Well integrity assessment by a 1:1 scale wellbore experiment: Exposition to dissolved CO2 and overcoring

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Well integrity assessment by a 1:1 scale wellbore

experiment: Exposition to dissolved CO 2 and overcoring

J.C. Manceau, J. Tremosa, C. Lerouge, F. Gherardi, C. Nussbaum, L.J. Wasch, P. Alberic, P.

Audigane, F. Claret Highlights

• A 1:1 scale experiment on well integrity is carried out in a rock laboratory.

• The stage B dedicated to well exposure to CO

-rich pore water is presented.

• The well system has been ultimately overcored for inspection.

• Key messages on processes affecting the integrity of a well are derived.

• Similar experimental set-up could be used for other aspects of well integrity.

Abstract

In this work, we present the results of a new in situ experiment to complete the existing scientific dataset on well integrity in the context of CO

storage. This experimentation has been designed to evaluate the sealing behaviour of a monitored well after mechanical and chemical stresses due to pressure and temperature changes (stage A) and due to the exposure to carbonated brine (stage B), before a final overcoring stage for retrieving the well system and the surrounding clay. The stage A has been the subject of a first publication (Manceau et al., 2015; Water Resour. Res., 51, 6093–6109) and the two latter stages are described in this paper.

Multidisciplinary methods (hydraulic tests and modelling, fluid sampling and modelling,

analysis of cement and clay samples on the overcore) are used to get better insight, in a realistic

wellbore context, on the interplay between the geochemical questions, and the operational and

construction issues. In particular, this study shows that when good integrity pre-exists before a

well is in contact with carbonated water, the exposure to dissolved CO

does not seem to lead

to a degradation of the well hydraulic properties but rather to their improvement.

cement shrinkage during curing are some of the construction issues frequently cited in the literature (see e.g. Gasda et al., 2008, Kuperschmied et al., 2015, Zhang and Bachu, 2011, Nelson, 1990, Bonett and Pafitis, 1996 and Choi et al., 2013). The well integrity quality also depends on how the operations that the well is experiencing (both during its active life and after its abandonment) modify the initial wellbore properties. Significant changes in formation- or casing-pressure and/or temperature could damage the near-well formation, the casing or the cement sheath as well as detach the bonding of those elements (Nelson, 1990, Zhang and Bachu, 2011, Carey, 2013, Bai et al., 2015 and Vrålstad et al., 2015). Integrity problems could arise from micro annuli formed due to such mechanical stresses. The wellbore can also be impacted by the geochemical environment in which it has been completed: abnormal caprock and cement degradation as well as casing corrosion may arise in some cases (highly saline and corrosive brine for instance, cf. (Nelson, 1990).

On a geological carbon dioxide storage site, decommissioned wells drilled through low- permeable caprock are potential connections between the CO

storage reservoir and overlying sensitive targets like aquifers. In sedimentary basins already used for oil and gas or geothermal resources, such wells are likely to be present in large numbers within the CO

storage complex.

In addition, materials commonly used for well completion (Portland cement and steel casing)

are known to react with the low-pH carbonated brine resulting from the CO

injection

operations (Bruckdorfer, 1986, Carey, 2013 and Walsh et al., 2014). Those geochemical

reactions, and their potential impacts on the creation/increase of flow pathways, have been the

subject of the majority of the research effort relative

towellintegrityinthefieldofCO₂storage(seee.g.thereviewby Carrolletal.,(2016)).

PortlandcementreactivityafterCO₂exposurehasbeenstud- iedbyexperimentsin batchreactors toassesstheevolution of thecementintrinsicproperties(hydraulicandmechanical)(Barlet- Gouédardetal.,2009;DuguidandScherer,2010;JungandUm, 2013;Kutchkoetal.,2007,2008,2009;Lestietal.,2013;Rimmelé etal.,2008).Theeffectof exposuretowetsupercriticalCO2 or toCO₂-saturated waterorbrine hasbeenobservedwithdiffer- ent cement types, and different experimental conditions (fluid composition,water/cementratio,curingconditions,pressureand temperature).Thestudiesagreeonasimilargeochemicalprocess (explainedforinstanceinAbdoulghafouretal.(2013)):thedif- fusion ofcarbonated brinewithin thecementfirstleadstothe dissolutionofportlandite(calciumhydroxide)andsubsequently ofcalciumsilicatehydratephases(CSH),bufferingthelow CO₂- richbrinepHandproducingcalciumions.Togetherwithcarbonate species,theseionsformcalciumcarbonate (carbonation).Espe- ciallyinanadvectiondominatedregime,calciumcarbonatecan re-dissolveandleaveanamorphoussilicagel(oramorphouszeo- lite,cf.Masonetal.(2013))afterthedissolutionoftheremaining calciumsilicatehydratephases.Threereactionzonesareconse- quentlyobservedaftercementexposuretoCO₂-saturatedbrine (e.g.Kutchkoetal.(2008)):Aregiondepletedinportlanditeclose tounaltered cement,a zone with calciumcarbonate precipita- tioncharacterisedbyanorangecolourationand,incontactwith thecarbonatedbrine,asilicarichamorphousregion.Eventhough thecarbonatedzone hasbeenshowntobeless permeableand mechanicallystronger, theopposite hasbeenmeasuredforthe zoneenrichedinamorphoussilica.Giventhedifferentconditions atwhichexperimentsareundertaken,theresultsoftheexisting studiesaremixed,butcementalterationdoesnotappeartolead toincreaseinpermeabilitycapableofcompromisingwellintegrity (Carey,2013).

Concernshave beenraisedabout theinfluenceof geochem- istryontheflowthroughexistingdefectswithincement.Works havebeenperformedtoassessthebehaviouroffracturedcement exposedtobrineand/orCO2 (Huertaetal.,2013,2016;Liteanu andSpiers,2011;Luquotetal.,2013;Walshetal.,2013;Wigand etal.,2009),aswellasofcompositematerialsrepresentingflow pathwaysalongwellboreinterfaces(Careyetal.(2010)forthecas- ing/cementinterface;Masonetal.(2013)andWalshetal.(2014) forthecaprock/cementinterface).Mostofthesestudiesobserved adecreaseofeffectiveabsolutepermeabilityofthecement.This couldbeduetofracturehealingbycalciumcarbonateprecipita- tion(Wigandetal.,2009;LiteanuandSpiers,2011),due tothe developmentoftheverylowpermeableamorphoussilica-richzone (Abdoulghafouretal.,2013),orduetomechanicaldegradationof thealteredlayersleadingtoadeformationofthefracture/leakage pathway(Masonetal.,2013;Walshetal.,2014).However,Luquot etal.(2013)showedamorecomplexbehaviour,withadependence ofthefracturepermeabilitynotablytothefractureapertureandto renewalrateofthecarbonatedbrinewithinthefracture.

SteelcasingcorrosionhasalsobeenwidelystudiedinaCO2-free context(thetypesofcorrosionthatcanbeencounteredindown- holeconditionsareforinstancedescribedinTalabanietal.(2000)) andinCO2-richenvironments(anexhaustivereviewisgivenin Choietal.(2013)).Allthosestudieshighlightedtheimportanceof thealkalineenvironmentprovidedbythecementsheaththatpro- tectsthecasingagainstcorrosion(passivation).Theyalsodescribed dissolvedCO₂asacorrosivespeciecapableofhighcorrosionrates incaseofdirectexposureonsteelcasing(underunfavourablecon- ditions,ratesupto20mm/yeararegiveninChoietal.(2013)).

Caprock alteration (close tothe cementsheath) is alsosus- pectedto play a role in wellintegrity evolution (Mason et al., 2013).Thegeochemicalreactionsinvolvingforinstancecarbonate-

richorclay-richcaprocksmayinducepore-waterormineralogical changesandcouldplayaroleinthereactivityofthewellcloseenvi- ronmentandconsequentlyinthechangesofhydraulicproperties.

The existing researchactivitiesin thewell integrity domain often conclude on the importance of considering this issue as a multi-physical problem with coupled hydrodynamic, geo- mechanical and geochemical phenomena. Understanding what couldoccurrealisticallyinthefieldisindeedadifficultpoint,as itimpliesreproducinginsituconditions.Thisdifficultyhasbeen tackledbycombiningobservationsmadeonindustrialCO₂storage analogueswiththeresultsoflaboratoryexperiments.Forinstance, toassessCO2interactionswiththewellbore,sampleshavebeen collectedfromanexistingCO₂-EORwell(35yearsofoperations withCO₂)(Careyetal.,2007)andfromanaturalCO₂producer(30 yearsofproduction)(Crowetal.,2010).Nevertheless,determining somekeypropertiesonrealwellsischallenging,suchasmeasur- ingeffectivepermeability(cf.Gasdaetal.(2008)),andtherefore thecomparisonbetweenfieldandlaboratoryoutcomescanonly bepartial.

