The impact of reduction of doublet well spacing on the Net Present Value and the life time of fluvial Hot Sedimentary Aquifer doublets

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Delft University of Technology

The impact of reduction of doublet well spacing on the Net Present Value and the life time

of fluvial Hot Sedimentary Aquifer doublets

Willems, Cees; Nick, H.M.; Goense, Twan; Bruhn, David

DOI

10.1016/j.geothermics.2017.02.008

Publication date

2017

Document Version

Final published version

Published in

Geothermics

Citation (APA)

Willems, C., Nick, H. M., Goense, T., & Bruhn, D. (2017). The impact of reduction of doublet well spacing on

the Net Present Value and the life time of fluvial Hot Sedimentary Aquifer doublets. Geothermics, 68, 54-66.

https://doi.org/10.1016/j.geothermics.2017.02.008

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ContentslistsavailableatScienceDirect

Geothermics

j ou rn a l h o m e pa g e :w w w . e l s e v i e r . c o m / l o c a t e / g e o t h e r m i c s

The

impact

of

reduction

of

doublet

well

spacing

on

the

Net

Present

Value

and

the

life

time

of

ﬂuvial

Hot

Sedimentary

Aquifer

doublets

C.J.L.

Willems

a,∗

_,

_H.M.

_Nick

a,b

_,

_T.

_Goense

a

_,

_D.F.

_Bruhn

a,c

a_Department_of_Geoscience_and_Engineering,_Delft_University_of_Technology,_Delft,_Netherlands

b_The_Danish_Hydrocarbon_Research_and_Technology_Centre,_Technical_University_of_Denmark,_Copenhagen,_Denmark c_Helmholtz_Centre_Potsdam_–_GFZ_German_Research_Centre_for_Geosciences,_Germany

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received8November2016

Receivedinrevisedform24January2017 Accepted28February2017 Keywords: Directuse Districtheating Lowenthalpy Fluvialsandstone Geothermaldoublet NieuwerkerkFormation

a

b

s

t

r

a

c

t

Thispaperevaluatestheimpactofreductionofdoubletwellspacing,belowthecurrentWestNetherlands Basinstandardof1000–1500m,ontheNetPresentValue(NPV)andthelifetimeoffluvialHot Sedimen-taryAquifer(HSA)doublets.First,asensitivityanalysisisusedtoshowthepossibleadvantageofsuch reductionontheNPV.TheparametervaluerangesarederivedfromWestNetherlandsBasinHSAdoublet examples.Theresultsindicatethatareductionofwellspacingfrom1400to1000mcouldalreadyimprove NPVbyupto15%.ThiseffectwouldbelargerinmoremarginallyeconomicHSAdoubletscomparedtothe WestNetherlandsBasinbasecasescenario.Thepossibilitytoreducewellspacingissupportedbyfinite elementproductionsimulations,utilizingdetailedfaciesarchitecturemodels.Furthermore,ourresults underlinethenecessityofdetailedfaciesarchitecturemodelstoassessthepotentialandrisksofHSA doublets.Thisfactorsignificantlyaffectsdoubletlifetimeandnetenergyproductionofthedoublet.

1. Introduction

Large potentialresources of heat are stored in sedimentary rocks.IntheNetherlandsalone,theDutchgeologicalsurvey esti-matedthetotalrecoverableheatfromthistypeofresourcetobe approximately55timeslargerthantheannualheatconsumption

(Kramersetal.,2012; CBS).HotSedimentaryAquifers(HSA)are

especiallysuitablefor‘directuse’orheatproduction,becausethey areoftenfoundin areaswithaveragethermal gradient(Boxem

etal.,2011;Pluymaekersetal.,2012).Intheseareastemperatures

forcommercialelectricityproduction(e.g.,Shengjunetal.,2011) arefound atdepths where pore space is generallydiminished. However,heatingaccountsforhalfofthetotalenergy consump-tionin,forexample,theEuropeanUnion(EuropeanCommission, 2016).Therefore,HSAshouldbeconsideredasimportantenergy resources.Unfortunately,alargegapcurrentlyexistsbetweenHSA potentialandexploitation.Thisgapisaresultofacombinationof highinitialinvestmentcostsandlargeuncertaintiesinboth dou-bletlifetime and capacity.The heightoftheinitialinvestment ismainlyinﬂuencedbyacombinationofhighdrillingcostsand

∗ Correspondingauthor.

