedimentsitoftenformsas

disseminatedgrainsandporeBillings[6].Differentformsofgashydrateoccurrencesare

showninFigure2.4.

4

Figure 2.3: Gas hydrate phase diagram [7].


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2.2.Detection of gas hydrates

TobeabletoplanhowtomoveforwardindrillingthroughHydrateearingSediments(HS),

itiscrucialtodetectthezonesbeforedrillingcommences.Inordertodothis,acoustical

methodsareused,speciBicallycollectionofseismicdata.Thepresenceofgashydratesin

sedimentswillbeclearlyindicatedontheseismicdata,asitsacousticvelocityisveryhigh

(around3.3km/s,whichisabouttwicethevelocityofnormalsediments)[9].Two

characteristicsinseismicdataareusedtodetecthydratelocations;ottomSimulating

ReBlection(SR),andaphenomenoncalledblanking.

SRsarethemostwidelyusedindicatorforgashydratezonesoffshore.TheSRsmarkthe

phaseboundarybetweentheGHSZandthefreegaszonebelow.Seismicdatarecordsthe

reBlectionstrength,whichisproportionaltothechangeofacousticimpedance(theproductof

velocityanddensity).Asboththedensityandseismicvelocityinfreegasismuchlowerthan

ingashydrates,theboundarybetweenthezonesproducesaverystrongreBlectioninthe

seismicdata[9].

Inseismicdata,blankingisalocationwherethereisreductioninseismicreBlections,

resultinginanearlyblankzone.Thepresenceofgashydratesinporousmediaincreasesthe

averageacousticalvelocityinthelayers,resultinginreducedvelocitycontraststhatare

5

Figure 2.4: Potential gas hydrate occurrences [6].


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requiredtocreatestrongreBlections.Ingashydratezones,blankingwillthereforeoccur

abovetheSR[9].lankingandSRsareshowninFigure2.5below.

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Figure 2.5: Blanking and BSR [9].


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3. Gas kick

ThedeBinitionofakickisBlowofformationBluidsintothewellboreduringdrilling.Formation

Bluidswillenterthewellborewheneverthewellborepressuredropsbelowtheporepressure

(giventhattheporesarepermeable).TheformationBluidwillkickthedrillingmudoutof

thewell,resultinginasuddenincreaseinmudpitvolumeonthesurface[10].

Kickscanbecategorizedintwogroups[11]:

Underbalancedkicks:ThistypeofkickistheresultofmudweightbeinginsufBicientof

keepingtheformationBluidtrapped.Thiscanhappenwhendrillingthroughzones

withhigherpore-pressurethanexpected,ifthedrillingBluidislightenedornotto

speciBicationtobeginwith.

Inducedkicks:Thistypeofkickoccursifdynamicandtransientpressureeffectslower

thepressureinthewell.Forexamplewhenpullingdrillstringoutofthewell.

Kicksduetohydratedissociationwillbeexplainedinchapter4.

Whendetectingakick,itisimportanttotakeactiontopreventfurtherlossofcontrolofthe

well.Fordrillersitisimportanttobeabletopredictgasbehavior,assmallvolumesofgascan

bepotentiallydangerousbecauseofthehugeexpansion.

Ifthemaximumallowedannularshut-inpressureishigherthanthecasingpressure,killing

thewellisthestandardprocedure.Inordertokillawell,anewoverbalanceintheborehole

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Figure 3.1: Formation fluid entering wellbore [10]


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mustberestored.Pumpingdrillingmudwithhigherdensityrestoresthisoverbalance.There

aretwomainkilling-methodsusedintheindustrytoday[12]:

Drillersmethod,inthismethodtheformationBluidisdisplacedbeforeinjectingthekill

mud.Thisisthemostcommonmethodofrestoringanoverbalanceafterakickhas

beendetected.

Engineersmethodorthewait&weightmethodasitisoftencalled,increasesthemud

weightandstartspumpinginkillmudimmediately.