Inthiswork,wepresenttheresultsofanewexperimenttofill thegapsintheexistingscientificdatasetonwellintegrityinthe contextofCO₂storage.Theinnovativenessofthisexperimentis thatthewellintegrityisassessedaccountingforallthecomponents ofawell(casing,cement,caprockformation),usingcommongeom- etryandmaterials,andwithsignificantlydetailedinstrumentation toenableaccesstoparametersusuallydifficulttomeasure.This experimenthasbeendesignedtoevaluatethesealingbehaviour ofawellaftermechanicalandchemicalstressesduetopressure andtemperaturechanges(stageA)andduetotheexposuretocar- bonatedbrine(stageB).ThestageAhasbeenthesubjectofafirst publication(Manceauetal.,2015)andthestageBisdescribedin thispaper.Thepaperalsopresentstheoutcomesoftheovercor- ingprocedure(similarprocedureshavealreadybeencarriedoutin othercontexts,e.g.Korolevaetal.(2011),Jennietal.(2014)).

Themainpurposeofthispaperistounderstandhowexposure toCO₂-richbrinecanaffectwellintegrityinarealisticenvironment and togetbetterinsight, inthis complex context,in theinter- playbetweengeochemistry,andtheoperationalandconstruction issues.

Afterpresenting theexperimental design, theresults of the firststage arebrieflyrecalled;theprotocol ofthesecondstage isexplainedandthehydraulicandgeochemicaldatasetsgathered duringthemonitoringoftheexperimentarepresented.Weinter- prettheseoutcomeswithdedicatedmodellingandwithadditional materialscollectedandanalysedaftertheovercoringofthewell system.

2. Previousoperations:designandwellintegrity assessmentundertemperatureandpressurestresses

Inthissection,werecapessentialinformationforunderstanding stageB.Foranextensivedescriptionoftheexperimentalconcept andofstageA,thereaderisreferredtoManceauetal.(2015).

2.1. Experimentalset-up

TheexperimenttookplaceintheOpalinusClayformation(shaly facies)oftheMontTerriUndergroundRockLaboratory(St-Ursanne, CantonofJura,North-WesternSwitzerland).TheMontTerriUnder- groundRockLaboratorycrossestheMontTerrianticlineformed duringthefoldingoftheJuraMountainsintheLateMioceneto Plioceneperiod,around10–2Millionyearsago.TheOpalinusClay consistsmainlyofincompetent,siltyandsandyshales,deposited around175My(Aalenien/Toarcian),buriedatleastat1350m(the presentoverburdenthicknessis300m)(BossartandThury,2008).

Fig.1. a)Conceptoftheexperimentationwiththesketchoftheovercoringoperations:ingreen,contourofboreholeBCS-5A;inred,contouroftheovercoreBCS-5OC(not toscale)b)technicallayoutofthecompletion;onlythehydrauliclinesusedforpressuremonitoring,fluidextractionandinjectionarerepresented(nottoscale)c)time-line ofthewholeexperiment.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

FiguremodifiedafterManceauetal.(2015).

Thisoverconsolidatedshaleformationcanbeconsideredasaref- erencecaprock-likeformation.

Theconcept and thetechnical layout ofthe experiment are providedinFig.1(aandb).Ashallowandsmallsectionofawell- bore(2.30m-long and 10.1m-deepfrom thelaboratory gallery groundlevel)hasbeenconstructedatscale1:1(198mmdiameter borehole,and5.5”casing−outsidediameter160mmandinside diameter:140mm),usingacarbonsteelcasingandclassGcement (commonoilandgaswellsmaterials).Twodifferentzonesofinter- esthavebeenobserved,afirstonewiththecasinginternalsideand thecementplug,andasecondonewiththecasingexternalside, thecementannulus/sheathandtheformationrock.

Withtheaimofcharacterizingthesezones,threedifferentinter- vals(i.e.avolumewithnomaterial)havebeendesignedfor:

-acontinuousmonitoringofthepressureandtemperaturecondi- tions.TemperatureismeasuredwithPT1000probesandpressure ismeasuredwithpressuretransducerslocatedatthesurfacecon- trolboard;

-injectingorextractingfluidstosetspecificpressureorfluxcondi- tionsandtosamplefluids.Theinjectionoffluids(liquidandgas) isperformedwithasyringepumpTeledyneIsco(model500D).

Thefluidsareextractedwithamassflowmeter(BronkhorstTher- malMassFlowMeter,0–10g/h,1–20bar).Boththepumpandthe

flowcontrollermoduleareremotelycontrolledwiththeDCAM software(SolexpertsAG).

Interval1(ca.30L)islocatedbelowthewell-section,interval 2 (0.17L)over thecement plug,and interval 3 (6.6L)over the well-section.Temperatureininterval1canbecontrolledbythe circulationofboricacidwithinthecentralstainlesssteeltubeof interval 1.The cementingof thesheath andthe plughasbeen allowedbyamechanical(grout)packersystem.Theisolationofthe wholesystemfromthesurfaceisinsuredbya2.5mlongpacker, placedoverinterval3.ThesystemGeoMonitor(SolexpertsAG)is usedfordataacquisition.

2.2. StageA:implementationandoutcomes

Theboreholedrillingstage(October1st,2012,atime-lineofthe wholeexperimentalworkisprovidedonFig.1c)hasshownarela- tivelylowgalleryexcavationdamagedzone(60cm).Thisimpacted zonewasthereforelocatedmuchshallowerthanthewell-system depth,andwasnotlikelytointerferewiththeexperiment.The well-systemhasbeeninstalledandcementedonOctober4th,2012.

Giventhatthevolumesoftheintervalsweresignificantcompared tothe natural pore water flow in the OpalinusClay, the three intervalswerefilledwithPearsonwater(syntheticporewaterrep-

resentativeofthenativeformationwater,Pearsonetal.(2011)).

Theartificialsaturationstagelastedtwomonths.

The wellintegrity hasbeen assessed duringstage Afor dif- ferentconditions.Thisassessmentwasperformedfirstatinitial conditions(period1:February–March2013,ca.10barand17^◦Cin interval1),thenafteranincreaseofthewell-systemtemperature (period2:May–August2013,ca.10barand52^◦Cininterval1)and aftersharpvariationsininterval1pressure(period3:September 2013–January2014,10–30bar,31^◦C).Theeffectivewellperme- abilitywasestimatedafterconstantheadhydraulictests(constant headininterval1)andaftercirculationsteadystatetests(injec- tionatconstantheadininterval1andextractionatconstantrate ininterval3).Onlytheeffectivepermeabilityoftheregionbehind casinghasbeenmeasuredsincetherewasnoconnectionthrough thecementplug(i.e.betweeninterval1and2).Thechemistryof thewellsystemhasbeencharacterisedafterregularsamplingin interval1and3.

ImportantconclusionshavebeenderivedfromstageA:

-The effective well permeability estimated all along stage A (between2×10⁻¹⁴m² and8×10⁻¹⁹m²)washigherthanthe cementorcaprockpermeability.Thissuggestedthatpreferen- tialflow pathwaysexisted,especiallyattheverybeginningof theexperimentwhentheeffectivewellpermeabilitywasatits highestlevel.Theassumptionofthepresenceofpathwayswas laterconfirmedbythegeochemicalevolutionofthefluidsand bytheobservationthattheeffectivewellpermeabilitydepended onpressure.Thiswouldmeanthattheeffectivewellpermeabil- ityispartly,possiblyentirely,duetotheflowattheinterfaces betweenthecaprockandthecementannulus,supportingthe acknowledgedimportanceofthecementingprocessduringwell construction.

-Thetemperatureandpressurestressesappliedtothewell-system duringstageAappearedtoinfluencesignificantlytheeffective wellpermeability.Increasingtemperaturedecreasedtheeffec- tivewellpermeabilitybyapproximately3ordersofmagnitude, whileincreasingpressureatthewellbottomincreaseditbymore than2ordersofmagnitude.Thiswouldindicatethatoperations- inducedstressescouldinfluencethewellintegrityasmuchasthe cementingprocess.