E-mailaddresses:c.j.l.willems@tudelft.nl(C.J.L.Willems),h.m.nick@tudelft.nl

(H.M.Nick),t.goense@student.tudelft.nl(T.Goense),d.f.bruhn@tudelft.nl

(D.F.Bruhn).

thereplacement of existing fossilfuel basedheating networks. ThesetwofactorsdecreasethecompetitivenessofHSA exploita-tionwithotherenergysourcesand therebylimit itsgrowth.In theNetherlands,forexample,twonewgeothermalprojectsare realisedinHSAeachyear,whilemorethan100explorationlicences havebeengrantedsince2007.Areductionoftheinitialinvestment costswouldreducetherisksfordevelopersandhencestimulatethe growthofHSAexploitation.Inthispaper,weevaluatetheeffectof wellspacingreductionontheNetPresentValue(NPV)ofaHSA doublet.Also,wediscusstheeffectsuchareductionwouldhave onthelifetimeofadoublet.Thecurrentwellspacing standard intheWestNetherlandsBasin(WNB)andParisBasin(Mijnlieff

andVan Wees,2009; Lopezet al.,2010;Mottaghy etal.,2011;

Daniilidisetal.,2016),is1000–1500m.Partially,thelargedistance

isusedtopreventearlycoldwaterbreakthrough.Overdesign, how-ever,couldleadtounnecessarilylongthermalbreakthroughtime. Forexample,nothermalbreakthroughhasyetbeenreportedin thepast40yearsofexploitationintheParisBasin.Awellspacing reductioncouldstillresultin sufficientlifetime,while improv-ingthefinancialsituationofadoubletintwoways.First,itmay reducethedrillingcostsbymakingtheoverallwelllengthshorter, atleastforthecurrentstandarddoubletlayoutwithtwodeviated wellsfromthesamesurfacelocation.Second,itcouldreducethe requiredpumpenergyduetoshorterflowpathsbetweenthewells. Anotheradvantageis thedecreaseof chanceongeologicalflow bafflesbetweenthewells,suchassealedsub-seismicfaults(e.g., http://dx.doi.org/10.1016/j.geothermics.2017.02.008

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Baileyetal.,2002)orpoorsandstonebodyconnectivity(Larueand

Hovadik,2006;PranterandSommer,2011).Finally,moredoublets

couldberealisedinthesameaquiferwhichincreasestheamount ofproducedgeothermalheat(MijnlieffandVanWees,2009).The numberofstudiesonoptimisationofgeothermalsystemsis lim-itedandoftenfocusonmaximizingenergyproduction(e.g.,Sauty

etal.,1980;EkneligodaandMin,2014;Adams,2015).Boththe

pos-siblefinancialadvantageofwellspacingreductionanditsimpact ondoubletlifetimeisevaluatedinthispaper.Inthefirstpart,the effectofavariationinwellspacingontheNetPresentValue(NPV) foratypicalWNBdoubletisdetermined.Thisisderivedfroma sen-sitivityanalysisinwhichbothproductionandfinancialparameters arevaried.ParameterrangesarederivedfromaWestNetherlands Basincasestudy.Theanalysisisbasedonfinite-element produc-tionsimulationsinhopermeableandimpermeablefaciesbodies intheaquifercouldsignificantlyaffectdoubletlifetimeand capac-ity(e.g.,Pranteretal.,2007;HammandLopez,2012;Poulsenetal.,

2015;Crooijmansetal.,2016).Homogeneousaquifermodelsdonot

capturethisuncertainty.Inourstudy,detailedﬂuvialfacies archi-tecturerealisationsaregeneratedutilizingaprocess-basedfacies modellingapproach(Cojanetal.,2004;Grappeetal.,2012;Hamm

andLopez,2012)basedonaWNBgeologicaldataset.Minimum

wellspacingisanalysedintermsoflifetimeandNPV.Theresults ofthisstudycouldbeusedasanincentiveforre-evaluationofHSA wellspacingstandards.Utilizingdetailedfaciesarchitecture mod-els,moreprofoundestimatesofdoubletlifetimeandcapacitycan bemade.Thisshouldpreventoverdesignandtherebyimprovethe competitivenessofHSAexploitation.

2. Dataandaquifermodelling

Theaquifermodelsinthis paperwerebased onageological datasetoftheﬂuvialLowerCretaceousNieuwerkerkFormationin theWestNetherlandsBasin(DeVaultandJeremiah,2002;Jeremiah

et al., 2010; Donselaar et al., 2015). This dataset was chosen,

becausemostoftheapproximately40explorationlicencestarget thisHSAintervalintheWNB.Parametervalues,whichwereusedin aquifermodellingandproductionsimulations,werederivedfrom thisdataset.Furthermore,productionratesandreinjection tem-peratureswerederivedfromWNBdoubletexamples.Byusingthis dataseta realisticrange ofheterogeneities wasderived to con-straintthesetoffaciesrealisations.Twotypesofaquifermodels werecreated:(1)detailedfaciesarchitecturerealisationsand(2) homogeneousmodels.Theﬁrsttypeofrealisationsweregenerated withaprocess-basedapproach.Thefollowingsectionsdescribethe geologicaldataandthemodellingapproach.