Failuretotakeproperactionwhenakickisdetectedcaninworstcasescenariosleadtothe

developmentofablowout.AblowoutistheuncontrolledBlowofreservoirBluidsintothe

wellbore.Themosttroublesomeblowoutsaretheundergroundblowouts,wherereservoir

BluidfromazonewithhighpressureBlowsintoazonewithlowerpressurewithinthe

wellbore[11].UndergroundblowoutsareverydifBiculttobringundercontrol,anditmay

takemonthstostoptheblowout.Ablowoutcanresultinseriousdamage.Lossofhumanlives,

largematerialandeconomicallossesarecommon.

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4. Drilling through hydrates

WhenHSaredrilledthroughachangeinpressureandtemperaturemayoccurandcausethe

hydratestobecomeunstable[13].Asexplainedinchapter2,if1m3ofmethanehydrate

dissociate,itwillproduce164m3methanegas.Whenavolumechangelikethathappensitwill

resultinakick,orinworstcasescenarioablowout.Attheshallowdepthswheregashydrates

usuallyareencountered,theblowoutpreventer(OP),riser,choke-andkilllinesare

normallynotinstalled[4].Hydratedissociationintheformationmaycauseproblemswith

wellborestabilityandsubsurfaceequipment,whichcanleadtoreducedefBiciencyandsafety

issuesofdrillingoperations.Gashydratescanalsobeencounteredatgreaterdepthswhenthe

OPandriserareinstalled.Equipmentonsurfaceandsubsurfacearemoreexposedtodanger

inthiscasebecausetherapidlyincreaseinvolumewillinvolveahugestrainonthe

equipment[4].Wellboreinstabilityduetohydratedissociationismainlycausedbytwo

problems:

Whenthehydratesdissociateinthewellbore,thedrillingmudwillexperiencea

reductionindensityandachangeinrheologyduetodissolvedgas.Thismayleadtoan

enlargementofthewellboreandevencollapseofthehole.

Whenhydratesdissociate,thesurroundingsedimentsmayexperienceanincreasein

permeabilityandareductioninstrength[13].

WhendrillingthroughHS,severalproblemscanbeencountered.Someoftheseare:

Subsidence,stuckpipe,gasleakingontheoutsideofthecasingandaninefBicientcementjob.

AnotherproblemisthatthedrillingwindowinHSisnotwellunderstood.Whendrilling

throughgashydratesyouneedtostayabovethecollapsepressure,belowthefracture

pressure,andatthesametimemanagethedissociationtemperatureandpressureofthe

hydrates[15].Failuretodothiscanleadtoagaskick.

WhendissolvedgasBlowstowardsthesurface,hydratesmayagainform.Someofthe

problemsthatmaybeencounteredonsubsurfaceequipmentaredescribedbelow[4]:

ChokeandKill-lineplugging:ThiscausesdifBicultyintheuseofthelinesduringwell

circulation

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PlugformationatorbelowtheOP:Well-pressuremonitoringbelowtheOPs

becomesimpossibleordifBicult

Plugformationaroundthedrillstringintheriser,OPsorcasing:Makesthedrill

stringmovementaproblem

PlugformationbetweenthedrillstringandOPs:Thiscausesproblemsinthefull

closureoftheOPswhennecessary

PlugformationintheramcavityoftheOPs:CausesdifBicultyinopeningtheOPs

fully.

UnawarenessofHSwhendrillingawelliscurrentlythemostprominentdrillinghazard.

[15].Inthefuturewearemostlikelyforcedtodrillindeeperwaters,arcticenvironmentsand

possiblyproducehydratesasasourceofenergy.Inordertodothisinasafemannerweneed

tounderstandandcontroltheproblemswecurrentlyarefacingregardinggashydrates.

4.1.Possiblehydrate-relatedkickinthedeepwaterGulfofMexico

AnappraisalwellontheShenziBieldinthenorthernGulfofMexicoexperiencedissuesrelated

topossiblehydrateformationsaftercementingthe22casing.Thewaterdepthatthis

particularwellis1323meters.Insuchgreatwaterdepthsthepressureandtemperature

conditionsareidealforhydratestoforminastablestructure.

