Cement Plugs.docx

Cement Plugs.docx

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CementPlugs

ReasonsforSettingCementPlugs

Byfarthemostcommonapplicationofcementplugs-particularlyinmatureareas-iswellabandonment.

Someotherreasonsforsettingplugsare:

∙tocurelostcirculationduringdrilling

∙toprovideameansofwellcontrol

∙tomaintainwellborestability

∙fordirectionaldrillingandsidetracking（orwhipstocking）

∙toprovidezoneisolation

∙toprovideaseatforopenholetesttools

Abandonment

Tosealoffadryholeordepletedwell,cementplugsareplacedatrequireddepths（Figure1,Cementplugsforabandonment）.

Figure1

Theseplugspreventzonecommunicationandanyfluidmigrationthatmightinfiltrateundergroundfreshwatersourcesorcauseundesirablesurfaceconditions.Wellpluggingregulationsaredriventoalargeextent,byrequirementsrelatingtotheprotectionofundergroundsourcesofdrinkingwater.Mostregulatoryagencies,forexample,requireabottomplug,aplugacrosscasingstubs,spacerplugs,andperhapsremedialsqueezesatabandonment.

Lost-CirculationControl

Ifdrilling-fluidcirculationislostduringdrilling,itcansometimesberestoredbyspottingacementplugacrossthelost-circulationzone（Figure2,Cementplugforlostcirculationcontrol）andlaterdrillingbackthroughtheplug.

Figure2

WellControl

Acementplugcanserveasameansofwellcontrolwhenthewellisatthepointwherethereisnomarginbetweenporepressureandfracturepressure.Incaseswherethedrillstringcannotbesafelypulledwithoutriskingablowout,itmaybeleftintheholeandcementedinplace（Crook,2006）

WellboreStability

Cementplugs,togetherwithpolymers,resinsorothermaterialsusedseparatelyorincombination,canbeusedtoconsolidateformations,fillinwashed-outholesections,andalterthenear-wellborestressenvironment.Oncetheintervalisstabilized,theplugcanbedrilledout.（Crook,2006）

DirectionalDrillingandSidetracking

Tosidetrackaholearoundunrecoverablejunkorforanundesirabledirectionorpoorstructuralposition,acementplugisplacedataspecificdepth.Thisplughelpssupportthewhipstockfordirectingthebitintothedesiredarea（Figure3,Cementplugfordirectionaldrillingandsidetracking）.

Figure3

ZoneIsolation

Inawellwithtwoormoreproducingzones,itissometimesbeneficialtoabandonadepletedorunprofitableproducingzonebyplacingacementplugaboveit（Figure4,Cementplugforzoneisolation）.Thisplugpreventspossibleproductionlossinto,orfluidmigrationfrom,thelowerinterval.

Figure4

FormationTesting

Acementplugissometimesplacedbelowazonetobetestedthatisaconsiderabledistancefromtotaldepth（Figure5,Cementplugforformationtesting）.Thisplugisnecessarywhenastraddlepackerwithsidewallanchororbridgeplugisnotpossibleorpractical.

Figure5

JobDesign

Cementpluggingconsistsofplacingacementcolumninopenorcasedhole.Althoughthissoundsrelativelysimple,manyproblemsmaybeencounteredduringthepluggingoperation.Majordifficultiesincludefluidmigration,cementcontamination,andpoorcement-slurrydesign.

Nevertheless,cement-plugfailuresmaybeminimizedby:

�usingacaliperlogtodetermineholegauge

�carefullydeterminingcement,water,anddisplacementvolumes,andalwaysplanningtousemorethanenoughcement

�usingspacerorpreflushaheadofandbehindthecementcolumntominimizecontamination

�rotatingdrillpipe/tubingandusingcentralizersandscratcherswhileplacingcement

�placingplugswithcare,andmovingpipeslowlyoutofcementtominimizecontamination

�usingdivertertoolsandviscouspillspacers

Inadditiontotheprecautionslistedabove,thereareseveralotherimportantfactorstoconsiderindesigningforacementplug:

slurrycompositionandvolume,governmentregulations,placementtechnique,andwellconditioning.

Cementvolumeneededforaspecificpluggingoperationdependsonpluglengthandholediameter.Governmentregulationsmayalsodictatepluglengthforwellabandonment.Forsidetracking,200to300ftisgenerallyrequiredtominimizedrilling-fluidcontaminationthatoccursattheplugtop.Moreover,allowancesareusuallymadefordressingoffdrilling-fluid-contaminatedcementbeforeattemptingtosidetrack.

Fluidspacersshouldbeusedbothaheadofandbehindcementslurrytominimizemixingofcementanddrillingfluid.Also,spottingaviscouspillspacerattheintendedplugbottomcanimprovecement-plugstability.Useofadivertertool,whichforcesfluidtoflowdirectlyatthewellboreface,providesforamoreuniformplacementofspacer,viscouspillspacerandcementslurry.Adiverter-toolapplicationtothebalancedmethodplacementtechniqueisillustratedbythesecementplugs:

Figure1（Idealizedcase）,

Figure1

Figure2（Experimentalresults）andFigure3（Recommendedtechnique）.