2.1. Geologicaldataset

AsubsurfacedatasetoftheﬂuvialNieuwerkerkFormationin theWNB formedthebasis for the geologicalmodelling in this study.Thesamedatasetandmodellingapproachasdescribedin

Crooijmansetal.(2016)andWillemsetal.(2017)wasused.The

datasetcomprisedofcoresandGamma-ray(GR)logsof geother-malwellsIntheWNB.Thecorestudyprovidedthicknessranges offaciesbodieswhichwereusedasinputparametersto gener-ateprocess-basedfaciesrealisations.Inapproximately75mofcore inMKP-11and25minQ13-09,fivedifferenttypesoffacies bod-ieswererecognized:floodplainfines,crevassesplays,single-storey channelbodiesandamalgamatedsandstonecomplexes.The maxi-mumfiningupwardsequencethatwasrecognizedinthecoreswas 4m(Fig.1).Thereforeitwasassumedthatthepaleobank-fullflow depthofthefluvialsystemthatformedtheNieuwerkerk Forma-tionwas4m.Basedonflowdepth,thepaleobank-fullflowwidth wasestimatedat40m(Williams,1986)(Fig.1).Furthermore,cores

providedporosity-permeabilityrelationsfortheaquiferproperty modelling.ThegammaraylogswereusedtoderiveN/G ranges oftheNieuwerkerkFormation.GRlogsintheWNBshowedthat theNieuwerkerkFormationsigniﬁcantlyvariesinthicknessand N/G.Thethicknessvariesfrom50toalmost200mandtheN/Gthe approximately15–70%indifferentsectionsoftheaquifer(Fig.1). 2.2. Process-baseddetailedfaciesarchitecturerealisations

Togeneratethedetailedfaciesarchitecturerealisations,a simi-larapproachasinCrooijmansetal.(2016)andWillemsetal.(2017)

wasused.Inputparametersfortheprocess-basedfaciesmodelling (Fig.2)were(1) channelwidthand depth, (2)maximum over-bankﬂooddepositthickness(Hth),(3)avulsionfrequency,(4)ﬂood

frequency,and(5)floodplaintopographyparameter(henceforth: FT-parameter)(Fig.2).Influvialsystems,thethicknessoffloodplain depositdecreasesawayfromthechannel.Thedistanceatwhich thethicknessdecreasedexponentiallyistheFT-parameter(Fig.2). AhighFT-parametermeansthattheflooddepositiswideandthick, whichincreasesthesedimentaggradationrateanddecreasesthe N/Goftherealisation.Thepaleobank-fullflowdepthwasderived fromthecoreanalysisandanalogues,respectively.Asitwasnot straightforwardderivevaluesoftheotherparameterfromcores suchasthefloodplaindepositthickness(e.g.,Bridge,2006),values rangeswereassumedtocapturetheuncertaintyoftheparameter valuesandtoobtainrealisationsthatrangeinN/G.Floodfrequency, maximumflooddepositthickness(Hth)andtheFTfactorwerethe

primarycontrolsonN/G.Toobtainrealisationswithawiderange ofN/Gvaluesbetween15and70%,overbankﬂoodfrequencywas variedbetween20and120years,Hthbetween0.2and0.6mand

theFT-factorbetween300and600m.Avulsionfrequencyisvaried from600to1600years(TörnqvistandBridge,2002).Duringevery simulatedfluvialflood,sedimentsweredeposited onthe flood-plainwithamaximumthicknessHthnearthechannel(Fig.2).In

thesimulations,sedimentaryprocessesdistributeandshape differ-entfaciesbodiessuchaschannellags,point-bars,crevassesplays, mudplugsandfloodplainfines.Realisationshavedimensionsof 1km×2km×50mandthepaleoflowdirectionisparalleltothe longedgeoftherealisations.Theprocess-basedFlumysoftware methodwasexplainedinmoredetailinCojanetal.(2004),Grappe

etal.(2012)andLopezetal.(2009).

