Effects in the solubility of CaCO3: experimental study and model description.

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ContentslistsavailableatSciVerseScienceDirect

Fluid

Phase

Equilibria

j o ur n a l ho m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / f l u i d

Effects

in

the

solubility

of

CaCO

3

:

Experimental

study

and

model

description

B. Coto

a,∗

_{, C.}

_Martos

a

_,

_J.L.

_{Pe ˜}

_na

b

_,

_R.

_Rodríguez

a

_,

_G.

_Pastor

a

a_Department_of_Chemical_and_Energy_Technology,_ESCET,_Universidad_Rey_Juan_Carlos,_C/Tulipán_s/n,₂₈₉₃₃_Móstoles,_Madrid,_Spain b_Technology_Center_Repsol,₂₈₉₃₃_Móstoles,_Madrid,_Spain

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received23December2011

Receivedinrevisedform13March2012 Accepted14March2012

Availableonline23March2012

Keywords:

Calciumcarbonate Scale

Solubility Electrolytes ELEC-NRTL

a

b

s

t

r

a

c

t

Crudeoilisusuallyco-producedwithreservoirwater,withincreasingcontentintheproductionfluid alongfieldlife.Changesintemperature,pressure,and/orchemicalcompositionmaycausesignificant precipitationofinorganicsalts(“scales”)duringproduction.Therefore,theknowledgeoftheinfluence thatdifferentvariablesmayhaveonsaltsolubilityiscriticaltoanticipateoridentifypotentialflow assur-anceproblemsrelatedtoscales.Thepresentworkisspecificallyfocusedinthestudyofcalciumcarbonate precipitateformationasamaincomponentof“scales”.Duetothenumberofvariablesinvolvedin cal-ciumcarbonateprecipitation(temperature,pressure,CO2partialpressure,othersaltcontent)andthe heterogeneityofreservoirconditions,thereareseriouslimitationstoperformafullexperimentalstudy coveringallthepossibleprecipitationscenarios.Solubilitydatapresentedinthiswork,bothpreviously reportedandexperimentallydetermined,coverawiderangeofexperimentalconditions.

Asimulationmodelthatallowsquantitativepredictionsindifferentscenariosisaninterestingtool. AversatilesimulationalgorithmwasdevelopedusingASPENPLUS®_7.1_from_Aspen_Technology,_Inc.,

thatallowsdifferentexperimentalconditionsandthequantificationoftheinfluenceoftemperature, pressureandpHinCaCO3solubility.Thissimulationschemewasappliedtodescribebothliteratureand newexperimentalsolubilitydata.Predictedresultswereinreasonableagreementwithexperimental information.Thesolubilityofcalciumcarbonatedecreaseswithtemperature,increaseswithpressure andshowsamaximuminpresenceofNaCl.TheCO2partialpressurehasstrongeffectbecauseitisdirect relationwithsolutionpHthatmodifytheamountofionicspeciespresentintheaqueoussolution,and henceincreasingthesolubilityofcalciumcarbonate.SpecialattentionwasdevotedtosuchpHeffect but,inordertohaveafullypredictivemodel,noparametersfitwascarriedout.Themainconclusionof thisworkisthesuitablesimulationschemetodescribeandpredictthesolubilityofcalciumcarbonate atdifferentconditions.

1. Introduction

Thestudyoftheeffectofdifferentvariablesonthesolubility

ofcalciumcarbonatein aqueoussolutionis importantto

antic-ipateand/ormitigatepotentialﬂow assuranceproblemsdueto

calciumcarbonatescaleformationanddepositionduringcrudeoil

andassociatedwaterproduction.Reliabledataofcalcium

carbon-atesolubilityasafunctionoftemperature,totalandpartialCO2

pressure,pHandothervariablesarerequiredtocalibrateaccurate

simulationmodelscapabletoreproducethiscomplexequilibrium.

Thesolubilityofcalciumcarbonateinwaterhasbeenpublished

indifferentsourcesatroomtemperatureandhighertemperatures

[1,2].

Ingeothermalﬂuids,signiﬁcantamountsofcarbondioxidemay

existat reservoirpressure and temperatureconditions[2].This

∗Correspondingauthor.Tel.:+34914887089;fax:+34914887068.