3. Exposureofthewell-systemtoCO₂-richporewater (stageB)andovercoringoperations

3.1. DissolutionofgaseousCO2withininterval1

StageBwasdedicatedtotheexposureof thewell-systemto CO2-richporewater.TheinjectedCO2gashadaspecificisotopic signature␦¹³C(+15.8‰vs.PDB¹), chosentobesignificantlydif- ferentfromtheisotopicsignaturesofthecarbonreservoirsofthe clayformation(carbonates,carbonspeciesdissolvedinporewater, organicmatter).Bromide,withanexpectedinitialconcentrationin interval1of1×10⁻³mol/L(3.087gofNaBrhavebeenintroduced) hasalsobeenusedasanadditionaltracer.Ithasbeeninjectedin theinjectionlinejustbeforethestartingoftheCO₂injection.

TheobjectivewassimilartostageA,i.e.toforceacontinuous migrationofpore-waterfromthebottomofthewelltothetop, but,for thatstage,replacingthesyntheticwater bycarbonated water.In order toensurestablepressure conditionsduringthe formation-waterreplacement,CO₂ingasphasehasbeencontinu- ouslyinjectedat19^◦Cand25barduringtwoweeks.Thisinjection hasbeenperformedatthebottomofinterval1toensureabub-

1PeeDeeBelemnite.

blingalongthe1.35m-longintervalandaprogressivedissolution oftheinjectedCO2.7560mLofCO2gaswasinjected(seeFig.2d), correspondingtoamassof402g(thedensitiesarecomputedafter Lemmonetal.(2013)).

3.2. Circulationsteadystatetestswithcarbonatedbrine

AfterMarch4,2014,circulationsteadystatetestshave been carriedoutsimilarlyasduringstageAwithconstantheadinjection ofcarbonatedbrineininterval1andextractionatconstantrate ininterval3(Fig.2).Forthecontinuousinjectionofcarbonated brine,thedissolutionofCO₂insyntheticporewaterwasperformed atthesurfaceinapressurevessel.Thedissolutionconditions(as constrainedbythepressurelimitationofthepressurevessel)were duringmostofstageB atapproximately19.5^◦C and14bar(the pressurewaslowerbetweenMarchandMay,2014).

Atthebeginning,testswereperformedwithapressureof28bar ininterval1andanextractionrateof0.16g/hininterval3.The extractionratehasbeendoubledonApril7th 2014inorder to favourthefluidflowthroughthewelltointerval3.Asdoneduring stageA-period3,threepressureincreasesininterval1havebeen imposedduringthisperiod(similartoHItests,buttheextraction ratesweremaintainedduringthoseincreases):A+2barstepon May19th,a+8barsteponJune3rdanda+6barsteponJuly10th.

Theeffectofthesethreeincreasesisvisibleintheinterval3pressure evolutionbuttheeffectisverysmall.

3.3. Leakageininterval3andcontinuationofthewellexposure tocarbonatedbrine

OnAugust6th2014,alossofintegrityininterval3occurred, leadingtoarapiddecreaseofthepressureininterval3,downtoval- ueslowerthan1bar(measuredatthesurface).Inordertofindthe locationoftheleakage,fourN2injectiontestswereperformedon August12th,onAugust21st,onSeptember10thandonSeptember 18th.Noleakagewasdetectedonsurfaceequipment.Duringtest4, thecentraltubeofthewellcompletion(whichreachesthesurface) wasinvestigated.Inthiscentralrod,noiseofbubblingwasheard anddustexhaustwasobservedatthesurface,indicatingaleakage comingdirectlyfrominterval3,morepreciselyviaaconnection betweeninterval3andthecentralrod.Oncethelossofintegrityof interval3occurred,theextractionrateininterval3,andtheinjec- tionininterval1werestoppedleadingtoaprogressivedecrease ofthepressureinthatinterval,downto19.5bar.Thepressurein interval1wassettothis valueonAugust19thuntilSeptember 18thwhenitwasincreasedupto25bar.Theleakageininterval 3nolongerallowsthequantificationoftheextractionratewithin thisintervalandthesubsequentwelleffectivepermeabilityofthe cementedannulus.Neverthelessstrongsealingbetweeninterval1 and3allowstocontinuetheconstantheadinjectionofCO₂-rich pore-waterwithininterval1at25barduring6moremonths(up toMarch18th2015).Atotalmassof498gofCO₂hasbeeninjected withininterval1duringtheentirestageB.

3.4. On-linefluidmonitoringandfluidsamplingforanalysesat laboratory

AttheendofthestageA,severalsensorshavebeenaddedtothe extractionline(betweentheflowcontrollerandthevial)toassess continuouslytheinterval3fluidpropertychangesduringstageB:

-Upstreamoftheflowcontroller,pHandEhprobes(digitaloutput;

HamiltonPolilytePlusArc120forthepHsensorandHamilton PolilytePlusORPArc120fortheEhsensor).Thesesensorscan

Fig.2. RawdatarecordedduringstageB—a)pressureofintervals1and3,andofthepressuretankinwhichCO2isdissolved/b)temperatureofinterval1and3,andofthe pressuretank(thetemperaturesensorofinterval1stoppedfunctioninginSeptember2014)/c)injectedvolumeofcarbonatedbrineininterval1andextractedvolumein interval3sincethebeginningofstageB/d)volumeofCO2injectedingasphaseininterval1atthebeginningofperiod4.

beusedfortemperaturesrangingbetween0and140^◦Candfora pressureofupto16bar;

-Downstreamtotheflowcontroller,onebromideprobe(analogue output0–2V,withacontrolunit).

Threeby-passes with peek/Plexiglas flowthrough cells have beeninstalledforpH,Ehandbromideprobes.

Inparallel,fluidshave beenregularlysampledin intervals1 and3fortheirchemicalanalysesatlaboratory.Somefluidsamples werecollectedbyopeningtheintervalflowlineandafterpurging theflowlineofitsvolume(directsampling).Fluidfrominterval3 wasmostlysampledaftertheflowmeterwherethevialisprogres- sivelyfilledbythesolutionextractedfromtheinterval(continuous sampling).

Chemicalanalysesincludedmajorcations(K,Na,Ca,Mg,Sr)and anions(Cl,Br,SO₄),dissolvedorganiccarbon(DOC),totalinorganic carbon(DIC)and␦¹³CofDOCandDICwhenpossible.Analytical proceduresaregiveninAppendixBofSupplementarymaterial.

3.5. Stoppingofthesystemandshorthydraulictestswithgas OnMarch18thand 19th2015,thesystemhasbeenstopped priortotheovercoringoperations.TheCO2-richwaterfillinginter- val 1 wasremoved to avoid any exsolution. The pressure was maintainedwithN₂at25barthroughlineP1(upperlineofinterval 1),whilethewaterwasextractedinapressurevesselthroughline Q1(lowerline).

Afterthisstep,mostofthewaterhadbeenremovedfromthe intervalsbut thetemperatureandpressureconditionswerenot significantlychanged.Giventhesesimilarconditions,ahydraulic testhasbeendonetocomparethehydraulicpropertiesofthewell under100%water-saturatedconditionsvs.undergaspresence.

Since larger flow paths were suspected at the top of the cementedannulus after the observations madeduring stage A, resin mixed withfluorescein (usedas a fluorescent tracer) has beeninjectedininterval3withtheaimofdetectingtheresinflow pathsandthusofimagingtheseinterfacesatleastatthetopofthe cementedwell-system.Theinjectionwasperformedatapproxi- mately2bar.

3.6. Overcoringoperationsandcomplementaryanalysesonfluid andsolids

Theinitialplanwastoovercorethewell-systemwitha350mm- diameter tool in order to retrieve the well system and the surroundingclaystone.TheoperationsstartedonMarch30th2015, fromtheinitialdepthofthepilotborehole(4.6m)tothebegin- ningofthecementedannulus(8.15m).However,itappearedthat theclayrockwasmilledduringthoseoperations,whilethecement sheathremainedintact.Theovercoringoperationswerestopped and anadditionalborehole (boreholeBCS-5A)wasdrilledfrom depth8.15mtodepth12.18monApril8th2015withthetriplecore technique (inside diameter 71mm/outside diameter 86mm)to sampleclayrockclosetotheclay/cementinterface(seeFig.1c).The distancebetweentheclaysamplingandtheinterfaceisestimated tobeapproximately15mm.Theoperationshavebeenresumedto completetheovercoringandthewell-systemhasbeenliftedoutof theboreholeonJuly1st2015(coreBCS-5OC).