2.3. Heterogeneousaquifermodels

Faciesgrid blocks in the realisations weredivided into two classes,aquiferand non-aquifer.Thenon-aquiferclass included ﬁnegrainedfaciessuchascrevassesplays,overbankalluviumand mudplugs.Thesebodieswereallassumedtoberelatively imper-meable.Theirassumedpermeabilitywas5mDandporosity10%. Sandyfaciesbodiessuchaspoint-barsandchannellagswereall assumedtobeaquifergridblocks.Porosityvalueswereassignedto theseblocksbasedonthecoreplugporositydata.Fromthisdata, abetadistributioncorrelationfunctionwasderived.The distribu-tioncharacteristicsincluding:mean,standarddeviation,skewand kurtosiswereequalto0.28,0.075,0.35and2.3,respectively. Sec-ondly,thepermeabilityofeachgridblockwasdeterminedbya porosity-permeabilityrelationobtainedfrompetrophysicaldataof wellMKP-11(TNO,1977):k=0.0633e29.5_._In_this_equation,_k_is_the

permeability[mD]andistheporosity[–]. 2.4. Homogeneousaquifermodels

FortheNPVsensitivityanalysis,homogeneousaquifermodels werecreatedbasedonthesamegeologicaldataset.Thesemodels hadanaverageporosityof28%andpermeabilitybetween250and 2000mD.ThispermeabilityrangewasderivedfromWNBHSAwell

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Fig.1.Gamma-raylogof1offshorehydrocarbonwellwithcore(well1)andtheassociatedsedimentarylog.Well2–5aregeothermalwells.

tests(Lingen,2014)andcoreplugmeasurements(TNO,1977).The thicknessofthehomogeneousmodelsinthesensitivityanalysis variedbetween50and120m.

3. Numericalproductionsimulations

Intheproductionsimulations,theaquiferwasconﬁnedbetween two50mthickimpermeableover-andunderburdenlayersthat providethermalrecharge.Theenergybalancewassolvedforarigid mediumfullysaturatedwithwater,inwhichthermalequilibrium wasassumedbetweentheﬂuidandsolidphases:

C

∂

tT+wCw

∇

·(qT)−

∇

·(I

∇

T)=0 (1)

Inthisbalance,t(s)istime,T(K)isthetemperature,isthemass density(kg/m3_),_C

w(J/kgK)isthespeciﬁcheatcapacity,(W/mK)

isthethermalconductivity,Itheidentitymatrix,andq(m/s)isthe Darcyvelocityvector.Thesufﬁxwreferstotheporeﬂuidandsto thesolidmatrix.Theheatcapacity,densityandconductivity val-uesforaquifergridblocksare730J/kgK,2650kg/m3_and_2.7_W/mK

respectively.Forthenon-aquiferblocksthesevaluesare950J/kgK, 2600kg/m3_and_2.0_W/mK._The_thermal_conductivity_and_the

vol-umetricheat capacityare described interms of a localvolume average.Heatconductivity,densityandheatcapacityareassumed tobeindependentoftemperatureforsimplicityanddescribedby =(1−␾)s+wandC=(1−)sCs+wCwinwhichisthe

porosity.ThisDarcyﬂowvelocityvectorcanbedeterminedby: q=(kP)/,whereisk(m2₎_the_intrinsic_{permeability,}_the

tem-peratureandsalinitydependentviscosityexplainedinCrooijmans

etal.(2016),andP(Pa)thepressure.Thepressureﬁeldisobtained

throughsolvingthecontinuityequation:(

∂

w)/

∂

t+(wq)=wS,

whereS(s–1₎_is_external_sinks_and_sources._The_detailed_modelling

procedurefollowstheapproach explainedin Saeidet al.(2014,

2015).Theproductionsimulationsyieldaproductiontemperature

developmentovertimeandtherequiredinjectionandproduction pressureforthedeﬁnedproductionrate.Thedifferencebetween thesepressuresforeach timestepi(Pi)wasusedtoestimate

pumpenergylosses(Epump,i)asindicatedinEq.(2)(e.g.,Willems

etal.,2016),whereQ istheconstantproductionrateandεthe pumpefﬁciency.Theproducedenergy(Eprod,i)wasestimatedby

Eq.(3)oneachtimestepi(e.g.,Willemsetal.,2016)inwhichw

isthewaterdensityof1050kg/m3_and_T

ithedifferencebetween

injectionandproductiontemperature.Thenetenergyproduction, orthedoubletcapacitywasdeterminedbythesumoftheproduced energyandthepumpenergylosses.

Epump,i=

QPi

ε (2)

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Fig.2. (1)Process-basedfaciesmodellinginputparametersinacross-sectionofapointbar.Lateralaccretionsurfacesandcoarsechannellagdepositsareindicatedinthe pointbar.(2)Mapviewoffaciesinthedepositionalenvironment.

Table1

EconomicparametersfortheNPVrealisationbasedonVanWeesetal.(2010).