E-mailaddress:baudilio.coto@urjc.es(B.Coto).

compoundhasastronginﬂuenceintheequilibriumofcalcium

car-bonatesolutions,andthesaltsolubilityisstronglyconditionedby

thepresenceofthisgas.

Thecomplexityofthedescriptionofthesolubilityofcalcium

carbonateinwaterisduetothepresenceofsimultaneous

chem-icalequilibriaofa highnumber ofspecies(CO32−,HCO3−,CO2,

etc.)involvedinthisprocessbetweensolid,liquidandgasphases.

Furthermore,ifhydrocarbonphasesarealsopresentinreservoir

conditions, CO2 willalso be present in the liquid hydrocarbon

phase, addinganotherphase totheequilibriatobeconsidered.

Accordingtothesecomplexequilibria,theeffectofbothCO2

par-tialpressureandpHcanmodifythedistributionofdifferentionic

species,affectingthedepositionorthesolubilizationofthecalcium

carbonate[3].

Theeffectofpressurealoneonthesupersaturationof

carbon-ate mineralsin water solutions hasusually minor signiﬁcance.

More important is the effect on the calcium carbonate

sol-ubility of carbon dioxide and the related chemical reactions

that lead to the formation of CaCO3 precipitate. Firstly, the

(2)

precipitationofCaCO3 iscontrolledbythefollowingequilibrium

(1):

Ca2+₊_2HCO

3−CaCO3(s)+CO2+H2O (1)

Thusaspressuredecreases,CO2iseffectivelylostfromsolution

tothegasphase.Consequently,thedrivingforceleadingtothe

pre-cipitationofCaCO3isincreasedaccordingtotheaboveequilibrium

[4].

Secondly,thelossofCO2fromthesolutionresultsina

reduc-tionintheconcentrationofcarbonicacidbythefollowingchemical

equilibrium(2),whichconsequentlyresultsinanincreaseinbrine

pH[2,3,5–7].

CO2+H2OH2CO3 HCO3−+H+ (2)

On theotherhand, thehighnumber of ionic species which

appearinthesolutionofcalciumcarbonatemakestheavailability

oftheinteractionparametermatrixofelectrolytethermodynamic

modelapriorityforpropersimulationmodels.

Formationwaters containedin sedimentaryrocksmay have

salinityvaluesinarangeofapproximatelyﬁveordersof

magni-tude[8],fromdilutemeteoricwaters,withtypicalvaluesofsalinity

below10gl−1_,_to_waters_with_salinity_higher_than₆₀₀_g_l−1_.

Consid-erableprogresshasbeenmadeinpredictingthesolubilityofmany

commonreservoirminerals,likecalciumcarbonate,insolutions

withhighionicstrengthbyapplyingthePitzer virial-coefﬁcient

approachtoestimateionactivities[9–12].AswellasPitzer

thermo-dynamicmodel,localcompositionmodelsthatexpresstheexcess

Gibbsfreeenergyduetoshortrangeintermolecularinteractions

suchastheWilson,NRTLandUNIQUACmodelshavebeenused

asactivitycoefﬁcientmodelsinthepredictionofsaltprecipitation

[13–15].

Asmentionedabove,calciumcarbonateisoneofthemainsalts

whichmaycauseﬂowassuranceproblems,creatingdepositsonthe

productionﬂowlines.Inthiscase,theslowkineticofthesolubility

processofcalciumcarbonateandtheinﬂuenceofpH,temperature,

pressure,etc.[16]maychangetheequilibriumofthissalt.Asall

thementionedvariableshaveabigeffectontheprecipitationof

calciumcarbonate,itisachallengetocarryoutafullexperimental

studycoveringallthepossibleconditionsandtodevelopsimulation

modelsabletodescribealltheexperimentalinformation.

Thecalculationofactivitycoefﬁcients ofsaltsin a saturated

solutionisthemain probleminorder topredictthescale

pre-cipitation.ThePitzerandTheElectrolyteNon-RandomTwoLiquid

(ELEC-NRTL)modelsarethemostwidelyusedmodelstoestimate

theactivitycoefﬁcientsforaqueouselectrolytesystems[17].

