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Fundamentals of Industrial Instrumentation and Process Control, Second Edition William C Dunn
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PREFACE
Wellcontrolproblemsarealwaysinteresting.Therawpowerthatisreleased bynatureintheformofanoilorgaswellblowingoutofcontrolisawesome. Certainly,wellcontrolisonething,andWILDwellcontrolissomething else.Therewillbewellcontrolproblemsandwildwellsaslongasthere aredrillingoperationsanywhereintheworld.Itjustgoeswiththeterritory.
Theconsequencesoffailurearesevere.Eventhemostsimpleblowout situationcanresultinthelossofmillionsofdollarsinequipmentandvaluable naturalresources.Thesesituationscanalsoresultinthelossofsomething muchmorevaluable—humanlife.Wellcontrolproblemsandblowouts arenotparticular.Theyoccurintheoperationsoftheverylargestcompanies andtheverysmallest.Theyoccurinthemostcomplexoperationssuchas deep,high-pressuregaswells,andtheyoccurinthesimplestshallowoperations.Menhavelosttheirliveswhenthingswentwrongatsurfacepressures of12,000psiandatsurfacepressuresof15psi.Thepotentialforwellcontrol problemsandblowoutsiseverpresent.
Timereallyflies!OnceIwasyoungandnowIamold.Itseemstohave happenedinthetwinklingofaneye.But,alothastranspiredinbetween. Throughthoseyears,Ihadtheprivilegeofbeinginvolvedinmostofthe industry’smostmonumentalandpublicwellcontrolevents,includingthe Al-AwdaProject—theoilfiresofKuwait—andthetragedyontheDeep WaterHorizon,theMacondoblowout.Iworkedoneverymajorblowout inCanadasinceLodgepoleincludingOldSalty,theindustry’soldestthat blewoutin1916onthePeaceRiverinnorthernAlberta.I’vebeeninvolved intheevolutionofoilwellfirefightingononesideoftheequationandthe introductionofengineeringintowellcontrolandhelpedpioneerthetechnicalaspectsofwellcontrol.
Lotshashappenedsincethistextwaslastupdated.Themostsignificantis this.AlltheLionsaregone!We’velosttheiconsofthewellcontrolindustry.
IstartedasadrillingengineerforHumbleOilandRefining,theforerunnerofExxon,inOklahomaCity.WewereworkingintheArkomaBasin andweredrillingwithair.Ofcourse,everywellwasablowoutwiththeonly differencebeinghowbig.Irecallsomethathadfireouttheendoftheblooie lineashighasthecrownonthederrick!Ilearnedalotaboutwellcontrolbut nothingaboutclassicproceduressincethesewerenoclassicoperations.
Shortlyafterthat,IleftHumbleandafewshortmonthslaterstartedin theconsultingbusinessintheTexasPanhandle.Thereweretwoconsultants inallofwesternOklahomaandtheTexasPanhandle—meandCWKelly. KellyhadmostofthebusinessbutIsqueakedby.
AroundthePanhandleandwesternOklahoma,itwascommonpractice todrillintotheMorrowsandswithoutsettingaprotectivestring.TheMorrowwasanabnormallypressuredstratigraphicsandandwasunpredictable. Asaresult,itwasstandardoperatingproceduretodrillintotheMorrow withoutsettingcasingontop.Everyeffortwasmadetominimizeexposure. Theplanwasusuallytodrillnomorethanacoupleoffeetofanydrilling breakintheMorrowandthencirculatetoseeifitcouldbemanaged. Thewellscouldnotbeshutinastherewasalwaysonlyafewhundredfeet ofsurfacepipesetandthousandsoffeetofnormallypressuredinterval betweenthecasingseatandtheMorrow.
Mostoften,thepermeabilityintheMorrowwaslowandthedeliverabilityequallylow.Wecouldhandlethegasweencountered.But,prettyregularly,wewouldhitagoodMorrowsandthatgetsourhatsblownoff!About allyoucoulddowasberiggedupwithagoodchokeandseparatorandletit blowtothepituntilyoucouldgetitkilled.Iwasveryfrustratedbecausewe didn’thavethetechnicalskillstoknowhowmuchmuddensityandpump rateweneededtokillthewell.Oftentimes,thewellwouldjustblowwhateverwepumpedbackintoourfacesuntilaftermanytrialanderrorefforts, werandomlylandedontherightcombinationofmudweightand pumprate.