Basecaseeconomicparameters

Heatprice D6.00 D/GJ

Electricitypriceforoperations D22.22 D/GJ

Discountrate 7 % CAPEX Wellcosts D1.5 MD/km Pump D0.50 MD Heatexchanger D0.10 MD Separator D0.10 MD Contingencycosts(10%) D0.89 MD SEI(insurance) D0.69 MD OPEX 5 %ofCAPEX/year

Tax 25.5 %oftaxableincome

Depreciationperiod 10 years Feed-intariff(SDE+)

Baseenergyprice(2015) D0.052 D/kWh Correctionprice(2015) D0.019 D/kWh ContributionSDE+ D9.17 D/GJ

4. NetPresentValuemodel

ANPVmodeldevelopedbyVanWeesetal.(2010)wasutilized torelateproductionsimulations toNPV. InputfortheNPV cal-culationswerenetenergyproductioninWattandtheeconomic parameters listedin Table1. In ourstudy,additional separator costswereincluded,becauseinmanyWNBdoubletsnaturalgas co-productionoccurs.TheNPVisthedepreciated,discounted, net-cumulativeincomeafter15years.Thisperiodwaschosen,because it is themaximumduration of theDutchfeed-in tariff scheme (SDE+)forgeothermalenergy(VanHeekerenandBakema,2013, 2015).0.25MD pumpwork-overcostsweretakenintoaccount everyﬁveyears,whichisequaltohalfoftheestimatedpumpcosts. 40%down-timewasassumedformaintenancethroughouttheyear. Becausedoubletwellsaredrilledfromonesurfacelocation, dou-bletwellspacinginﬂuencesthewelllength(Fig.3).Inthisstudy,

thelengthofa singlewell(WL)wasapproximatedbythesum ofavertical(Dvert)andadeviatedsection(Ddev),WL=Dvert+Ddev.

Theverticalsectionwasassumedtobe1500m.Thelengthofthe deviatedsection,wasapproximatedby:

Ddev=

(TVD−Dvert)2+

₁ 2L

2 (4) InEq.(4),TrueVerticalDepth(TVD)isthetotalwelldepthwhich is2.2kmandListhedoubletwellspacing.Reductionofthewell spacingfrom1000mto800mcouldthereforeresultinareduction ofthetotalwelllengthby2%.Becauseoftheassumedwellcostper kilometerinTable1,drillingcostsreduceaccordingly.For exam-ple,typicallydrillingcostsofa2.2kmdeepgeothermaldoubletis

∼7.2MD(VanWeesetal.,2010;NielsonandGarg,2016).Utilizing

equation4,thereductioninspacingfrom1000to800mcouldlead toareductionofinvestmentofupto0.16MD.Areductionfromthe 1500mstandardto800mwoulddecreasethetotalwelllengthby approximately9%andtheassociatedwellcostsbyapproximately 0.66MD.

5. Analyses

5.1. NPVsensitivityanalysis

Inthesensitivityanalysis,theeffectofa100mand400mwell spacingvariation froma1000mbasecase scenarioontheNPV wasevaluated.Thiswascomparedtotherelativeimpactofother parameters.In thisanalysis,homogeneousaquifermodelswere utilized.Twocategoriesofparameterswerecompared,ﬁnancial andproductionrelatedparameters.Theﬁnancialparameterswere variedby±10%ofthebasecasevalue.Productionrelated param-eterrangeswerederivedfromthegeologicaldataandcurrently activeWNBdoublets.TheparameterrangesarelistedinFig.6.In theanalysis,eachparameterwasvariedindividually,whilekeeping theothersatthebasecasevalue.

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Fig.3.Welllay-outexampleofaconceptualdoubletintheWNB.Theblackdottedlinesindicatehowawellspacingreductioninfluencesthetotaldoubletwelllength.The wellstargetafluvialsandstonefaultaquiferboundedbyfaultsdrilleddeviatedfromonesurfacelocation.Yellowcoloursindicatesandstoneandgreenishandgreycolours indicateimpermeablefloodplainfines.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)

Fig.4. (A)Exampleofafaciesrealisationusedinaproductionsimulationwithfourwellspacingdistances,paralleltothepaleoﬂowdirection.Homogeneousmodelswith adjustedheighttocompensatefortheuncertaintyinnetaquifervolumefor(B)15%and(C)70%N/G.