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Figure 4.1: Graphical sketch displaying the problems that may be encountered when drilling through HBS [4]


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Thedrillingoperationwasperformedasfollowing:The22casingwasdrilledtoadepthof

2203metersMDandcementedinplace.WhenthecementreturnsreachedtheseaBloor,a

minorannularBlowofBluidizedsedimentsdroppingfromthecementportsweredetectedbya

subsearemotelyoperatedvehicle(ROV).Furtherdrillingoperationswhereputonholdfor

overaweektocheckandrepairequipment.DuringthisweektheROVdetectedadecreasein

BlowofBluidizedsediments,untilonlybubbleswereBlowingslowlyoutofthecementports

whichwerepartiallycloggedwithhydrates.TheROVcollectedsomesamplesofthebubbles

anditwasdeterminedtobe100%methane.Thedrillingwasresumedand16casingwasset

insaltat3560meters.TheROVdetectedthattheBlowrateofbubblesincreasedduringboth

drillingoperationsandwhenthecasingwascementedinplace.Ontheotherhand,adecrease

ofbubbleBlowrateweredetectedwhendrillingoperationswereputonholdforleakofftest

andequipmentrepairs.AccordingtotheinformationfromtheROVtheBluidizedsediments

seemstobearesultofanexothermicreactionduringcementingandthecirculationofwarm

drillingmudthroughthewell.ThisevidencestronglysuggeststhattheBluidizedsediments

andsubsequentgasbubbleswerearesultofdissociatedin-situgashydrates[17].

asedonthefactthatBluidizedsedimentsandbubbleswereBlowingoutofthecementports,

animproved3DseismicobtainedfrommeasurementswhiledrillingwasscreenedforSRs

andothergeophysicalindicationsofhydratedeposits.Amaximumnegativeamplitudeextractionwasgeneratedonthesurfacetoapproximately914metersbelowmudlineto

identifypossiblehigh-amplitudegasanomalies.Theresultsgaveseveralindicationsof

shallowgasanomalies,theshallowestatonly405metersbelowmudline.Aplotofthegas

anomalieswheremadeandiscalledaSR,whichisexplainedinchapter2.2.

Althoughhydratedissociationinthisparticularcasedidnotnegativelyimpactthedrilling

operation,besidesstoppingdrillingforaweek,itisworthnoticingthatmassivedissociation

ofhydratescouldpresentapotentialrisk.Earlyobservationsofpossiblehydratedissociation

arethereforepreferred.Possibletechniquesfordeterminingtheamount,distributionand

presenceofhydratesaregeotechnicalinvestigations,MeasuringWhileDrilling,borehole

wirelineloggingandseismicinversions[17].

.

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4.2.DeepwaterHorizondidhydratescausetheblowout?

Thesemi-submersiblerigDeepwaterHorizon

experiencedagasreleaseandasubsequentexplosionat

21:49CentralTimeonthe20thofApril2010[18].The

blowoutoccurredwhendrillinganexploratorywellin

theMacandoBieldintheGulfofMexicoat1500meters

seadepth.Therewere126personsworkingontherig

atthetimeoftheaccident,whereaselevenpersonslost

theirlives[19].

Complicationsfromthecementjobareconsideredas

oneofthereasonstotheaccidentatDeepwaterHorizon.Halliburtonperformedthecement

job[20].Just20hoursbeforetheaccident,cementwaspumpeddownthewelltosetcasing[21].TheseabedintheGulfofMexicoiswellknownforcontainingvastamountsofgas

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Figure 4.2: 3-D seismic showing gas anomalies, this kind of plotting is called a BSR. [17]

Figure 4.3: Incident site [19]


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hydrates.Aswillbeexplainedinchapter5.3,thesettingofcementleadstoreleaseofheat.