TheELEC-NRTLmodelwasoriginallyproposedbyChenetal.

[18,19], for aqueous electrolyte systems and later extended to

mixedsolventelectrolytesystems[5,20].TheELEC-NRTLmodelis

aversatilemodelfor thecalculationof activitycoefﬁcients that

canbeappliedforalltheconcentrationrangeusingexclusively

binaryparameters,whilePitzermodelisrestrictedtoelectrolyte

concentrationuptoapproximately6m.

Despitethelimitationsofthemodels(absenceofbinary

inter-actionparametersofsomeelectrolytes),theirapplicabilitytothe

calculationofthethermodynamicequilibriumofelectrolyte

solu-tionshavebeentestedbymanyauthors[5–7,10,11].

Duetothechallengesandcomplexityofthecalciumcarbonate

system,thepresentworkisfocusedonthecompilationofreliable

experimentaldatatoevaluatethesolubilityofcalciumcarbonate

inaqueoussolutioninawiderangeofoperatingconditionsandthe

validationofathermodynamicmodelforcalciumcarbonate

solu-bilityevaluation.Theinﬂuenceofparameterssuchastemperature,

pressure,andpresenceofNaClinthesolutionwillbestudiedbya

simulationmodelunderASPENPLUS® _v7.1_software._This_study

focusesinpredictingrealistic oilﬁeldwatersystems,and

there-fore,thecalciumcarbonatesolubilitywasevaluatedathighvalues

ofionicstrengthusingbothexperimentalandtheoretical

proce-dures.Noliteraturedatawerefoundatthesamerangeofionic

strengthvalues.Thevalidationofthismodelwascarriedoutwith

calciumcarbonatesolubilitydataalreadyreportedintheliterature

andexperimentaldataobtainedinthepresentwork.

2. Experimental

Table1summarizesCaCO3 solubilitydatareportedinthe

lit-eraturebydifferentauthors[1,12,21–47]atdifferentconditions

oftemperature,ionicstrength,etc.Theexistingreporteddataare

relativelyscarce,and notalltheexperimentalconditionsofthe

describedexperimentswerereported.

Thesolubilityofcalciteishighlyinﬂuencedbytheamountof

carbonicacidpresentinthesolution(CaCO3 solubilityincreases

withCO2concentration).Inthatcase,calcitesolubilitystudiescan

bedividedintodifferentgroupsaccordingtotheamountofcarbon

dioxideemployed.Manystudiesarefocusedoncalcitesolubilityin

purewater,withoutthepresenceofCO2[21–25].Toachievethat

purpose,alltheCO2dissolvedinthewateriseliminatedbyusing

vigorousboiling.

Ontheotherhand,otherauthorsstudythecalciumcarbonate

solubilitywithdissolvedCO2.Forthatpurpose,pureCO2 is

bub-bledintothesolutionkeepingconstanttheamountofCO2,while

thetemperatureand/orpressurearechanged [15,26–37].Many

oftheseauthorsmeasuredcalcitesolubilityinCO2–H2Omixtures

(CO2concentrationislowerthansaturationvalue).Anotherstudies

usedagasphaseconstitutedbycarbondioxideandN2orairin

dif-ferentproportions.Wehaveusedliteraturedatawithexperimental

conditionssimilartoourwork.

Thesolubilityofcalciumcarbonatewasexperimentally

deter-mined in aqueous solution at different ionic strengths and

temperature.Purecalciumcarbonateandsodiumchloridereagent

gradeACS,providedbySCHARLAB,wereused.Thesolutionswere

preparedwithdeionizedandultrapurewater(MilliQ).

TheexperimentalsystemisaRADLEYScarrouselwithsix

her-meticclosedsphericalglassreactors(250mLeachone)thatcan

bestabilizedatthesameconditionsofpressureandtemperature

(temperaturewas controlled within±0.1◦C). An excess of

cal-ciumcarbonateinwaterorinaNaClsolutionwasstirredat95◦C

andatmosphericpressureineachreactoruntilphaseand

chemi-calequilibriawerereached(approximately4days)asdenotedby

conductivity,pHandcalciumconcentrationvalueswereanalyzed

along timeand usedas equilibriaindicators.A continuous

stir-ringof650rpmwasmaintainedduringalltheexperiment.Every2

days(stabilizationtime)asmallaliquotofsupernatantliquidwas

removed(∼1mL),weightedandﬁlteredthrougha0.45␮mﬁlter.