Figure2

Figure3

Cement-SlurryDesign

Cement-slurrydesigniscloselyrelatedtoobjectivesoftheplugandalsodependsonwelldepth,BHCT,anddrilling-fluidproperties.Somekeydesignconsiderationsare

�rheology

�densityandcompressivestrength

�thickeningtimeandWOCtime

Forlost-circulationplugs,slurrydensityshouldbecontrolledtoreducebulkslurryloss.Bentoniteandsilicateextendersareusefulslurry-density-reductionadditives,andtheylowerultimatecementcompressivestrength.Gilsonite,agranularhydrocarbon,isalsoalightweightcementadditiveusedinlost-circulationplugs.Lowcompressivestrengthisdesirableinalost-circulationcementplugbecausesidetrackingismorelikelytooccuroffahigh-compressive-strengthplug.Thixotropiccementslurriesarealsousedaslost-circulationcementplugs.Self-supportingpropertiesoftheseslurrieshelppreventtotalcementlossintothelost-circulationzone.

Asidetrackingorwhipstockplugissettoenableanewholetobedrilledawayfromtheoriginalhole.Sincethesuccessofawhipstockplugdependstoalargeextentonhighcompressivestrength,useofadensifiedcement（APIClassA,G,orH）isrecommended.Densifiedcements:

�providehighstrength

�toleratedrilling-fluidcontamination

�expanduponsetting

�decreasefluidloss

�havenegligiblepermeabilitywhenset

�providestrongbonding

Drilling-fluidcontaminationisalwayspossiblewhenplacinganopenholecementplug.Whendrillingfluidcommingleswithcementslurry,slurryretardationaswellasdilutionofthecementplugcanoccur.Theeffectofdrilling-fluidcontaminationontwodifferentdensityslurriesisshowninTable1（below）.

Becauseofthepropertieslistedabove,densifiedorreduced-water-ratiocementsgenerallyproducemoresuccessfulresults.Table2（below）showstypicalcompressivestrengthsofdensifiedClassGandHcementsusedforsettingsidetrackingcementplugs.

Additivessuchascalciumchloride,dispersant,orretardercanbeusedindensifiedslurries.However,becauseslurrieswithdispersantshavelowerconsistencyandviscosity,theamountofdispersantshouldbekepttoaminimumforbetterplugstability.Laboratorytestingformaximumviscosity,amplethickeningtime,andcompressivestrengthshouldbeperformedbeforeeachjob.

Someoperatorsusingwhipstockplugsclaimthathighercompressivestrengthsandbettersuccessratiosareachievedwhen10to20%sandand/orsilicaflourareincludedinthecementcomposition.Apossibleexplanationofthisphenomenonisthatsandmayimprovedrilling-fluidremovalbyitsscouringaction,andthusmayreducedrilling-fluidcontamination.Therefore,sandand/orsilicaflourmayaffectcompressivestrengthinawayunrelatedtosilica/cementreaction.

Tofurtherreducemudcontamination,itisadvisabletopumpaspacerfluidaheadofandbehindthecement.Thedensityofthespacerfluidshouldbeequaltoorgreaterthanthatofthedrillingfluid.Itisimportanttonotethatbecauseofthedensitydifferencebetweenmudandcement.Thecementwilltendtomigratedownwardinthewell.Thisisanotherreasonforincludingapreflushinthepluggingtreatment.

Drilling-Fluidcontamination（%）

Compressivestrength（psi）at230oFfor12hours

15.6lb./galslurry

17.4lb./gal*

0

2,910

7,010

10

2,530

5,005

30

1,400

2,910

60

340

2,315

*containsdispersant

Table1:

Effectofdrilling-fluidcontaminationoncementcompressivestrength.

Slurryweight

（lb./gal）

100�F

140�F

170�F

200�F

1,600psi

3,000psi

3,000psi

3,000psi

After12hours

16.5

2,075

4,000

7,800

9,035

17.0

2,850

6,535

8,375

10,025

17.5

3,975

6,585

8,550

10,675

After24hours

16.5

5,475

8,985

9,750

10,460

17.0

6,035

9,060

11,075

12,660

17.5

7,025

10,125

11,860

12,875

Table2:

TypicalcompressivestrengthsofAPIClassGandHcementsatAPIcuringconditions.

Mixing

Apoorlymixedcementslurrymayleadtocement-plugfailure,soahigh-qualityslurryshouldbeprepared.Becauseofthesmallcementvolumesassociatedwithplugcementing,theslurryshouldbebatchmixedormixedthrougharibbonblender,ifpossible.Thesemethodsleadtoaveryuniformcementslurry.Shouldeitherofthesemixingtechniquesbeimpractical,thecementshouldbemixedataratethatwillensureuniformslurrydensity.

PlacementTechniques

Therearethreebasictechniquesforplacingcementplugs:

thebalancedmethod,thedump-bailermethod,andthetwo-plugmethod.

TheBalancedMethod

Thebalancedmethodinvolvespumpingcementslurrydownthedrillpipeortubinguntilthelevelofcementoutsidetheworkstringisequaltothatinside.Thework