Thecementwillbeindirectcontactwiththeformation,asitispumpedbetweenthecasing

andtheformation.Onetheoryisthattheheatreleasedfromthecementcouldhaveaffected

thestabilityofgashydratesintheformation,leadingthemtomelt.Anotheristhatmeltingof

hydratesduringdrillingcreatedlargecavesinthewell,sothatwhencementwaspumped,it

wasnotsufBicienttocementtheentirecasinginplace.Furthermeltingofthehydrateswould

thenleadtoagaskickoutsidethecasing,andwithnocementtostopthegas,itwouldreach

thesurface.Survivorsoftheaccidentexplainedthatjustbeforetheexplosion,thepressure

hadbeenreducedinthedrillcolumnandheatwasappliedtosetthecementsealaroundthe

wellhead.Anexpandingbubbleofmethanethenbrokethroughvarioussafetybarriersbefore

explodingontheplatform[22].

Gashydratesalsoplayedamajorroleinthefailureofcontainingtheleakduringdrillingofthe

reliefwell.Pconstructeda100tonnedomeplanningtoplaceitovertheleak,guidingtheoil

throughafunneltobecollectedonavessel,asshowninBigure4.4.Thedomewasplacedover

theleakthe7thofMay.Tendayslatertheyconcludedthattheoperationwasafailure.Asoil

andgaswastransferredupthefunnel,thegasreactedwithwatertoformgashydrates,and

quicklypluggedthepipe[20].

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Figure 4.4: Dome placed over the leak [23]


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5. Avoiding gas hydrate problems

HShasbeendrilledthroughsuccessfully,eventhoughtherehavebeenreportedcasesof

blowoutscausedbyhydratedissociation.othinitialformationcharacteristicsandbottom-

holeconditionslikemudtemperatureandpressure,willdecidetheamountofgashydrate

thatisdissociated.

TodrillsafelythroughHS,wehavetodevelopabetterunderstandingofgashydrates,so

thatweareabletoidentifyproblemsthatmayoccurandhowtopreventthem.Some

techniquesadoptedsofar[13]:

CoolingthedrillingBluid.

Increasingthemudweighttostabilizethehydrates,butavoidingfracturingtheHS.

AddingchemicalinhibitorsandkineticadditivestothedrillingBluidtopreventhydrate

formationandtoreducehydratedestabilizationintheformation.

Acceleratingdrillingbyrunningcasingimmediatelyafterhydratesareencountered

andusingacementofhighstrengthandlowheatofhydration.

Tostaywithinasafeoperationalenvelopewhendrillingthroughgashydrates,youhaveto

assesstheallowabledrillingparameters:

MudWeight

Composition

Temperature

Youalsohavetoevaluatefactorslike[13]:

EffectofdrillingBluidonheatingtheformationandchangingthestressesandpore

pressure

Effectofheatontheformationsthermodynamicstabilityofthehydratesandwellbore

stability

EffectofreductioninHSstrengthandlossofcohesionduetohydratedissociation.

Therefore,modelingwellborestabilityinHSrequiresconsiderationofvariousmechanisms

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GashydratescancauseproblemsbothduringandafterHSareencountered.Whendrilling

through,piecesofhydratearecarriedupbytheBluidandturnedintogasbecauseofpressure

andtemperaturechanges.Oncepastthezone,gashydrateatthefrontoftheholecancontinue

todissociateduetotemperatureandpressureinthemud.othscenariosreleasesgasintothe

mud,alteringitsrheology,whichcanbedangerousifnottakenintoconsiderationwhen

designingthemud.Soitisimportanttochoosetherightdrillingmethodandmud,todeal

withtheissuebothduringandafterdrilling.

5.1. Drilling techniques to help prevent problems with hydrates [24]

ManagedPressureDrilling(MPD)

MPDhaveauniqueapplication,itisadrillingprocessusedtocontrolthepressureproBile

throughoutthewellbore.MPDtechniquesmaypreventformationinBluxbyascertainingthe

pressuresinthewellborewithintheenvironmentlimits.Itallowsforfastercorrectiveaction,

asitmayincludecontrolofbackpressure,Bluiddensity,Bluidrheology,annularBluidlevels,

circulatingfrictionandholegeometry.ThisprovidesbettercontrolifformationinBluxis

encountered,whichmeansthatthistechniquecanbeusedinwellswithnarrowpressure

windows.