Theﬁltermustbewarmedpreviouslyattheexperimental

temper-aturetoavoidanyprecipitationorsolutionofsolid.Afterwards,the

glassreactorstemperaturewasdecreased10◦Candthementioned

experimentalprocedurewasrepeatedagainuntilthetemperature

reached25◦C.Inordertostudytherepeatabilityoftheprocedure,

threedifferentsamplesweretakenateachtemperature.ThepHof

thesampleswasmeasureddirectlyfromeachreactoratevery

tem-peraturewithaCRISONMM40multi-meter,calibratedpreviously

withtwobuffersolutionofpH7and4.01.

Theamountofcalciumindissolutionwasanalyzedby

Induc-tivelyCoupledPlasmaAtomicEmissionSpectroscopy(ICP-AES)on

aVARIANVistaAXAxialCCDSimultaneousICP-AES

spectrome-ter.Calciumwasanalyzedattwowavelengths,315.887nm−1_and

317.933nm−1_,_and_the_average_value_was_used._Calibration_curve

wasobtainedfromStandardcalciumsolutions(concentrationof

1000mg/L,providedbySCHARLAB).Twodifferentlinear

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Table1

Literaturecalciumcarbonatesolubilitydataandexperimentalconditions.

Author Year Reference T(◦C) PCO2(bar) CNaCl(moldm−3)

Segnitetal. 1962 1 75–200 40 0

Segnitetal. 1962 1 100 1–60 0

Miller 1952 27 0–105 1–100 0–0.5

Ellis 1963 31 100–300 2–150 0

Milleroetal. 1984 12 25 – 0.1–6

Ponizovskiietal. 1980 41 25 0.02 –

ShterminaandFrolova 1945 43 – – –

ShterminaandFrolova 1957 42 – – –

Kaasaetal. 2005 44 – – –

Nagy 1988 45 – – –

FrearandJohnston 1929 36 25 1 –

Hastingsetal. 1927 46 38 – 0.024–0.2

Yanatéva 1955a 28 0–55 1 –

MacDonaldandNorth 1974 24 1–25 25.3–962.6 –

Weyl 1959 37 10–70 1 –

Mitchell 1923 26 25 1–24 –

LeatherandSen 1909 33 15–40 1 –

Johnston 1915 34 16 1 –

LyashchenkoandChuragulov 1981 25 25 1–1000 –

Kindyakovetal. 1958 22 25–75 1 –

Kendall 1912 21 25–100 1 –

Morey 1962 23 25–350 200 –

Yanatéva 1960 30 70 1 –

Ellis 1959a 29 98–302 1.9–142.3 –

Malinin 1963 32 100–300 2–150 –

SharpandKenedy 1965 38 197–300 20–1000 –

Nakayama 1968 47 25 0.01–1 –

Plummeretal. 1982 39 0.1–90 0.30–0.98 –

Wolfetal. 1989 40 10–60 0.01 0–6

3. Simulationmodel

Thesolubilityequilibriumofcalciumcarbonatewassimulated bymeansofﬂash separator (Fig.1)using ASPENPLUS®_,_which

allowstheeffectofthedifferentparametersintheCaCO3solubility

tobeevaluated.

Theﬂashseparatorisfedbytwostreamscontainingvapourand

liquidphases.Theinletliquidstream(LIQ-IN)containscalcium

car-bonatesaturatedwater(introducedinthesystemasCaCO3solidin

excesspluswater).Theinletvapourstream(V-IN)canbeformed

bycarbondioxideand/orair,andmaybesaturatedwithH2Oat

thesamepressureand temperatureconditionsoftheﬁrstﬂash

separator.

Pressure,temperatureandstreamcompositionsaresetupin

thisﬁrstﬂashseparator.Attheseconditions,thechemicalis

sim-ulatedandthreephasesequilibrium(liquid,gasandsolidCaCO3).

Thefluidoutletfromthefirstseparatorisfedintothesecondflash,

inwhichadifferenttemperatureisﬁxedatthesamepressure

con-ditions.