Claystonesampleswerecharacterisedatsixdifferentdepths of BCS-5A: 8.45–8.59, 9.46–9.58, 10.02–10.10, 10.15–10.18 (only for CO2 partial pressure measurement), 10.65–10.74 and 12.08–12.13m. Cation exchange capacity, aqueous leaching, carbonand oxygenisotopesofcalcite,and CO₂ partialpressure measurementswereperformedonthesesamples.Thethreecore

Fig.3. EstimatedeffectivewellpermeabilityovertimeduringstageB;stageA,period3(fromManceauetal.,(2015))isalsoshownandthepressurestepssetininterval1 arealsoindicated.

samplesnearinterval1ofCO₂injection:10.02–10.10,10.15–10.18 and 10.65–10.74m werecut intotwo half vertical partscorre- spondingtoahalfneartheinterval1andahalffurtherfromthe interval1.ThecoreBCS-5OCwassampledatfivedifferentdepths:

8.32, 8.84, 9.54, 9.79and 10.12m. Microscopic observations as wellascarbon and analysesofoxygenisotopesof calcitewere carriedout.Theanalyticalprotocolsfollowedfortheanalysesare providedinAppendixBofSupplementarymaterial.

4. Results

4.1. HydraulicobservationsduringstageB 4.1.1. Welleffectivepermeabilityinversion

For stage B, theeffective well permeability hasbeen deter- minedbasedonthesameinversemodellingprocedurethanthat usedforstage Aandpresented inManceauetal. (2015).A2D- radialnumericalmodelhasbeendevelopedusingTOUGH2code (Pruess etal.,1999).Themodel radialextensionis100m, with 3901cells. Thecementsheathpermeability wasusedtorepre- sent theeffectivewellpermeability;the otherinfluentialinput parameters were the caprock permeability, the intervals’ com- pressibility,andtheboundary conditionsin termsof formation porepressure.Similarvaluesofcaprockequivalentpermeability tothoseusedattheend ofstageAwerere-usedfor thebegin- ningofstage B(i.e.1.5×10⁻⁴mDforthecaprockin connection withinterval3, and3.×10⁻⁵mD forthecaprockin connection withinterval1).Thedynamicmodelresultsobtainedattheend ofstageAwerelogicallyconsideredasinitialconditionsforstage B.Amajoruncertaintyconcernedthecompressibilityofinterval1 and3,whichcouldhavebeeninfluencedbythepresenceofgas thatwouldnothavedissolvedproperly.Interval1compressibil- ityhasbeencomputedduringthethreepressurestepsperformed inthatinterval:1.29×10⁻⁹,1.18×10⁻⁹and1.28×10⁻⁹Pa⁻¹have beenmeasured,showingsimilarvaluestovaluesmeasureddur- ingstage A(1.3×10⁻⁹Pa⁻¹ hadbeenconsidered) andtherefore confirmingthecompletedissolutionoftheinjectedgas.Sinceno waterhasbeendirectlyinjectedininterval3duringstageB,the compressibilityofinterval3couldnothavebeenmeasuredwith precision.ThisparameterhasbeenconsideredequaltoitsstageA value(2×10⁻⁸Pa⁻¹),aftertheverificationthatnogasexsolution occurredduringfluidmigrationtowardstheinterval3.

Thisexsolutionpotentialhasbeenevaluatedwithamulti-step modellingprocedureassumingthatfluidflowprevalentlyoccurs intheexperimentalapparatusthroughachannelizedpathatthe

cement-caprockinterface(annulus).Thecalculations have been donewithPHREEQCv.3.(ParkhurstandAppelo,2013),acodeable tosimulatereal gasbehaviourby meansof thePeng-Robinson equationofstate.Thegoalofthesecalculationsistoevaluateif, and,underwhichconditions,thewaterinjectedintotheexperi- mentalapparatus(viainterval1)canrelease(bydepressurization) a free gasphase, and if this free gas phase can accumulate in theinterval 3. Fixing thechemical composition ofthe aqueous solutionspresentintheintervals1and3,thenumericalresults sensitively dependon(i) theinjectionpressure of gasesin the refillingtank,(ii)theconfiningpressureofinterval1,(iii)thepres- sure/temperaturepathofthefluidspropagatingalongtheannulus, and(iv)theamountoffluidsfrominterval1effectivelytransferred throughtheannulusintointerval3.Theminimumamountofinter- val1waterrequiredtohavefreegasaccumulationintheinterval 3hasbeenestimatedtoabout1.5L,significantlyabovethevol- umeestimatedviahydraulicmodelling(0.220Lover6months).

Thisorder-of-magnitudeestimationcorrespondstoaconservative scenariothatconsidersthatallthegaspossiblyreleasedfrominter- val1waters,duringalinearP,Tdecreasingpathfrominterval1 (30.5^◦C, 28–36bar)tointerval3 (21.7^◦C,3.9bar) averageinitial conditions,hasbeeneffectivelytransferredintointerval3,without anylossintotheadjacentcaprockformation.Anypossibledilution withcaprockporewater,and/orgaslossintothecaprockwould increase(evensignificantly)thisthresholdvalue.

TheresultsofthehydraulicmodellingforstageBareprovidedin theappendices(Fig.A1ofSupplementarymaterial);onlytheperiod beforethelossofintegrityininterval3couldhavebeenmodelled.

Notethatthecaprockpermeabilityattheinterval1levelhasbeen consideredconstantthroughouttheexperiment.Thegoodmatch- ingbetweentheobservationsandthemodellingresultsintermsof injectedwatervolumevalidatesthisassumption.Theequivalent caprockpermeabilityattheinterval3levelhasbeenconsidered constantatthebeginningofstageB,butaroundday50,theresults oftheinitialmodelstartedtodeviatefromtheobservations;aslight increasehasbeensettomimictheobservedbehaviourofinterval 3pressure(2×10⁻⁴mDinsteadof1.5×10⁻⁴mD).Onepotential explanationforthatnecessarymodificationislikelytobedueto thechangeofflowdirection(above5–6barininterval3,thepore waterflowsfrominterval3towardsthecaprock),whichcannotbe properlymodelledwiththeinitialparameters.

TheevolutionofeffectivewellpermeabilityduringstageBis displayedinFig.3togetherwiththatofstageA/period3,corre- spondingtosimilartemperatureconditionsinthewell(onlythe fluidcompositionhasbeenchanged).WeseethatduringstageB,

Fig.4.Scaled-volumeflowingalongthewell-systemwithpore-waterandgasatsimilarconditions(2scenariosofpore-watervolumeareshowncorrespondingtotwo fractionsofthetotalpore-watervolumeinjectedininterval1,respectively4.5and9%).

theeffectivepermeabilityvaluesremainlowerthanduringstage A,period3eventhoughthelowestvaluesareinthesamerange forthetwotimeperiods(ca.1×10⁻³mD).Themajordifferenceis theresponsetothepressurestepsimposedininterval1duringthe circulationsteadystate.Threepressureincreasesininterval1had beencompletedduringstageA,period3similarlytoduringstageB.

Thesechangeshadledtosignificantincreaseintheeffectivewell permeability,whiletheseincreasesseemsmuchlowerduringstage B(seeFig.3,notethatthefirstpressureincreaseofstageBinMay 2015doesnotleadtoanyobservablechangeandisnotrepresented onthefigure).

4.1.2. Hydraulictestswithgas

Justbeforestoppingthesystem,thewell-systemhasbeenkept atthesamepressureandtemperatureconditions(25bar,30^◦C), butwiththereplacementofthewaterfillingtheintervalsbygas (N2).GiventhefactthatthecapillarypressuremagnitudeinOpali- nusClayislikelytobesignificantandthereforetorestraingasflow towardstheformation(seeforinstanceFig.4ofCroiséetal.(2006)), onecanconsiderinafirstapproachthatmostofinjectedgasflows alongthewell-system.Thisvolumeinjectedafterthereplacement, hasbeencomparedwiththevolumeestimatedtoflowalongthe well-systemjustbeforethereplacement.Sincetheinjectedpore- waterwasflowingbothtowardsthecaprockformationandthrough thewell-system, thefraction of the injected carbonated water flowingthroughthewell-systemhadtobeestimated.Duringthe modelledperiodofstageB,thisfractionhadanaveragevalueof 4.5%andwasalwayslowerto9%.Sinceourpurposewastocompare permeability-likequantities,thevolumeshavebeenscaledbythe inverseofthefluidviscosity.Thesescaled-volumesareplottedin Fig.4forafractionofinjectedbrineflowingalongthewellsystemof 4.5%andof9%.Forbothcases,thescaled-volumeofinjectedwater appearstobelowerthanthatmeasuredduringgastestsbutofthe sameorderofmagnitude.Despitetheuncertainties linkedwith thisanalysis(somegasmighthaveenteredinthecaprockforma- tion,thepore-waterfractionflowingalongthewell-systemmight bedifferentfromtheestimations),itwouldindicatethatthewell effectivepermeabilitymeasuredallalongthisstudyinfullywater- saturatedconditionsisnotsignificantlydifferentfromtheeffective permeabilitythatwouldbeusedtoquantifygasadvancementfront migration.