IrememberonewhenweworkedonforasmallcompanyoutofDallas calledFilonExploration.WeworkedforthenicestguyinTexas,buthe didn’tknowmuchaboutdrillingandnothingaboutwellcontrol.Wewere drillingaMorrowwellnearHiggins,Texas,andwehadonlyashortstringof surfacecasingset.WeweregettingclosetotheMorrow,andIcalledour contactandtoldhimthatifwefoundwhatwewerelookingfor,weregoing togetourbuttsblownoff.Ourcontacttoldmethatwewereoffsettingawell thathadnoMorrowandyoucouldthrowarockandhitthatlocation.Itold himthatdidn’tmeananythingintheMorrow,andwehadbettergetrigged uptohandlesomegas.Heaskedwhatweneeded.Itoldhimagoodseparator,chokemanifold,drillingchoke,andnaileddownflowlines.Hesaid hewouldcallmeback.
Hecalledbackandtoldmehehadtalkedtothedrillingcontractorand thedrillingcontractorassuredhimthatifwedidhitsomegas,wecouldrig allthatstuffup.Itoldhimthatwasnotright,butwewoulddowhatever hewanted.
Icalledourguyattherigandtoldhimtoputanoteontheboardinthe doghousetodrillnomorethan2ftofanydrillingbreakfromthatpoint forward.Hedid.
Laterthatnight,Igotacallfromourguy.Thedrillerhaddrilled26ftof Morrowsandin18min.Hesaidwehadabout800psionthecasing.Itwasa fewdaysbeforeChristmas.IkissedmywifegoodbyeandtoldherIdidn’t knowwhenIwouldbeback.IhadnothadtomissChristmasathomebut wassurethiswastobethefirst.
Iwenttotherig.IcalledJerryandhemetmethere.Wegotriggedup andopenedthewellupthroughthechokes.Itwasscreaming!WehadaBIG Morrowwell.Weriggedupandpumped16ppgmudatabout13bpm.The wellneverblinked!Weregroupedandcalledtheoffice.Ourcontactsaidwe werenotdoingitrightbecausewewerelosingcirculationandshouldbe pumping12ppgmud!Weaboutfainted.Jerrytoldhimhewasnotsure whatwouldworkbutthatwouldnotwork.Filonbackedoffandletus gobacktowork.Wegotallthehydraulichorsepowerwecouldfind,went backouttothewell,mixedupafewhundredbarrelsof20ppgmud, pumpeditat20bpm,andkilledthewell.
IsadlylamentedtoJerrythatFilonwouldinsistthatweworkover Christmas.Wegotitkilledandcementedthedrillcollarsabout3:00am onChristmasEve.Weweresittinginthetrailerattherig.Jerryasked theHalliburtonengineerhowlongweshouldwaitonthecement.The engineerpausedandthoughtforaminute.Jerryinterjectedandproceeded toremindhimhowmuchworkGSMdidaroundthePanhandleduringthe courseoftheyear.JerrytoldhimthatifHalliburtoneverwantedtodo anotherjobforGSM,hisanswerwouldbe72h!Theengineerthought foraminuteandthensaid—Ithinkweshouldwaitoncementabout 72h!WethencalledFilon.Ofcourse,theywantedustogotoworkimmediatelytowhichwereplied,wewantedtogotoworktoobutHalliburton saidweneededtowaitonthecementfor72h.Filonreluctantlyagreedand wewenthomeforChristmas.
Thatjobmadeabigimpressiononme.Iresolvedtosolvethosefluid dynamicmysteriesinthefuture.WhenIwasatHumble,Ilearnedthatthere wasabigseparationbetweenoperationsandengineering.WhenIgottothe field,Ifoundthatchasmwasthegreatestinwellcontrol.Nooneeven attemptedtoapplyengineeringprinciplestowellcontroleventsbeyond theclassicalwellcontrolprocedures.Iharboredtheopinionthatblowouts werenodifferentthananyotherengineeringproblem.Wejustoftendidn’t havealotofdatatoworkwithwhichmeantourzoneofuncertaintywould bebigger.
CHAPTERONE EquipmentinWellControl Operations
“ … Icouldseethatwewerehavingablowout!Gastothesurfaceat 0940hours.”
0940TO1230HOURS
Naturalgaswasatthesurfaceonthecasingsideveryshortlyafterroutingthe returningwellborefluidthroughthedegasser.Thecrewreportedthatmost oftheunionsandtheflexlinewereleaking.A31 2 -in.hammerunioninthe linebetweenthemanifoldandtheatmospheric-type“poor-boy”separator wasleakingdrillingmudandgasbadly.Theseparatorwasmountedinthe endofthefirsttank.Gaswasbeingblownfromaroundthebottomofthe poor-boyseparator.Atabout1000hours,themotorsontherigfloorbegan torevasaresultofgasintheairintake.Thecrewshutdownthemotors. At1030hourstheannularpreventerbeganleakingverybadly.The upperpiperamswereclosed.