ThepHoftheoutletliquidstreamfedintothesecondﬂash,is

adjustedmodifyingthemolarﬂowofapureCO2streamuntilthe

experimentalpHvalueisreached.

Thisschemeisrepeatedtoreproduceexperimentalconditions

fromthehighesttemperature95◦Ctothelowest25◦C.Thestreams

atthermodynamicequilibriumthatleavethelastﬂashseparator

areconstitutedbytheCO2 and asmallamountofH2O(vapour

phase)andbyatwophasesstreamthatcontainsallionicspecies

insolution(liquidphase)andtheprecipitatedcalciumcarbonate

(solidphase)ateverypressureandtemperaturevalues.

Thechemical reactionstakeninto accountin thesimulation

processarethefollowing:

CaCO3(s) CO32−+Ca2+ (3)

CaCO3→ CO32−+Ca2+ (4)

H2O OH−+H3O+ (5)

2H2O+CO2HCO3−+H3O+ (6)

H2O+HCO3−CO32−+H3O+ (7)

CaOH+Ca2++OH− (8)

NaCl→ Cl−+Na+ (9)

TheNRTLactivitycoefﬁcientmodelmodiﬁedfor electrolytes

systems(ELEC-NRTL)wasusedforthesimulations.Thecalcium

carbonatesolubilitywascalculatedascalciummolarconcentration

consideringallthecalciumspeciesinaqueoussolutiononcethepH

wasadjustedateachtemperaturestep.

4. Resultsanddiscussion

Initially,theinﬂuenceoftemperatureontheCaCO3 solubility

processwasevaluatedforbothexperimentalandliteraturedata,

comparingtheavailableCaCO3solubilityprovidedbythe

simula-tionmodel.Table2showsthevalueofpHandcalciumcarbonate

solubilityin the differentcases studied,thetemperature range

studiedwasbetween25and95◦Cat1barofpressureanddifferent

concentrationsofNaClfrom0to2.4moldm−3_were_studied._The

thermodynamicmodelwasusedtosimulatetheexperimentaldata

attheconditionsdescribedinSection2(differenttemperaturesand

atmosphericpressure).Fig.2illustratesthiscomparativestudy.As

itisshown,thethermodynamicmodelsimulatesthe

experimen-taldataofthecalciumcarbonatesolubilityinwateraccurately,the

errorsassociatedwiththemeasureofcalciumconcentrationwas

calculatedbydifferentrepetitionsofthesameexperimentinthe

sameconditions,inFig.2areplottingthedifferenterrorsobtained

ineachpointandaswecanseeinthisﬁgurethevaluesof

cal-ciumcarbonatesolubilityobtainedineachpointareveryaccurately

withthethermodynamicmodelused.Fig.3showsthe

compar-isonbetweensimulatedcalciumcarbonatesolubility(expressed

ascalciummolarconcentration)withdatareportedinthe

liter-ature [1,27,31].In thesimulation scheme,thetemperaturewas

variedfrom10to170◦Cataﬁxedpressureof40barofCO2 by

usingELEC-NRTLmodel.Strongtemperaturedependenceforthe

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Fig.1.ASPENsimulationmodelforthesystem.V-IN=inletvapourstream;LIQ-IN=inletcalciumcarbonatesolution;V-OUT=outletequilibriumvapourstream; LIQ-OUT=outletequilibriumliquidstream(calciumcarbonatesolution);CO2-IN=inletCO2supporttoﬁxedpHvalue;LIQ-PH=obtainedcalciumcarbonatesolutionwithpH

adjustedtoexperimentalvalues.

seen,theseresultsarein reasonableagreementwiththose

ref-erencedinliterature.Afteranalyzingbothﬁgures(Figs.2and3)

itisremarkablethatthesolubilityofthissaltdecreaseswiththe

temperature,andtherefore,adecreaseofthisvariablereducesthe

precipitationofcalciumcarbonate,asexpected[14].Theeffectof

pressureintheCaCO3illustratedbythesetwoﬁguresshowsthatif

thepressureincreasesthesolubilityofcalciumcarbonateincreases

too.