5.2. Impactofﬂuvialfaciesarchitectureonlifetime

Inthesecondpartofthispaper,theeffectofaquifer architec-tureontheadvantageofreductioninwellspacingwasevaluated.In thispart,productionsimulationswerecarriedoututilizingdetailed ﬂuvialfacies architecturerealisations.In thesetof realisations, N/Gvariedbetween15and70%N/G.Doubletwellswereplaced at400,600,800and1000m(Fig.4A)undertheconditionthatboth wellsintersectthesameamountofsandstonegridblocks,likein

Crooijmansetal.(2016).Doubletwellpairswereplacedparallelto

thepaleoﬂowdirectionasthisreducestherequiredpumpenergy losses(Willemsetal.,2017).Forcomparison,twohomogeneous modelsweregeneratedinwhichthethicknesswasadjustedby15 and70%respectively(Fig.4BandC).Inthisway,bothmodeltypes hadthesamenet-aquifervolumerange.Twolifetimescenarios wereconsidered.IntheﬁrstscenarioA,thelifetimewasreached aftertheproduction temperaturedecreased by10%ofitsinitial

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Fig.5. (A)Basecasescenariocumulativediscountedcashﬂowandnetannualincome.(B)Productiontemperaturedevelopmentfor1000mwellsspacing,100m3_/h_production

rateinthebasecasescenarioandina50mthickaquifer.

Fig.6. TornadoplotsillustratingtheimpactofseveralparametersonNPVinpercentages.Theparameterrangesandtheirbasecase(BC)valuesarepresentedontheright sideofthetornadoplots.

value.InthesecondscenarioB,thelifetimewasreachedwhenthe temperaturedecreasedby1◦C.

6. Results

6.1. Basecasescenario

Thethermalcapacityofourbasecasedoubletis4.1MW(Eqs. (2)and(3)).Theassociateddiscountedcumulativecashﬂowand thenetannualincomearepresentedinFig.5A.Forthebasecase scenario the NPV is approximately 3.7MD and representsthe

cumulative discounted cash ﬂowafter 15 years. Theassociated InternalRateofReturnonInvestments(IRR)would be approxi-mately8years.Thethermalbreakthroughmomenthappensafter morethan60years,evenifaquiferthicknessisreducedto50m (Fig.5B).Afterthisthermalbreakthroughtheproduction temper-atureonlydecreasesbyapproximately1–2◦Cperdecade.

6.2. NPVsensitivityanalysis

TheresultofthesensitivityanalysisispresentedinFig.6.A tor-nadoplotshowsthechangeinNPVinpercentages(NPV)which

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Fig.7.DiscountedcumulativecashﬂowforscenarioA(basecaseparameters),B (L=600,Q=100/h)andC(L=1000,Q=110m3_/h)_in_the_sensitivity_analysis.

isdeﬁnedas:(NPVBC−NPVx)/NPVBC×100%,asaresultofthe

vari-ationofasingleparameter.NPVBCisthebasecaseNPV,andNPVx

istheNPVasaresultofadjustmentofaparameterinthe sensi-tivityanalysis.Theparameterrangesarepresentedontheright sideofthetornadoplot.Thisanalysisindicatesthatvariationofthe temperaturedifferencebetweeninjectionandproduction water (T)hasthemostsigniﬁcantimpactonaHSAdoubletNPV.Aﬁve degreeCelsiusvariationcouldchangetheNPVbyapproximately 50%.A10m3_/h_variation_of_the_production_rate_could_vary_the_NPV

byupto40%.Reducingthewellspacingfrom1400to1000mcould improvetheNPVupto15%.Becausewelllengthisestimatedby equation4,thewelllengthdecreasesby5.5%ifthewellisreduced from1400to1000m.Anadditionalreductionfrom1000to600 changeswelllengthby4.2%.Thereforethetornadoplotshowsa slightasymmetricalresult.Anadditionalwellspacing reduction from1000to600mincreasestheNPVbyapproximately13%.The 100mwellspacingvariation hassmallerimpactontheNPV of approximately4%.Permeability,aquiferthicknessandpump effi-ciencyhave alimitedeffectontheNPV.Becauseoftheinverse relationbetweeninjectionpressureandpermeability,thenegative effectofa750mDpermeabilityreductiononNPV islargerthan thepositiveeffectofa1000mDpermeabilityincrease.Variation ofthepumpenergyefficiencyby±10%didnothaveasignificant impactonNPV(lessthan1%).Otherparametersthathavea sig-nificantimpactonNPV areCAPEXandtheheightofthefeed-in tariffsubsidy.10%variationofdiscountrate,heatpriceandOPEX affecttheNPVbyapproximately10%.Inouranalysis,wellspacing istheonlyindependentparameter,whichisdeterminedpriorto drilling.ProductionrateandTdependongeological uncertain-ties,surfacefacilityefficiencyandonrequiredheatconsumption. Also,allfinancialparametersdependoneconomyorgovernment policy.Therefore,theseresultsalsoindicatetheuncertaintyinNPV asaresultofuncertaintyingeologicalandfinancialcircumstances. Notethattheseresultsarebasedonhomogeneousaquifermodels, theeffectoffaciesandpropertyheterogeneitiesarenotincluded.