1230TO1400HOURS
Continuingtoattempttocirculatetheholewithmudandwater.
1400TO1500HOURS
Thecasingpressurecontinuestoincrease.Theflowfromthewellisdrygas. Thelinebetweenthemanifoldandthedegasseriswashingoutandtheleak isbecomingmoresevere.Theflowfromthewellisswitchedtothepanic line.Thepaniclineisleakingfromnumerousconnections.Flowistoboth thepaniclineandtheseparator.
Thegasaroundtherigignitedat1510hours.Thefirewashigherthan therig.Thederrickfellat1520hours.
Thisexcerptisfromanactualdrillingreport.Wellcontrolproblemsare difficultwithoutmechanicalproblems.Withmechanicalproblemssuchas thosedescribedinthisreport,anotherwiseroutinewellcontrolproblem escalatesintoadisastrousblowout.Inareaswherekicksareinfrequent,it
thewellheadorflowlines.Thicknesstesterswereusedtomonitorcritical areasandshowednosignificantthicknessreduction.
AtadeepblowoutinthesouthernUnitedStates,theflowratewasdeterminedtobewellover100mmscfpdthroughthe31 2 in.drillpipeby7in. casingannulus.Theflowingsurfacepressurewaslessthan1000psi.There wasgreatconcernabouttheconditionofthedrillpipe:woulditbeeroded orperhapsevenseveredbytheflow?Afterabout10daysexposure,thedrill pipewasrecovered.Attheflowcross,thedrillpipewasshiny.Otherthan that,itwasunaffectedbytheexposuretotheflow.
Unfortunately,theindustryhasnoguidelinesforabrasion.Erosionin productionequipmentiswelldefinedbyAPIRP14E.Althoughproductionequipmentisdesignedforextendedlifeandblowoutsystemsare designedforextremeconditionsovershortperiodsoftime,theAPIRP 14Eoffersinsightintotheproblemsandvariablesassociatedwiththeerosion ofequipmentunderblowoutconditions.Thisrecommendedpractice relatesacriticalvelocitytothedensityofthefluidbeingproduced.The equationsgivenbytheAPIareasfollows:
where
Ve ¼ fluiderosionalvelocity,ft/sec
c ¼ empiricalconstant
¼ 125fornoncontinuousservice
¼ 100forcontinuousservice
ρ ¼ gas/liquidmixturedensityatoperatingtemperature,lb/ft3
P ¼ operatingpressure,psia
Sl ¼ averageliquidspecificgravity
R ¼ gas/liquidratio,ft3/bblatstandardconditions
T ¼ operatingtemperature, °R
Sg ¼ gasspecificgravity
z ¼ gascompressibilityfactor
A ¼ minimumcross-sectionalflowarearequired,in2/1000bbl/day
Pipe sizing to avoid erosional velocities for continuous two-phase flow
Based on Ve = 100 / √rm
Where ve = Fluid erosional velocity, ft/s rm = Gas/liquid mixture density at flowing pressure and temperature, lb/ft3
Assumptions: Flowing temperature = 75° f
Gas specific gravity = 0.65
Liquid specific gravity = 0.85
Sand free stream
Gas compressibility = 1.0
Eqs. (1.1)–(1.3) havebeenusedtoconstruct Fig.1.1,whichhasbeenreproducedfromAPIRP14Eandoffersinsightintothefactorsaffectingerosion. Becausethevelocityofacompressiblefluidincreaseswithdecreasingpressure,itisassumedthatthearearequiredtoavoiderosionalvelocities increasesexponentiallywithdecreasingpressure.
Itis,however,interestingthatpursuantto Fig.1.1 andEqs. (1.1)–(1.3),a highgas/liquidratioflowismoreerosivethanalowgas/liquidratioflow.
Thepresenceofsolidscausesthesystemtobecomeunpredictable.Oilfieldservicecompaniesspecializinginfracturestimulationandthose involvedinslurrypipelinesareveryfamiliarwiththeerosionaleffectsof solidsinthepresenceofonlyliquids.Testingofsurfacefacilitiesindicates thatdischargelines,manifolds,andswiveljointscontainingelbowsand short-radiusbendswillremainintactforupto6monthsatavelocityof approximately40ft/sevenatpressuresupto15,000psi.
Furthertestshaveshownthat,inadditiontovelocity,abrasionisgovernedbytheimpingementangleorangleofimpactoftheslurrysolids andthestrengthandductilityofthepipeandthehardnessofthesolids. Atanimpingementangleof10degreesorless,theerosionwearforahard, brittlematerialisessentiallyzero.Inthesetests,themaximumwearrate occurredwhentheimpingementanglewasbetween40and50degrees.
