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RoleofMicrostructureinSuckerRodStringFailuresinOilWellProductionSofianeBenhaddadandGlenLee(Submitted12December2000;inrevisedform20February2001)Sucker-rodpumpsareoperatinginveryaggressiveenvironmentsinoilwellproduction.Thecombinedeffectofacorrosiveenvironmentandsignificantmechanicalloadscontributetofrequentcasesoffailureoftherodstringduringoperation.Standardsandrecommendationshavebeendevelopedtocontrolandavoidthosefailures.Thisstudypresentsvariousfailurecasesofsuckerrodsindifferentapplications.Theheattreatmentofthesteelmaterialandtheresultingmicrostructureareanimportantfactorinthebehaviorofthesuckerrod.Aspheroidizedmicrostructurepresentsaweakerresistancetocorrosionaffectingtherodlife.Non-metallicinclusionsareapittingpreferentialsiteleadingtofatiguecrackinitiation.Heterogenousmicrostructureasbandedmartensiteandferrite/pearlitedecreasestheductilityofthematerialaffectingthefatiguepropagationresistance.Keywords:corrosion-fatigue,inclusions,pitting,steelmicrostructure,suckerrod1.IntroductionTheoilproductionwellsinwesternCanadagenerallyoperateinaggressiveenvironments,includingcorrosiveelements,highvelocityofthefluids,andconsiderabledepthforcompletion.Determinationandanalysisofthenecessarydataarerequiredforselectionofmaterialsforwellcomponents.Materialsforthetubing,casing,andsuckerrodstring,aswellasasuitablecoatingorchemicalprotectionprogramareselectedappropriatelyforeachoperatingwell.Insuchanaggressiveenvironment,themostvulnerabledownholecomponentisthesuckerrodstringduetoitsfunction.Specificationsandstandards[1-4]havebeenwrittenfortheimprovementoftheperformanceandlifeexpectationofsuckerrods.Therecommendationsofthesecodesandstandardsaredirectedtowardsabetterchoiceofmaterialforgoodcorrosionresistanceandabetterdesignoftherodstringforanevendistributionofstresses.Theresultingrecommendationsarerestrictiveandmaypaysignificantattentiontoeconomicfactors.Therefore,manyoperatorscontinuetousecarbonoralloysteelcomponentswithanappropriatechemicalinhibitionprogram.Carbonsteelispreferredoverothermaterialsduetoitsmachinability,highstrength,andlowcost.Theuseofcarbonsteelrodstringsinaggressiveenvironments,withoutproperlyconsideringthecomplexinteractionsbetweenthematerialanditsserviceenvironment,leadstofailures.Inthiscontext,numerousrodstringfailureshavebeenstudiedthroughmetallurgicalinvestigationthatincludedmacro-observation,chemistry,fractography,microscopy,andhardnesstesting.Thispapersummarizesseveralofthestudiedcasesanddividesthecasesaccordingtothemainrootcauseoffailure:mechanicallyinducedfailureandenvironmentallyinducedfailure.2.MechanicallyInducedFailuresAllsuckerrodfailurecaseswererelatedtothemostcommonlyusedmaterials:carbonoralloysteel.Theserodstringswereoperatingunderoneofthreeconditions:1)inconjunctionwithachemicalcorrosioninhibitionprogram;2)inconjunctionwithaprotectivecoating;and3)withoutanyprotectionfromtheserviceenvironment.Casesaresummarizedalongwiththedetailsofthefailureanalysis.2.1RoleoftheMicrostructure2.1.1Observation.Threesuckerrodsbrokeatthepinend(Fig.1)andproducedthesamemicroscopicrupturewithidenticalfeatures.Therodswerefabricatedfromcarbonoralloysteel.Theobservationofthesurfaceoftherupturerevealedthreedistinctzones(Fig.2).Thefirstzone,correspondingtothecrackinitiation,revealedpitsandcorrosion.Thesecondzonewasflatandsmooth,indicatingamacroscopicallybrittlefracture.Thesecondz...