Fig.4depictstheinﬂuenceofthepartialpressureofCO2onthe

solubilityprocessofcalciumcarbonate.Thisﬁgurecompares

refer-enceddata[1,27]at100◦Canddifferentpressurestothesimulated

values.Asitisshown,thesimulationoftheprocessvaryingthe

pressureintherangeof1–70barataﬁxedtemperatureof100◦C

yieldsanincreaseofthesolubilitycurveofcalciumcarbonatein

agreementtodatareferencedintheliterature.

The variation in the CaCO3 solubility with pressure can be

explainedbythesimultaneousequilibriumsinvolvedinthis

pro-cess.Asstatedabove,thepresenceCO2isofhighimportanceas

itmodiﬁestheamountofthespeciespresentinaqueoussolution.

Thus,calciumcarbonateisslightlysolubleinpurewater,butthe

solubilityincreaseswhencarbondioxideispresentinthesolution

[3].Therefore,higherpartialCO2 pressuresmaketheamountof

calciuminthesolutionincrease[1].

Fig.5plotsthesimultaneousinﬂuenceofthetemperatureand

thepressureofCO2onthesolubilityofcalciumcarbonatevalues

wereobtainedbysimulation.Themostimportanteffectobservedis

theincreaseoncalciumcarbonatesolubilityintwoordersof

mag-nitudeinpresenceofCO2,asstatedabove.Theeffectoftemperature

oncalciumcarbonatesolubility,seemstoincreaseathigherpartial

pressureofCO2,decreasingastemperatureincreases.

AsitismentionedinSection1,thepresenceofcarbondioxide

inthegasphaseisassociatedinadditiontochangesinpHchanges.

Fig.6showstherelationshipbetweenCaCO3solubilityandpHin

theaqueoussolutioncomparingbothsimulationandliteraturedata

reportedbyNakayama[47]ataﬁxedtemperaturevalueof25◦Cat

aﬁxedpressureof1bar.AqueoussolutionpHisofagreat

impor-tanceaslowervaluesofpH(highPCO2)modiﬁestheamountofthe

speciespresentintheaqueoussolution(increasingtheamountof

Table2

Experimentalcalciumcarbonatesolubilitydataandexperimentalconditionsdeterminedinthiswork.

Experiment T(◦_C) _P_(bar) _C_NaCl_(mol_dm−3₎ _pHa _C

Ca2+b×104(moldm−3)

1 25 1 0.0 8.31 5.26±0.65

2 35 1 0.0 8.34 4.53±0.36

3 45 1 0.0 8.31 3.7±0.43

4 55 1 0.0 8.34 3.34±0.43

5 65 1 0.0 8.35 2.75±0.22

6 75 1 0.0 8.33 2.39±0.27

7 85 1 0.0 8.35 2.27±0.27

8 95 1 0.0 8.34 1.83±0.22

9 25 1 0.6 9.34 9.23±0.37

10 25 1 1.2 9.41 11.01±0.07

11 25 1 1.8 9.67 10.70±0.30

12 25 1 2.4 9.95 10.10±0.46

13 95 1 0.6 8.43 7.92±0.40

14 95 1 1.2 8.54 9.11±0.56

15 95 1 1.8 8.80 8.16±0.33

16 95 1 2.4 9.00 7.40±0.15

a_ı

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100 90 80 70 60 50 40 30 20 0,0 5,0x10-5 1,0x10-4 1,5x10-4 2,0x10-4 2,5x10-4 3,0x10-4 3,5x10-4 4,0x10-4 4,5x10-4 5,0x10-4 5,5x10-4 6,0x10-4

MCa

2+

T(ºC)

Prediction

Experimental data (this work)

Fig.2. InﬂuenceofthetemperatureontheCaCO3solubility(P=1bar).

200 180 160 140 120 100 80 60 40 20 0,0 5,0x10-3 1,0x10-2 1,5x10-2 2,0x10-2 2,5x10-2 3,0x10-2

MCa

2+

T (ºC)

Experimental data [1] Experimental data [27] Prediction

Fig.3. InﬂuenceofthetemperatureontheCaCO3solubility(P=40bar).