SlimandInsulatedMarineRiser

Drillingindeep-seaandcoldwater,thereisaneedforinsulatedrisers.Slimmerrisermeans

thereturnswillhavehighervelocity.WhendrillingthroughHStherewillbelesstimefor

heattransfertowarmthereturns,whichagainminimizesthedissociationofhydrates.

Drillingwithcasing

Theformationmightberatherfragileandthewellboreshouldbecasedasquicklyaspossible.

Aone-tripdrillingsystem,thatcarriescasingwithitandthepossibilityforfastcementing

couldbetheanswer.Drillingwithcasingsolvesthis.Insteadofdrillpipe,casingisusedand

cementedinplaceassoonasthesectionisdrilled.Thisprotectswellborefromtheformation

andHSandpreventsinBlux.

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5.2. Drilling fluid design [25]

WhendesigningadrillingBluidtobeagashydrateinhibitor,characteristicsincludedshouldbe[26]:

Lowestpossibledensitywithmaximumhydratesuppression

CompatibilitywithmostcommondrillingBluidcomponents

CompatibilitywithmostsaltstobalancehydratessuppressionandBluiddensity

Provideshaleinhibitionforadequateboreholeanddrillcuttingsstability

Ashydratescanforminpipes,OP,risersetc.itisimportantthatonceawellisintheGHSZ,

hydrateinhibitors/suppressorsareaddedtothedrillingBluid.

Wellsarebeingdrilleddeeper,andtheneedforimprovedandcustomizeddrillingBluidto

handlethevaryingconditionisneeded.Itmustdealwithchangingmudlinetemperaturesto

preventhydratesfromformingandmaintainingthedrillingBluidproperties.Agoodwaytodo

thisismakingsalinedrillingBluids,whichmeansaddingsalts.Thishasprovensuccessfulin

wellsdowntoabout2300minthegulfofMexico.HighperformancedrillingBluidsisagoodwaytohelpstophydratesfromblockingtheOPandchokelines.TheNorthSeahasalower

mudlinetemperature,sothermodynamicinhibitorsmaynotbesufBicienttopreventhydrates

fromformingandadditionaladditivesmightbeneeded.

Syntheticbasedmud

Syntheticbasedmudisalowtoxicityoilbasedmud.Dependingonthepressure,themudwill

absorbthegasandreducetheamountofgasreachingtheOPsandchokelines.

KineticInhibitorsandCrystalModiiers

Anotherwayistoslowdowntheprocessandpreventtheagglomerationofgashydrate

formation,onewaytodothatisbyusingcrystalmodiBiers.Thisisaprocessthatusesmostly

polymericandsurfactant-basedchemicalstoalterthenucleation(aphysicalreactionthat

occurswhenpartsofasolutionstartstoprecipitateout)andgrowthofhydratecrystals.Can

alsousekineticinhibitorstopreventcriticalnucleifromappearingandbythatinhibiting

formation.Thesechemicalswillnotstopgashydratesfromdissociating,butinhibiting

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formationofitinpipes,OPetc.Thechallengewiththismethodisthesurfaceacitivityofthe

kineticinhibitorandchoosingtherightchemicalsinhighsalinitydrillingBluid.

Gashydratepills

Gashydratepillscontainhighlyinhibitiveformulationsandcanbeutilizedwhenneeded.This

meansthatthepillsareplacedinOPs,chokeandkilllinesetc.andwhenagaskick,shut

downorabandonmentoccurs,thesepillsareactivated,preventinggashydratefromforming.