4.2. GeochemicalobservationsduringstageB

4.2.1. Evolutionofpore-watercompositionininterval3

Thecompositions ofthe solutionssampled inInterval3 are reportedintheAppendixCofSupplementarymaterial(TableC1).

Thebromidetracercontent(Fig.5a)isofparticularinterestsinceits evolutionisrelatedtoanarrivalofwaterflowingfromtheinjection interval.Thebromideconcentrationofinterval3wasstablebefore CO2 injectionandduringthe2firstmonthsafterCO2 injection.

Then,itsconcentrationstartstoincreaseupto2–2.5timestheBr initialconcentration,about3monthsafterCO₂injectionstarts.The Brconcentrationthatwasreachedduringthearrivalofthetracer stillremainsintherangeofBrconcentrationintheOpalinusclay porewater,buttheevolutionoftheBr/ClratioconfirmsthattheBr increaseisduetothearrivalofaBrrichwaterwithadifferentBr/Cl ratiothantheOpalinusclayporewater.TheBrevolutionininterval 3isthenmostlikelytheindicationofanarrivalofBr-markedwater frominterval1.

IncontrasttotheBrnon-reactivetracerevolution,theisotopi- callymarkedinjectedCO₂ isnotobservedininterval3(Fig.5b).

Fig.5.a)EvolutionofBr/Clratioininterval3;b)Evolutionofinterval1and3water isotopicsignature.ThedashedverticallineindicatesthebeginningofstageB.

Indeed,the␦¹³C-TICsignatureremainsconstantovertheexper- iment duration, in the range of its initial signature, while the

␦¹³C-TICsignatureininterval1showshighervalues,notablydif- ferentiatedfrominitialporewater.ItsuggeststhattheinjectedCO2

hasreactedalongitsflowpathbetweeninterval1andinterval3 (potentiallybytheprecipitationofcalcite).

InparalleltothischangeofBrconcentration,someelements(Ca, Na,Mg,Sr,Si,SO₄,Cl)presentsomesmallvariationsintheirconcen- tration,displayingeitheranincreaseoradecrease.Thevariations arerelativelymoderatedbuttheyareobservedsimultaneouslyand correspondtothesamplesinwhichaBrarrivalhasbeenevidenced.

ThepHis stableininterval3(AppendixCofSupplementary material,Fig.C1) during theperiodwhere CO₂ wasinjected in interval1.ThepHmeasuredbythein-linesensorisconsidered asmorereliablesinceitismeasuredattheintervalpressureand, then,doesnotsufferCO₂outgassingduringthesamplingandthe analysis.Theredoxpotentialwasmeasuredin-lineattheoutlet ofinterval3(AppendixCofSupplementarymaterial,FigureC1) andwasalsocalculatedfromthemeasuredFe²⁺andtotalFecon- centration(AppendixCofSupplementarymaterial,TableC1).The in-linemeasuredEhfirstshowsadecreaseandthenstabilizesata valueofabout−560mV.Thedecreaseisduetothearrivalofwater frominterval3,whichreplacesthewaterusedforthesaturation oftheEhsensorby-passlines.ThestabilizednegativeEhvalues, correspondingtoreductiveconditions,arethenconsideredasrep- resentativeoftheredoxconditionsintheinterval.TheEhsensor wascalibratedatseveraltimes,confirmingthemeasuredEhvalues.

MeasuredEhvaluesarelowerthantheEhobtainedinotherexper- imentsatMontTerri(usuallybetween−250and−400mV(Wersin etal.,2011)).Thepositivecalculatedpevaluesobtainedfromthe watersamplingsarenotconsistentwiththein-linemeasurements and,then,indicatethatthewaterwereoxidisedoncesampledfrom theinterval.

4.2.2. Insightsfromporewatercompositionevolution,and expectedmineralogicalevolutionofthewellsystem

Based onP,T conditions of the experimentalapparatus, and constraintsfromhydraulicmodelling,asetofreactivetransport calculationshasbeenperformedwithTOUGHREACTv.2(Xuetal., 2012)toevaluatethegeochemicalprocessesinducedbytheprop- agationofcarbonatedwatersthroughtheannulus.Themodelhas apseudo-2Dgeometry(Fig.6)andconsiders asimplificationof thedifferentdomainsthroughamulti-porosityapproach,includ- ingpropertiesreasonablyrepresentativeoftheaverageproperties of the compartments of interest (intervals 1 and 3, annulus, caprock).Inparticular,theannulushasbeenmodelledasasingle domainwith a verticalpermeability of 1.5×10⁻¹⁸m². The dif- ferentdomainsarealignedalong theverticalextentofthewell andmassexchangeatlateralboundaries(suchasattheannulus- caprockinterface)isallowed.Theinitialgeochemicalconditionsare givenintheappendices(TableA1).Themodelconsiderstheannu- lustohavetheinitialgeochemicalpropertiesofcement,whichis byfarmore reactivethantheadjacentcaprock.Volumes,inter- faceareas,internodaldistances,effectiveporosity,permeabilityof thedifferentcompartmentsarekey parameters thatsensitively affectthenumericaloutputs,calibratedbytrial-and-errortoobtain reasonable fitting with experimentaldata on tracerconcentra- tion.Thewaterflowbetweentheintervals,thecaprockandthe annulusobtainedbyinversionofthepressureevolutionusingthe hydraulicmodelwereconsideredasinputparametersofthereac- tivemodel.Thematchingbetweennumericaland experimental datais intrinsically non-linear, and given thequitelarge num- berofparametersinvolved,itisnotpossibletoidentifyaunique

“bestfittingsolution”.Fig.A2ofSupplementarymaterialshows anexampleofgoodfittingconditions,achievedbyfollowingthis

Fig.6.Conceptualmodelusedforreactivetransportsimulations.Bluearrowsrep- resentmassexchangebetweendifferentcompartmentsconsideredinthemodel:

1a=frominterval1(injectionchamber)toannulus;1b=fromannulustointerval3 (monitoringchamber);2=caprock-annulus;3=fromcementsheathtointerval3;

4=fromnear-apparatusenvironmenttointerval3.(Forinterpretationoftherefer- encestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthis article.)

iterativeapproach.Massbalanceininterval3iscontrolledbythe predominantcontributionoffluidspropagatingfromtheinterval 1(injectionchamber),andflowingthroughboththeannulusand theportionof thecaprockadjacent tocementinterface.Under theseconditions,thepropagationofpH-acid,CO₂-richfluidscom- ingfrominterval1causesasignificantdecreaseofthepHinthe aqueoussolutionpermeatingtheannulus.In thiscompartment, thepHrapidlystabilizesbetween8.2–8.3,wellbelowtheinitial valueof∼12.4.Thereactiveflowthroughtheannulusalsocauses anincreaseofthetotalconcentrationofmostoftheaqueouscon- stituents(Si,S,Mg,Sr,Br).OnlyCatendstodecreasebecauseof mineraldissolution/precipitationprocesses.Overthe6monthsof simulation,themainmineralogicaltransformationspredictedby thecode(Fig.7)areaminordissolution ofmostoftheprimary solidconstituentsofthecement(portlandite,CSH(1.6),ettringite, hydrogarnets:C₃FH₆andC₃AH₆),andtheprecipitationofcalcite, hydrotalcite,andtracesofamorphoussilica.Overall,themineralog- icalfaciesofthecementsheathisnegligiblyaffectedbywater-rock interactions,andaminordecreaseinporosityispredicted(−4‰) overthe6-monthperiodofhydraulictest,whichcanbeconverted toanetvolumeof2.2×10⁻⁷m³.Itisnoteworthythatthereac- tivemodelimplementedhere(wherefluidsofmixedcomposition fromcaprock,cementandinterval1interactwiththeprimarymin- eralogy ofthecementitiousannulus)underestimates exchanges betweentheannulusandthelateraldomains(caprock,andpos- siblycementmatrix).Thus,thelateralinflowofcaprockfluidsis underestimated,whichalsomeansthatanadditional‘sealingpro- cess’couldbeneglectedifadditionalphasesformbyreactionofthe caprockwaterandCO2.

