T
heoilandgas industryhas learnedhowtosolvecountless
problemsovertheyears.Manyprocedures,methods,
chemistriesandproductshavebecome industrybest
practicesusedbybothservicecompaniesandexploration
andproduction(E&P)companiesaroundtheworld.These
bestpracticesofferanexcellentstartingpointforsolvingnew
problems,butaproper investigationstillneedstobecompleted
whendetermininghowtoovercomespecificchallenges.
Inordertosolveagivenproblem,thesourceofthe issue
needstobeunderstood,aplanneedstobedevelopedand
executedandanevaluationneedstobeconducted.While
drawingonbestpracticeswithinthe industryknowledgebase is
recommended,eachmustbeusedwithcautionandapplied in
thepropermannerforthespecificchallenge.There isnosilver
bulletorone‑size‑fits‑allsolutionformostofthechallenges
facedbythe industry,buttherearewaysto improveproducts,
proceduresandresults inrealworldapplications.
Becomingabestpractice
Onesuch improvementthathasbecomeabestpractice isthe
useofhigh‑ratepumpingduringprimarycementingoperations.
TherehavebeenanumberofstudiesbybothE&Pandservice
companiesscrutinisingtheeffectsofhigh‑ratepumpingduring
cementing.Thiscan includetherateatwhichcement ismixed
andpumpedaswellastherateatwhich it isdisplacedfromthe
casing.Thereasonbehindpumpingathighrates is increased
annularvelocitytooptimiseholecleaning.Theannularvelocity
isaffectedprimarilybythewellboreconfiguration. Ifthe
wellboreconfiguration isvery large,thepumpratetoachievea
givenannularvelocitycanbemagnitudes largerthanthatofa
smallerwellboreconfiguration.Forexample, inawellborewitha
configurationof177.8mm(7 in.)casinganda222.3mm(8
¾
in.)
openhole,theannularvelocity is71.5m/min. (234.6ft/min.)ata
pumprateof1.0m
3
/min. (6.3bbls/min.). Iftheopenholesize is
increasedbyonly25.4mm(1 in.),theannularvelocitydecreases
byapproximately40%.
AstudywasconductedbyHart,W.A.,&Smith,T.R. (1990),
ShellCanada, (JCPT90‑06‑01S)todefinetheparametersbywhich
thebestzonal isolationcouldbeachieved. Indoingthis,they
continuedtocomebacktoarequiredannularvelocityduring
pumpingoperation.Thetargetedannularvelocity is
80m/min. (262.5ft/min.),whichtypically increasedthe
displacementefficiencyand improvedthecementbondresulting
inbetterzonal isolation.Thegoalandtheeffectsofthestudyare
brieflydefined inFigure1.
Thisapproachhasbeenadopted inmanydifferentareas
oftheoilandgas industry.Throughcontinuingtofollowthis
bestpractice,newchallengesfacingthe industryhavesurfaced.
Twoofthesenewchallengesaredealingwiththe increasetothe
equivalentcirculatingdensity(ECD)andtheabilitytoachieve
therequiredpumpratetomaintaintheprescribedannular
velocity.Theannularvelocity isusedtoaid inachievingzonal
isolationthroughdisplacementefficiencywhereas lossescan
occur iftheECDbecomestoohigh.Atthetimeofthisstudy,the
wellboresweretypicallysmallerthanwhat isseen in industry
today.This includesboththecasingandopenholesizes.Asa
result,thepumprate,orannularvelocity,waseasilyachievedby
aconventionalpumpingunit.
Implementationdifficulties
The industryhaschangedsincethetimeofthe initialstudy,and
althoughthe idearemainsstrong,thereare instanceswhere
it isdifficultto implement.Oneoftheseareaswhere itcanbe
difficulttoachievetherecommendedannularvelocity is inthe
Canadianoilsands.Thecasingstringsandholesizesarevery
large,whichresults indramatic increasestotherequiredpump
rates.Acomparisonofaconventionalandsteamassistedgravity
drainage(SAGD)wellboreconfigurationscanbeseen inTable1.
Thesepumpratesonlytake intoaccountthewelldrilled
exactlyasthedimensionsstated inTable1. It iswellknown
thatthewellbore isnotdrilledto ‘gauge’hole.Thisrequiresan
excess,ornon‑gauge,volumetobeused inthecalculation.The
differencebetweenthegaugeholeandtheexcess isshown in
Figure2.
Whentheexcess is included inthepumpratecalculations,
thepumpratescontinueto increaseasshown inFigure3.
Table2showsthatatapumprateof2.4m
3
/min.
(15.1bbl/min.)and0%excess,anannularvelocityof
82.44m/min. (270.5ft/min.) isachieved. Iftheexcess is increased
to150%,theannularvelocitydecreasesto32.98m/min.
(108.2ftmin.).Thisposesthechallengeofhowtoachievethe
requiredannularvelocities.Onemeansofensuringthatthe
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