Pipe cross-sectional area, sq. in /1000 bbls fluid per day
Fig.1.1 Erosionalvelocitychart.
Thewearrateincreasedwhenthesolidsintheslurrywereharderthanthe tubularsurface.Sandisslightlyharderthansteel.Bariteismuchlessabrasive thanhematite.
Thereisnoauthorityfortheerosionandwearratewhensolidssuchas sandandbariteareaddedtogasanddrillingmudinablowoutorawellcontrolsituation.Therecanbelittledoubtthatthesteelsareerodingundermost circumstances.APIRP14Emerelystatesthattheempiricalconstant, c, shouldbereducedifsandproductionisanticipated.
Solidsintheflowstreamcanbedisastrousforthewellcontrolsystem. OnecaseinpointoccurredattheApacheKey1–11inWheelerCounty, Texas.ManycalledtheblowoutattheKeythebiggestblowoutinthehistoryofthestateofTexas.Itwascertainlyoneofthemostbaffling,spectacular,andunpredictable.Withover90ftofMorrowsandandanopenflow potentialinexcessof90mmscfpd,theKeywasoneofthebestwellsever drilledintheAnadarkoBasin.On4Oct.1981,whilewaitingonpipeline connection,thewellinexplicablyerupted,launchingthewellhead,80ftof 27 8 in.tubing,80ftof75 8 in.casing,and12ftof103 4 in.surfacecasing.The wellflowedessentiallyunrestrictedintotheatmosphere.
TheKeywasroutinelycappedbytheendofOctober.However,3days later,thewellcratered.Alltheventlineswereopenedinanattemptto relievethepressure.Asillustratedin Fig.1.2,the45degreeturnswere cutoutcompletely.Inaddition,acloselookat Fig.1.2 showsthatthe 7 1 16 in.by10,000psivalvebodytotheventlinehadalsocutout.
Thecappingstackwasremoved,andthewellcontinuedtoflow,while subsequentwellcontroloperationswereconducted.Theparticulatefrom thewellhadadistinctivecolorandwasidentifiedascomingfromazone thatwasoriginallyseparatedfromtheflowstreambythetubingandtwo stringsofcasing.
AtanotherpointinthecontroloperationattheKey,a20in.by 10,000psistackwasbeingriggeduptocapthewell.Duetothesizeand weight,thestackhadtobeplacedontothewellinsections.Whilebringing thesecondsectionintoplace,thecrewnoticedthattheboltsinthefirstsectionwereloose.Thefirstsectionwasremovedandexamined.Asillustrated in Fig.1.3,thecasingheadwascutoutbeyondtheriggroove.Thissevere erosionhadtakenplaceinthetimerequiredtomakeuptheboltsonthe spoolandpickupthesecondsection,whichcouldhavebeennomorethan acoupleofhours.
Timeandagain,theflowfromtheKeydemonstrateditsdevastating nature.Whenthemomentumkillprocedurewasimplemented,several
jointsof51 2 in.casingwererunintothe75 8 in.casing.Therewasa10,000psi gatevalveontopofthe51 2 in.casing.Afterthemomentumkillwasaccomplished,thewellremaineddeadforabout1h,andthenflowcommenced again.Thevalvelastedonlyafewminutes.Theconnectionsofthe51 2 in. casinghadbeenerodedtotheextentthatthethreadsonthepinswerevisible throughtheboxes.Imagine!Allthatfromacasedhole!
Theinsideofalltheequipmentinthestackandtheflowlineshadtobe protectedwithaspecialstellitematerial.Towardtheendofthewellcontrol operations,anaverageof2000yd3 ofparticulatematerialwasbeingseparatedfromtheflowstreamandremovedeachmonth.
Fig.1.2
Wellsdonothavetobedeepandhigh-pressuredtodemonstrate suchfury.On12Mar.2000,awelllocatednearTabor,Alberta,Canada, gotoutofcontrol,despitebeingonly3500ftdeepandnormallypressured.Thevolumerateofflowwasestimatedtobebetween20and 40mmscfpd.
Within30minaftertheblowout,holesareerodedinthechokeline,fillingthemanifoldhousewithgas.Laterinspectionsrevealedseveralholesin thechokemanifold.Oneisshownin Fig.1.4.Within2h,thedrillingcross cutout(Fig.1.5).
(ReproducedwithpermissionfromConocoCanadaLimited.)
Fig.1.3
Fig.1.4