Fig.7.Porosityandmediumvolumefractionvariationsforselectedsolidcon- stituents(CSH(1.6),portlandite,C3AH6,calcite)intheannuluscompartment.

Considering theaverage effectivewell permeability inferred fromthehydraulictestsandusedinthereactivetransportsim- ulations(1.5×10⁻¹⁸m²),theequivalentthicknessoftheannulus computed by thecubic law would be 7.7×10⁻⁷m²; with that geometry,theestimatednetvolumechangewouldcorrespondto averticalextentofcompletesealingof46cm.

4.3. Observationsafterovercoring

4.3.1. Observationsduringtheovercoringstageandinspectionof theretrievedwell-system

4.3.1.1. Goodqualitycementingprocess. Thecementsheathappears tobeanimpressionoftheinitialboreholewalldrilledintheclay- rock,indicating a good cohesion betweencementand clayrock eveninfractures/shearplanes(seeFig.8-1).Thiswouldindicate that1)thecementingwasmostlyofverygoodqualityand2)the cementbulkwasnotcrushedduringtheovercoringoperations,i.e.

theexternalpartofthewell-systemrecoveredaftertheovercoring operationswouldbeclosetothecement/clayinterface.

4.3.1.2. Presenceofamicro-annulusatthecement/clayinterface. As indicated inSection 3.6, claywasmilled duringtheovercoring whilethecementremainedquasi-intact:thisclaycrushingcould belinkedwiththeinterfaceclay/cementweakness,(andbythe differenceofmechanicalpropertiesbetweenclayandcement).To checkthehypothesisofamicro-annulusexistenceatthatinter- face,avideowasmadeduringtheovercoringoperations,whenthe overcoredrillingwasatadepthof8.15m(i.e.intheupperpartof thecementedwellsystem).InFig.8-2,theinterfaceclay/cementis visibleandmicro-annuluscanbeobserved.However,itisdifficult toknowifthisspaceexistedanteriortotheovercoringoperations orifithasbeencreatedbythisstage.

Asdescribed inSection 3.5,resin and fluoresceinhave been injectedininterval3beforestoppingthesystemwiththeaimof localizingtheseflowpaths.Theexternalupperpartofthecemented wellsystemcorrespondingtothecement/clayinterfacelocated closetointerval3hasconsequentlybeenobservedunderUVillu- minationafteritsretrieval:agreen colorationhasbeennoticed (seeFig.8-3),indicatingthepresenceoffluoresceinandtherefore confirmingtheexistenceofaflowingpathatthislevel.

Theobservationofflowpathwaysattheclay/cementinterface appearstobeinlinewiththeconclusionsofstageAobtainedby hydraulicandgeochemicalmodelling.

in contactwiththeclay): this wasconfirmedwithHCl testing.

However,theappearanceisslightly differentfor the40cmsec- tionlocatedatthebottomofthecementedwellsystemwherea yellow/orangecolouringcanbeseen.

4.3.1.5. Signsofcementcarbonationalongthecement/steelinterface.

Afterremovingsomeofthecementsheathfromthecasing,the interfacecement/steelwasobserved.Awhiteprecipitatehasbeen observedatthatinterface.Nocleardifferencehasbeenobserved betweendifferentsamplestakenatdifferentdepths,whichindicate arelativelyhomogeneousprecipitation.HCltestingconfirmedthe presenceofcarbonates.

Asindicatedabove,thecementsheathintegritywasverygood, andnosignof significantcracksthroughthis sheathhavebeen found.However,onesmallfractureacrossthecementsheathhas beenobserved.Interestingly,carbonationwithinthefracturecan beobserved,which wouldindicateaself-sealingofthefracture (Fig.8-5).

4.3.1.6. Signsofcasingcorrosion. Attheverytopthewell-system (depth7.82–7.95m),azonewithnocementhasbeennoticed.The casingwasdamagedandcorrodedatthatlevel,andabreachhas beenobservedwithinthesteel.It is unlikelythat this holehas beencreatedduringtheovercoringoperationsgiventheverygood stateofthewell-systemthathasbeenliftedout,anditsappearance seemstoindicatethatthisholehasbeencreatedbycorrosion.Inter- estingly,theobservationofthecasingsteelonalargerzoneshows thatthecorrosionwasverylocalizedtothe“no-cemented”zone andthatthesteelcasingwasverywellprotectedbythecement sheathevenveryclosetothisdamagedzone(seeFig.8-6).

4.3.2. Characterizationoftheclaystonecore(BCS-5A)

4.3.2.1. Carbonandoxygenisotopesincalcite. Calciteextractionand carbonisotopesofcalcitewereperformedtotracepotentialmod- ifications of thecarbonate fractiondue toCO₂ injectionin the claystone.The␦¹³Cand␦¹⁸Oofthecalciteintheclaystonesrange between0.3and0.4‰(PDB),andbetween24.4and25.0‰(PDB) respectively.Theseisotopicvaluesarehomogeneouswhateverthe positionoftheclaystonesrelativelytotheexperimentsystem,and arealmostsimilartothecarbonandoxygenisotopiccompositions ofcalciteinunperturbedOpalinusclaystones.

4.3.2.2. Cation exchange capacity (CEC) and aqueous leachingon claystone. Cationexchangecapacities(CEC)weremeasuredinclay- stones in order to test potential perturbations of the sorption capacity and of thesorbed-cation distribution of clay minerals (TableC2ofSupplementarymaterial).TheCECandtheconcentra- tionsofexchangedcations(giveninmeq/100gofrock)measured inclaystonesfromthisexperimentdifferslightlyfromreference

Fig.8.Inspectionoftheretrievedwellsystem:1—a)shearplanes“footprints”ontheexternalpartofthewellsystem;b)Noimpactoftheshearplaneswithinthesheath(the shearplanfootprintishighlightedwithdottedline)2—Boreholeinspectionmadeaftertheovercoringthesystemuntildepth8.15m3—Photographofthesamepieceofthe cementedwell-systemundernormallight(top)andultravioletlight(bottom).Thepicturecorrespondstothecement/clayinterfaceatdepth8.16–8.36m(theadvancement frontuntilwhichtheresinseemstohavehardenedisindicatedwithadottedline)4—Halfcrosssectionofthecementedwell-systemsawedatdepth10.05m)5—smallcrack acrossthecementsheathsampledatdepth9.92m(dottedzone)6—Signsofcasingcorrosion.a)Breachinthecasingatanon-cementedzonelevelb)samepictureafter cementremoval:intactcasingoutsideofthedamagedzone.

valuesbyalowerMgconcentrationandahigherKconcentration.

Thesevariationsarelowandcouldbeduetothenaturalhetero- geneitiesoftheclayformation.TheSrconcentrationsmeasuredin thesampleBC5-12.08mareabnormallyhigh,andcombinedwith highsulfateconcentrationsintheleachatesstronglysuggestthe presenceofcelestite(strontiumsulfate)inthesample.

Aqueousleachingwasperformedinclaystonetoaccesstothe anionconcentrations(Cl,SO4,Br)inporewater,andtodetectBr tracer.Brmeasurementsweretoolowtodetectanydiffusioninto theclaystone.TheClconcentrationsmeasuredinsamplesfromthis experimentarealmostcomparablewithorslightlyhigherthanref- erencevaluesofOpalinusclaystoneatthelevelofthecasing.They slightlyincrease inclaystonessamplesneartheinterval 1(CO₂ injection).Valuesmeasuredinthesamecoresamplesatthelevelof theinterval1butnearerandfurtherfromtheintervalalsoexhibita decreaseoftheClconcentrationwiththedistance.Thesulfatecon- centrationsmeasuredinsamplesfromthisexperimentarealmost comparablewithreferencevaluesofOpalinusclaystone,exceptthe sampleBCS5-12.08m.

According to these analyses on claystone samples, the CO₂ injectiondidnotinducednoticeabledegradationof thecaprock formation.