TheparametersinthesensitivityanalysisaffecttheNPVofa doubletbecausetheyinfluencethediscounted cumulativecash flow.Thisisbecausetheinitialinvestmentcostsarechangedorthe netincomeis affected.Examplesofdiscountedcumulativecash flowinthreescenarios(A,BandC)fromthesensitivityanalysis arepresentedinFig.7.ScenarioAisthebasecasescenario.In sce-narioBthewellspacingisreducedby400mfrom1000to600m whileallotherparametersarekeptattheirbasecasevalue(Fig.6). Asaresult,theinitialinvestmentscostsarereducedwhilethenet

incomeisconstant.Therefore,thediscountedcumulativecashﬂow curveisshiftedupward,increasingtheNPVafter15years(Fig.7). InscenarioCtheproductionrateisincreasedfrom100to110m3_/h

whileallotherparametersarekeptattheirbasecasevalues.The higherproductionrateincreasedtheslopeofthediscounted cumu-lativecashﬂow,whichalsoincreasesNPV(Fig.7).

ThetheoreticaladvantageofawellspacingreductiononNPV inFig.6,appliestoatypicalWNBHSAdoublet.Toevaluatethe effectofwellspacingvariationonNPVindoubletsinotherbasins orcountries,thesensitivityanalysisisrepeatedfordoubletswith differentdrillingcosts,productionorreinjectiontemperaturesand productionrates.Thesethreeparametersarechosenbecausethey havethemostsigniﬁcantimpactonNPVaccordingtoour analy-sisresultsinFig.6.Inscenario1and2,drillingcostsarevariedby

0.3MD/kmfromtheWNBbasecase(Table1).Inscenario3theT

is5degreeshigherthaninourbasecasescenario.Thiscouldbea resultofahigherproductiontemperatureorhigherheatextraction efﬁciencyandaffectsthenetenergyproduction(Eq.(3)).Incontrast inscenario4theTis5degreelower.Finallytheproductionrateis changedby20m3_/h_in_scenario₅_and_6._In_these_six_scenarios_only

thebasecasevalueoftheindicatedparameterinscenario1–6is variedwhileallothervaluesarekeptatthesamevalueasinFig.6. Foreach scenario,a newNPV valueiscalculated.Subsequently, theeffectofa100mwellspacingvariationfrom1000monthis NPViscalculated.TheresultsarepresentedinFig.8.Theresults indicatethatwellspacingvariationhasgreaterimpactonNPVin areaswithhigherdrillingcosts.Ifthedrillingcostsare0.3MD/km higherthanintheWNB,the10%wellspacingreductionchanges NPVbyapproximately10%whichisapproximately5%more com-paredtotheresultinFig.6.Ifthedoubletcapacityislowerbecause ofthe5degreereductionofT,a10%wellspacingreductioncould increasetheNPVbyupto15%.Wellspacingreductionhasaneven higherimpactonNPVindoubletwithlowerproductionrates.Ina doubletwithan80m3_/h_production_rate,_a_10%_well_spacing

reduc-tionincreasedtheNPVbyapproximately25%.In doubletswith lowerdrillingcosts,higherT,orhigherproductionrates com-paredtheWNBbasecase,theimpactofwellspacingreductionon NPVremainsafewpercentages.Theseresultsimplythatreducing thewellspacingtoimproveNPVismorerelevantformarginally economicdoublets.

6.3. Impactoffluvialfaciesarchitectureondoubletlifetime The resultsof theproduction simulationswith detailed flu-vialfacies architecturerealisationsarecompared tosimulations inhomogeneousaquifermodelsinFig.9.Twolifetimescenarios andtwoproductionratesarecompared.Threeobservationscan bemade.First,800mspacingissufficienttoobtaina15yearslife timeinbothlifetimescenariosandaproductionrateof100m3_/h._If

theproductionrateis150m3_/h,₈₀₀_m_is_only_sufﬁcient_when_T

is7.5◦C.With600mspacingthelifetimeexceeds15yearsonly witha100m3_/h_production_rate_and_with_T_of_7.5◦_C._Secondly,

theseresultsalsoindicateanunderestimationoftheuncertainty inlifetimebythehomogeneousaquifermodels.Therangeoflife timeresultsperwellspacingdistanceislarger,whendetailedfacies architecturerealisationsareused.Thisuncertaintyincreasesfor largerwellspacing distances.In contrast,it is lowerfor higher production rates.The uncertainty in lifetime increases slightly forhigherallowedproductiontemperaturedrop(T).Thirdly,the resultsimplythatlifetimeestimationsarelowerinthe homoge-neousmodels.Thepeaksofthedistributionsof lifetimeofthe detailedfaciesrealisationsareclosertothelifetimeofthe70% homogeneousaquifermodel,especiallyforlargerwellspacings.