100 90 80 70 60 50 40 30 20 10 0 0,0 2,0x10-3 4,0x10-3 6,0x10-3 8,0x10-3 1,0x10-2 1,2x10-2

MCa

2+

P (bar)

Experimental data [1] Experimental data [27] Experimental data [31] Prediction

Fig.4. InﬂuenceofthepressureontheCaCO3solubility(T=100◦C).

HCO3−andCO32−),causingthesolubilityofcalciumcarbonateto

increase.ThesimulateddatausingELEC-NRTLmodelaresimilar

tothetheoreticalvaluesreportedbyNakayama[47].Theseresults

ofcalciumconcentrationareobtainedknowingCO2partial

pres-sureandthermodynamicconstantsinthesameconditionsused

intheASPENsystem.BothresultsemphasizetheinﬂuenceofpH

(asafunctionoftheamountofCO2inthesystem)inthechemical

equilibriainvolvedatthesolubilityprocess.

Increasesofthesolubilityofasaltincreaseswhenothersaltis

presentisamaintopicinordertosimulaterealformationwater

systems,withsigniﬁcantamountsofNa+_and_Cl−_ions._The

inﬂu-enceofthepresenceofNaClintheCaCO3solubilitywasstudiedin

thiswork.Table2showsthevalueofpHandcalciumcarbonate

sol-ubilityatdifferentNaClconcentrations.Fig.7showsthevariation

ofCaCO3solubilityagainstthemolarconcentrationofaddedNaCl

atapressurevalueof1barandthreedifferenttemperatures(T=25

and 95◦C) bothexperimentaland simulatedvalues areplotted.

Asitisshown,thethermodynamicmodelaccuratelyreproduces

experimentaldatawhenpHisﬁttedtoexperimentalvalue.

More-over,analyzingFig.8it canbeseenthat,asexpected,solubility

valuesarehigherforthesimulationsperformedwithaddedNaCl.

AmaximumsolubilityvalueisobtainedforanapproximateNaCl

(6)

7,2 7,0 6,8 6,6 6,4 6,2 6,0 0,0 2,0x10-3 4,0x10-3 6,0x10-3 8,0x10-3 1,0x10-2 1,2x10-2 1,4x10-2

MCa

2+

pH

Prediction

Experimental data [47]

Fig.6.EffectofthepHontheCaCO3solubilityat25◦Cand1barairpressure.

concentrationof1.25moldm−3_in_solution_for_all_the_{temperatures.}

ThatNaClconcentration,atwhichsolubilityreachesthemaximum

value,issimilartotheseawatersalinity.Therefore,itisinteresting

toremarkthattheextractionofthecrudeoilwithhighsalinity

for-mationwaterisprobablycarriedoutatmaximumCaCO3solubility

conditions.Thepresenceofmaximumvaluesforallthe

tempera-turesinFig.7leadstotheconclusionthatnoanincreaseofthe

solubilitycanbe reachedalthoughhigher amounts of NaClare

addedtothesolution.

Additionally,thethermodynamicmodelwasusedtopredictthe

solubilityresultsreportedbyMilleroetal.[12]atdifferentNaCl

concentrationataﬁxedpressureof1barasillustratedinFig.8and

sothecomparisonwithMilleroetal.[12]datacouldonlybemade

todescribetheionicstrengthinﬂuence.Afteranalyzingsimulated

andexperimentalresultsreportedinthis ﬁgure,nomeaningful

differencesamongthesecurvesareshownatlowconcentration

values,until1moldm−3_of_sodium_chloride_{concentration,}_whereas

athighervaluesofionicstrength (NaClconcentration)the

sim-ulationcurveshowsadeviationprobablyduetothedifﬁcultyof

theELEC-NRTLtoreproducehighNaClconcentrations,closetothe

solubilitylimit.

WhenFigs.2,4and8areconsideredtogether,important

dif-ferencesin Ca2+_{concentration} _can_be_observed. _Fig.₂ _shows _a

calciumcarbonatesolubilityvalue around5×10−4_mol_dm−3 _at

25◦C and 1bar of air pressure whereas in Fig. 8 solubility is

2.5 2.0

1.5 1.0

0.5 0.0

0.0 1.0x10-4 2.0x10-4 3.0x10-4 4.0x10-4 5.0x10-4 6.0x10-4 7.0x10-4 8.0x10-4 9.0x10-4 1.0x10-3 1.1x10-3 1.2x10-3

MCa

2+

M_NaCl Prediction

Experimental data T = 25 ºC (This work) Experimental data T = 95 ºC (This work)

Fig.7. InﬂuenceofthepresenceofNaClontheCaCO3solubilityat1barairpressure.