5.3. Cement [27]

Ascasingisruninthehole,ithastobecementedinplaceasfastaspossibletostabilizethe

wellbore.Cementhasanexothermicreactionwithwater,heatingthecement.Thehydration

ratedependsonthevariouscomponentsofthedrillingBluidandalsoonthetemperatureof

thesurroundings.Deepwaterwellsarearealchallengeconsideringthelowsurrounding

temperatures.Lowertemperatureswilldecreasethehydrationrate,causingthecementtoset

moreslowly.Theheatreleasedwillthenhavemoretimetodestabilizethegashydratesand

possiblyweakenthecement.Tobeabletodealwiththisproblem,cementdesignedto

preventdestabilizationofgashydrateshastobeused.Ingredientshavetobechosen

dependingonwellpropertiesandtestingtheadiabatictemperaturerisetoBindamixthatdo

notreleaseheatabovethehydratedestabilizationpoint.OneexampleofthiscanbeusingC2S

insteadofC3S,asC2Sonlyreleasesalmostathirdoftheheattothesurroundingscompared

toC3S.

2C3S+6HC3S2H3+3CH;H=-114KJ/mole

2C2S+4HC3S2H3+3CH;H=-43KJ/mole

IfrunningcasingasquicklyaspossibleafterdrillingpasttheHSzoneandchoosingtheright

cementslurry,youcanavoidalotofproblemsandmakeasaferwell.

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6. Discussion

Therehasonlybeenreportedminorincidentsofkicksduetohydratedissociationduring

drilling.Asweareforcedintocolderenvironments,suchasthearctic,thismaybecomea

morecommonproblem.Hydratescanbeencounteredatshallowdepthsandprettymuchall

overtheglobe.Thevastdistributionofhydratesgivesreasontobelievethatkicksdueto

hydratedissociationoccurfairlyoften.

Dissociationofhydratesmayhavebeenacontributingfactorintheblowoutaccidenton

DeepwaterHorizon.Whetherornotthisactuallywasacontributingfactor,israther

uncertain.ItisdocumentedandwellknownthattheGulfofMexicocontainsvastamountsof

gashydrates,whichposestheopportunitythathydratesmayhavedissociatedandcreateda

largebubbleofmethane.IfthiswasthecaseonDeepwaterHorizon,itmayalsohaveoccurred

inotherplacesaswell.

InordertoBindmorehydrocarbonsinthefuturewearemostlikelyforcedintoareaswhere

hydratesarestronglypresent.Whetherornothydrateswillposeadrillinghazardinthese

areasissomewhatunclear,butiflargeamountsofhydratesareencountereditmaypose

someseriousrisksthatshouldbetakeninaccountduringwellplanning.

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7. Conclusion

Thispaperhasexplainedanddiscussedhydratechallengeswhendrillingforoffshorenatural

gasandoil.Gaskicksasaresultofhydratesarenotcommonintheindustrytoday,butitis

likelytoseemoreofthisinthefutureastheindustrypushesforwardtolookfor

hydrocarbonsinmoreextremelocations.

HydrateswerealsoinvestigatedasapotentialcontributorintheDeepwaterHorizonaccident.

Hydrateswasfoundtobeapossiblecontributingfactortotheaccident,anditmadeattempts

oflimitingtheextentofthedamagesharder.Attemptstoplaceadomeontopofthewell

failedduetohydratespluggingthepipe.

Differentmethodsofalteringthemudpropertyanddifferentdrillingtechniqueswere

investigatedaspreventivemeasuresagainsthydrateforming.EarlyidentiBicationofhydrate

zonesbycollectionofseismicdatawasalsofoundtobekeytoensurethatpreventiveactions

canbetakenbeforehydrateproblemsoccur.

Althoughsomesolutionswereproposedinthispapertopreventhydrateproblemsitwas

foundthatafurtherunderstandingofhydratesisneededtoeffectivelysolvethechallenges

ahead.

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8. Abbreviations

OP-lowoutpreventer

SR-ottomSimulatingReBlection

GHSZ-Gashydratestabilityzone

HS-HydrateearingSediments

MPD-ManagedPressureDrilling

ROV-RemotelyOperatedVehicle

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9. References

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