4.3.2.3. CO2 partial pressuremeasurements. MonitoringCO2 out- gassingofcoreclaystonesamplesandanalysing␦¹³CofCO₂wasa waytodetectanydiffusionoftheCO₂tracerintotheclaystone.CO₂ partialpressures(pCO2)and␦¹³CofCO2werecomparedwithref- erencevalues.ThepCO₂measuredinunperturbedOpalinusClay rangesbetween3.6and8.9mbar(minimumtimeofequilibrium of 3months), and corresponding␦¹³C of CO2 is about−11.5‰

(Gaucheretal.,2010;Lerougeetal.,2015).Inthisexperiment,the equilibriumregardingpCO2 betweenmineralsandporewateris notexpectedtobeachieved.

Theanalyseswereperformedashorttimeafterconditioning(for analysingthemostavailableCO2)andalongertimeaftercondition- ing.TherawdataareprovidedintheAppendixCofSupplementary material(TableC3).TheCO₂outgassingofthenineglasscontain- erswerecomparedtooutgassingofOpalinusclaystonereferences.

At27days,foursamplesatdepth8.55m,10.15m(near),10.65m (near)and12.08mprovidedevidenceofabnormalhighCO₂out- gassing.At161days,allthepCO2valuesarehigherthanreferences valuesobtainedforunperturbedclaystones.TheCO2extractedat 2daysshowedabnormalhigh␦¹³C inclaystonesneartheinter- val1,andspecificallyatthebottomofthisinterval1,andatthe levelofthecasing−depth8.45–8.55m.After69daysandafirstgas extraction,the␦¹³Cremainsabnormallyhighinclaystonesnearthe bottomofinterval1.Twopopulationsofsamplesaredistinguished:

1)Afirstpopulationofsamples,theleastperturbed,exhibitan almostsimilarrangeofpCO2around14–16mbarandthelow- est␦¹³CofCO₂ between−12and−7‰;valuesof14–16mbar remain higher than reference values measured in previous works(Lerougeetal.,2015).Recentexperimentalworkprovided evidenceofanincreaseofpCO₂withtherelativehumidityand thesaturationstateofthesample(Lassinetal.,2016).Reference pCO2weremeasuredonsamplesfromadrillholedrilledwith Arfluxandwithoutanyresaturation,withamaximumrelative humiditymeasuredat∼95%.Inthepresentsystem,theinitial boreholewasentirelyresaturatedwithsyntheticPearsonpore water.

Fig.9.Aspectatdifferentscalesofthecementedannulusatthetopofthewell-system(indexT)andatthebottom(indexB):(a)Polishedcross-sectionshowingthedifferent textureofthecement;(b,candd)Focusonthecementontheexternalsideincontactwithclay.Chemicalcompositionofthecementedannulusclosetothecontactwith clay:(e)CaandSicontents(giveninweightpercent),measuredonsurfacesalongaradialprofileatthetopandbottomofthewell-system(cf.figuresc);(f)corresponding bulkCa/Siatomicratiocalculatedalongaradialprofileatthetopandbottomofthewell-system.

2)A second population of samples exhibit significantly higher pCO_2.Attheleveloftheinterval1,thesehighpCO₂areassociated toabnormalhigh␦¹³CofCO2,correspondingtothemigration oftheinjectedCO₂.

The sample BCS5 8.45–8.55m has a first ␦¹³C_CO2 value of+24.6‰andasecondoneof−6.2‰;thesedatacombinedwith O₂andN₂concentrationsindicatethattheclaystoneatthisdepth washighlyperturbed.TheBCS512.08–12.12m(belowtheinterval 1)showingaleakoftheglasscontainerdoesnothaveabnormal

␦¹³CCO2value(−7‰).

Toconclude,theimpactoftheCO₂injectionisevidencedatthe surroundingoftheinterval1,intervalinwhichCO₂-richwaterwas directlyinjected;elsewhereintheupperpartofthesystemthere isnoevidenceofimpactsoftheinjectedCO₂.

4.3.3. Characterisationoftheovercorecement(BCS5-OC)

4.3.3.1. Microscopicobservations. Microscopicobservationsofpol- ishedcross-sectionsofthecementcasingat7.8–8.8m(top)andat 10.12m(bottom)providedevidenceofalmosthomogeneoustex- tureandchemistryofthecementexceptonthecementinterfaces withclayandwithsteel(Fig.9).Thecarbonationdetectedbymicro- scopicobservationsandHCltestsisconfirmedbySEM.Thecement incontactwithsteelisjust characterisedbytheformation ofa 20␮m-thickcalcitecoating.Thecementincontactwithclaystone exhibitsavisibleexternal5mm-thickzonewithadifferenttexture probablyduetocreepduringhardeninganddifferentrateofhydra- tionofthecement(Fig.9a).Thecementonthissideshowsasurface perturbedbythedrillingwhichconsistsofamixingofcrushedclay andcementpaste.Inaddition,thefirst200–300␮mthickzoneof cementcloseofthecement/claystoneinterfaceismoreporousand

Fig.10.␦¹³C–␦¹⁸Odiagramshowingdatafromcalciteextractedinclayformation andincementatbothinterfaces(clayandsteel).DatafromthesametypeofclassG cementincontactwithCallovian-Oxfordianclayformationaregivenforcomparison (Gaboreauetal.,2012).Thedatacomingfromthisexperimentarenamed“ULTCO2”.

presentsaCa/Siratiolowerthanintherestofthecement(Fig.9c–f).

ThisdataisconsistentwithpreviousstudiesoncementclassGand classicalPortlandmaterialsincontactwithclay(Gaboreauetal., 2012).

It hasto benoted that this perturbation appears,on Fig.9, lessobviousatthebottomthanatthetopofthewell.However, themicroscopicobservationshaveshown,atthetopofthewell, somelow-perturbedzoneaswell,indicatingnon homogeneous processes.Noclearobservationofgeneralizeddifferencesbetween thetopandthebottomofthewellcouldhavebeenmade.

4.3.3.2. Carbonandoxygenisotopesincalcite

Inordertoquantifyandtodeterminetheoriginofcalcitein cementonbothinterfaces(externalinterfaceincontactwithOpal- inusClayandinternalinterfaceincontactwithcasing),the␦¹³C and␦¹⁸Oofcalciteweremeasuredafterperformingalocalattack ofthecementwithphosphoricacid(TableC4ofSupplementary material).

The␦¹³Cand␦¹⁸Oofcalciteincementincontactwithclaystone issignificantlydifferentfromthoseofcalciteincementincontact withsteel(Fig.10).The¹³Cenrichmentincementincontactwith claycouldbeduetointeractionswithCO2fromtheclayformation, ortointeractionswithinjectedCO₂(␦¹³C∼+20‰).Comparisonof thesedatawiththoseobtainedinsimilarclassGcementincontact withCallovian-Oxfordian clayformation(Gaboreauetal.,2012) providesevidenceofsimilarrangesofvalues.Consequently(C,O) isotopicdataoncementdonotsupportanyformationofcalcite linkedtoamigrationoftheinjectedCO2,butrathershowthatthe calcitewasformedbyprecipitationofcarbonatesduetoreaction withtheclaystoneorwiththesyntheticsolution.Howeveritis noteworthythatcalcitecontentincementatinterfacewithclayis loweratthebottomofthecasingneartheinterval1ofCO2injection (TableC4).Alowercalcitecontentcouldbeexplainedbyhetero- geneitiesofclay/cementinteractionsorbyapartialdecarbonation ofthecementduetotheinjectedCO2migration.

5. Interpretationanddiscussion

Theovercoringstephasprovidedtheopportunitytogaininfor- mationusuallynot accessibleorathighcosts.It hasconfirmed thepotentialmigrationpathwaysthatwereexpectedbyhydraulic andgeochemicalobservations,andmodellingperformedduring theoperations.Inourcase,themigrationalongthewellseemsto haveoccurredmainlyalongtheclay/cementinterface.Theincrease oftemperatureatthebottomofthewellhasclosedthatinterface progressivelybut theinterpretationmadeafterthefirstyearof experiment(seeManceauetal.,2015)andtheobservationsmade

duringandaftertheovercoringindicatethatamicro-annuluswas still presentat thetopof thewell.Itcan beassumedthat this micro-annuluswaspresentattheverybeginningoftheexperi- mentallalongthewelltoexplaintheinitialpermeabilityofthe well(20mD),despitethecaretakentoperformthecementing.The explanationofthatcanbeashrinkageduringthecementcuring.