6 5 4 3

2 1 0 1,5x10-4 2,0x10-4 2,5x10-4 3,0x10-4 3,5x10-4 4,0x10-4 4,5x10-4 5,0x10-4 5,5x10-4 6,0x10-4 6,5x10-4

MCa

2+

M_NaCl

Prediction

Experimental data [12]

Fig.8. InﬂuenceofthepresenceofNaClontheCaCO3solubilityat25◦Cand0.97atm

CO2pressure.

around9×10−3_mol_dm−3_at₂₅◦_C_and₁_bar_of_CO₂_pressure_when

MNaCl=0moldm−3. On theother hand,Fig.4 shows a value of

solubilityaround2×10−3_mol_dm−3_at₁₀₀◦_C_and₁_bar_of_CO₂

pres-sure.Thisdifferentmagnitudeorderinsolubilityvaluecannotbe

explainedintermsofpressureortemperaturechanges.A

signiﬁ-cantdifferenceisobtainedbyanalyzingpHvaluesinthesethree

cases.FordataplottedinFig.2,theinvolvedpHwas8.33

deter-minedexperimentallyandreproducedbysimulation.However,for

dataplottedinFigs.4and8,theinvolvedpHwere5.48and6.07,

respectively,bothdeterminedbysimulationfromthe

experimen-talconditionsreportedinliterature.Therefore,thepresenceofCO2

isamainvariablebecauseitmodiﬁessigniﬁcantlythepHofthe

solutionwhichhasastrongeffectonsolubilityvalueinagreement

withthatshowninFig.6.

Asithasbeendemonstratedduringthesimulationstudy,this

thermodynamicmodelmaybeappliedtorealcrudeoilproduction

schemes,beingausefultooltopredictpotentialscalingproblems.

Consequentlythetestedmodelcanbesatisfactorilyconsideredto

describetherealCaCO3solubilitychemistryandbeusedtoevaluate

conditionstoavoidorminimizeoilformationrisk.

5. Conclusions

In the present work,a thermodynamicmodel under ASPEN

PLUSsimulationenvironmentwasconsideredtoevaluateCaCO3

solubilityinwater.ELEC-NRTLmodelandseveralionicequilibria

wereconsidered.Inthisschemeasimplephaseseparatorwasused,

beingthesystempressureiscontrolledbyintroducingavapour

phasestreamwheretheamountofCO2canbemodiﬁed.The

inﬂu-enceof pressure, temperature,NaCl concentrationand pH was

studiedforbothliteratureandexperimentaldata.Theincreaseof

solubilityofCaCO3withthedecreaseoftemperature,aswellasthe

increaseofpressureandincreaseofCO2partialpressure,arewell

describedandinagreementwithexperimentalvalues.Solubility

showsamaximumagainstNaCltestedconcentrationforsalinity

valuessimilartothathighsalinityformationwaters.The

relation-shipbetweenCO2partialpressureandpHisalsodescribed,leading

totheexpecteddecreaseinsolubilityofcalciumcarbonatewith

increasingpH.

Simulationresultsarealsoinreasonableagreementwiththose

reportedinliterature.However,theeffectofNaClispoorly

pre-dictedbythismodelprovidinghighersolubilityvaluesthanthose

reportedbyMilleroetal.[12].However,themainconclusionofthis

workisthesuitablesimulationschemetodescribeandpredictthe

(7)

The application of this model could beused topredict the

amount of calcium carbonate deposited in equilibrium at any

conditions of pressure, temperature, etc. in realistic conditions

during crude oil production, and should be a valuable tool

for ﬂow assurance risk assessment studies to minimize scale

formation.

Acknowledgment

The authors thank REPSOL S.A. for their ﬁnancial support

throughtheresearchproject“Aseguramientodeﬂujodecrudos

depetróleo:Estudiodelacompatibilidaddeaguas”.

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