Cementshrinkagecanbeduetoseveralreasonsasdetailede.g.in Dusseaultetal.(2000):Waterexpulsionbeforehardening,lessvol- umeoccupiedbythecementafterthehydrationprocess,presence ofdissolvedgases,osmoticdewateringinstronglysaltedenviron- ment,highcuringtemperaturesandearlysetaresomeofthegiven reasons.Thethreeformerreasonscannotberuledoutbutthethree latterdonotcorrespondtotheconditionsoftheexperiment.In addition,thecement/clayinterfacesappearedtobeafingerprintof theinitialboreholewallandthereforetheshrinkage,ifitoccurred, tookplaceafterhardening.

Thecontinuoushydraulictestshaveshownadiminutionofthe permeabilityincreasepotentialwithpressureincreaseafterCO2

wasinjected,comparedwiththepreviousperiod(stageA,period 3).Thisobservationhasbeendoneinaspecificcontext:Inpar- ticular,themodellingofthehydraulictestshasshownthat the effectivepermeabilitywasverylowduringstageBandtheadvec- tivefluidflowthroughthenear-wellenvironmentwaslimited.In suchconditions,theexperimentindicatesthatevenwhenforcing CO2 richwatertoenterthenear-wellenvironment, thecontact withCO₂leadstosealingoftheinitialflowpathways,andthere- foreanincreaseofthewellintegrity.Furthermorewedidnotnotice anydegradationofthewellhydraulicproperties,asithasbeen sometimesnoticed,inadvectiondominatedcontext(seeSection 1).Thequestionofhowthewellwouldhavebehavedwithdifferent pre-CO2-contacthydraulicconditionsisanopenquestion.Never- theless,theexperimentprovidesinterestinginsightregardingthe effectofCO₂whenrelativelygoodintegritypre-existsbeforeawell isincontactwithaCO2plume.

The precipitation of carbonates along flow pathways is suggestedtoexplainthedecreaseofthewellboreeffectiveper- meabilityandtheapparentsealingoftheclay/cementinterface.

However,aninfluenceoftheinjectedCO2inthecarbonationcould nothavebeenmeasuredinthecementsampledinthewell-system.

Thiscouldbeexplainedby1)theprecipitationofcalciteoccurring inlimitedamountsregardingthebalancebetweentheCO2insolu- tion,thelimitedvolumeofmigratingfluids(giventhelowinitial effectivewellpermeability)andtheclayrockandcementvolumes incontactwiththeflowingwater;or2)theabsenceofprecipitation ontheanalysedcementsamples.Themostprobablecausecould bethatthecarbonatemineralsthathaveprecipitatedattheinter- facebetweenOpalinusClayandcementhavebeenlostduringthe overcoring(werecallthattheclaystonesurroundingthecemented casinghasbeencrushedduringtheoperations).Theprecipitation ofcarbonatesalongflowpathwayscanneverthelessbesupported bytheconsumptionoftheinjectedCO2duringitsmigrationalong thewell.Thishasbeenconfirmedbythecomparisonbetweenthe non-reactivetracerevolution(thathasreachedtheinterval3after itsco-injectionwithCO2)andthereactivetracerevolution(the isotopicallymarkedCO₂wasnotdetectedintheupperinterval).

Moreover,accordingtothereactivetransportmodellingresults, theprecipitationextentwaslimitedbutitcouldhaveinducedthe cloggingofasmall-sizepathwayonanon-negligibleheight.

Theclaystonesamplesanalysedshowedaverylowcirculation ofCO2-richwaterthroughthatrock,limitedtothezoneslocated veryclosetothehigh-pressureinjectioninterval.Eveninthatzone, thedetectionoftheinjectedCO₂mainlyduringthefirstoutgassing suggeststhattheCO2wasintheclaystoneporosityandnotincor- poratedinthesolid phase.No evidenceofmigration througha potentialBDZhavebeenfound,nordegradationofthecaprockclose tothecementsheathbeforeorafterthewell-systemexposureto

corrosionnoticedinonenon-cementedzonehighlightstheimpor- tanceofcorrosionissues.Itshouldbenotedthatthecontextofthe experimentwasveryspecific:Onlythecementsheathwasusedas aprotectionagainstcorrosion,andthesteelcasingsectionwascon- nectedtotheexperimentalset-up(packers,tubes)madeofother materials(notablystainlesssteel)in anelectrolyteenvironment (saltedwater,accordingtotheanalysestheinjectedCO2isnotlikely tohavereachedthatzoneandthustobeinvolvedinthecorrosion), whichcouldhavefavouredagalvaniccorrosiontype.Theobserved corrosionneverthelessshowstheimportanceofcorrosionpreven- tivemeasuresimplementation, suchascorrosioninhibitors,and cathodicprotection(Talabanietal.,2000;Choietal.,2013).

6. Summaryandconclusion

A1:1scaleexperimentsupportedbymulti-disciplinaryanalyses hasallowedimprovementoftheunderstandingofthewellintegrity issueinthecontextofCO₂geologicalstorage.Awellsectionhas beenconstructedintheMontTerriundergroundlaboratoryand thehydraulicandgeochemicalevolutionofthissectionhasbeen followedwiththeapplicationofdifferentstressesonthesystem (temperature,pressureandgeochemical).Thewellsystemhasbeen ultimatelyovercoredforinspection.Attheendoftheexperiment,a significantquantityofinformationofdifferenttypesareaccessible:

Rawdataorinterpretedresultsfrom2.5year-longhydraulictests, watercompositionatdifferenttimeperiods,observationsandmin- eralogicalanalysesonovercoredsamplesofthewellsystemgive theopportunitytoexplainthebehaviourofthewell-systemduring theexperiment.

The first year of the experiment (stage A), when different pressureandtemperatureconditionswereappliedisextensively describedinManceauetal.(2015).Thispaperhasbeenfocusedon thesecondyearoftheexperiment(stageB)dedicatedtotheexpo- sureofthesystemtoCO₂-richporewaterandtotheovercoring stage.

Twotypesofoutcomescanbeextractedfromtheresultspre- sentedinthatmanuscript.

1)Keymessagesregardingtheunderstandingofprocessesaffect- ingtheintegrityofawellcanbederivedfromtheexperimental results:

Therelativelyhighinitialwellpermeability(atthebeginningof stageA)hasbeenexplainedbyamigrationalongthecement/clay interface,possibly due tocement shrinkage, while the cement matrixandtheclay-richcaprockclosetothewellappearedtobea verygoodbarriertounwantedfluidmigration.Thisshowedthatthe

2)Thisstudyconfirmstheabilityofthistypeofexperiment(1:1 scaleinacontrolledenvironment)toimproveourknowledge oncomplex phenomena(thermal, mechanical,hydraulic and geochemical)occurringatarealisticscale,whichisespecially important for well integrity. Such experimental works may indeed allowan integration of a maximum of realistic pro- cesses,themonitoringofparametersusuallyonlymeasurable inthefield(e.g.effectivewellpermeability)andthevalidation oftheupscalingoflaboratoryresults:Similarexperimentalset-up couldbeusedtostudytheimpactofdifferenttypesofcontrolled- defectsinawell,ortotestmonitoringandremediationstrategies todetect/correctanyundesiredfluidmigrationsalongawell.

AsforstageA,itshouldbenotedthatthepressure,tempera- tureandgeochemicalconditionsrangecoveredduringstageBas wellastheexperimentdurationwerelimited,andthattheforma- tionunderstudytogetherwiththewellconstructionstageswere specifictothisexperiment.Anygeneralizationorextrapolationto longertime-scalesoftheexperimentsoutcomesshouldtherefore beconsideredwithcaution(Manceauetal.(2015)).

Inaddition,itshouldbenotedthatthepresentstudyisfocused onwellintegrityunderstanding:Fortheassessmentofpotential migrationimpactsinaspecificsituation,thewellintegrityshould beevaluatedinparalleltodrivingforcesinplace(buoyancy,pres- suregradient)andtothevulnerabilityofsubsurface/surfacestakes.

Acknowledgments

Theresearchleadingtotheseresultshasbeencarriedoutin theframeworkoftheULTimate-CO2Project,fundedbytheEuro- peanCommission’sSeventhFrameworkProgram[FP7/2007-2013]

undergrantagreementno281196.TheULTimate-CO₂consortium wouldliketothankSwisstopoandObayashiforfundingapartof thisexperimentation.

We thank Olaf Ukelis for his helpful comments on the manuscript.

AppendixAtoC. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttp://dx.doi.org/10.1016/j.ijggc.2016.09.

012.

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