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Fig.8.Effectof10%wellspacingreductiononNPVfrom1000to900m,fordifferentbasecasescenarios.Inscenario1–6allotherparametersareequaltothebasecase valuesofFig.6.Tisthedifferencebetweeninjectionandproductiontemperatures.Forcomparison,theeffectof100mwellspacingvariationinthebasecasescenariois presentedontop.

Fig.9.Lifetimefordifferentwellspacinginhomogeneousmodels(blueandblackline)andfaciesrealisations(reddots)utilizinglifetimescenarioofa7.5◦Cproduction temperaturedrop(leftcolumn)ofa1◦Cdrop(rightcolumn).N/Gvariesbetween15and70%inthefaciesrealisations.Theﬁrstrowrelatesto100m3_/h_production_rate,_the

secondrowto150m3_/h_production_rate._The_horizontal_black_dotted_line_indicates_the_minimal_required₁₅_year_life_time_for_the_feed-in_tariff_scheme._The_red_line_indicates

thelifetimedistributionoflifetimeperwellspacingforthefaciesrealisations.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredto thewebversionofthearticle.)

6.4. ImpactofﬂuvialfaciesarchitectureonNPV

TheNPVishigherwhenhomogeneousaquifermodelsareused (Fig.10A).ThiscouldbeexplainedbyFig.10B.Thisﬁgurepresents theratioofproducedheat(Eprod)andpumpenergylosses(Epump)

ineachdoubletscenario.Itshowsthatinthedetailedfacies realisa-tions,pumpenergylossesaresigniﬁcantlyhigherwhichdecreases thenetincome.Fig.10Bshowsthattheﬂuvialfaciesarchitecture

couldsignificantlyinfluencethenetenergyproductionandthereby alsotheNPV.Thisisaresultof therisk offlowpathformation betweenthewells.Thereforenocleartrendisobservedbetween theenergyratioandwellspacinginthefaciesmodels.Incontrast, aslightreductionoftheenergyratioisrecognizedwithincreasing wellspacingforthe70%N/Ghomogeneousmodel.Theenergyratio isapproximately5%lowerfor400mspacingcomparedto1000m spacing.Forthe15%homogeneousmodelthisreductionisnot

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rec-Fig.10. (A)NPVrelatedtowellspacing.NPVofdoubletsinfaciesrealisationsareaveragedperwellspacingdistance(reddottedline).(B)Ratiosofproducedenergyover pumpenergylossesaftertheﬁrstyear,associatedto(A).Reddotsindicatetheenergyratioofadoubletinasinglefaciesrealisations.(C)Pumpenergyandheatproduction developmentovertimefor25%N/Ganda(D)45%N/Grealisationwith1000mwellspacingand100m3_/h_production_rate._Note_that_the_same_scale_on_the_vertical_axes_in_C

andDwhiletheabsolutepumpenergyvaluesvary.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthearticle.)

ognizableinthisfigure.Asimilartrendwouldhavebeenobserved forthedetailedfaciesmodelsifsufficientrealisationswereused. Nevertheless,theeffectofwellspacingonpumpenergylosseswill nothaveasignificantimpactonNPVinthiswellspacerange.Note thattheratiosinFig.10Bapplytoa singlemomentintime as pumpenergyvaluesaretimedependent(Fig.10CandD).Thisis becauseofthetransientpressurestabilisationintheaquiferand becauseofthetemperaturedependentviscosityofthebrineinour simulations.Pumpenergyvariesmoresignificantlyovertimein lowerN/Grealisations(Fig.10C)comparedtohighN/Grealisations (Fig.10D).Heatproductionremainedconstantuntilthermal break-through(Fig.10C).Thevariationinpumpenergylossesandheat productionwere,however,smallandvariedbylessthan0.02MW in20years.

Aconsequenceofreducingthewellspacingisareductionof themaximalpossibleamountofrecoverableheat.Fig.11shows NPVandthemaximumrecoverableenergyinhomogeneousaquifer modelswithaN/Gof15,45and70%assuminglifetimescenario A(Tis7.5◦C),fortwoproductionrates.Thiscomparisonshows thatanincreasingproductionrateincreasesNPVbutdecreasesthe recoverableenergy.Incontrastlargerwellspacingincreasesthe recoverableenergyasitincreaseslifetime.Largerwellspacingis thereforefavourableinlowerN/Gaquifersorindoubletwithhigh productionrates.Theseresultssuggestthatacompromisebetween financialefficiencyandrecoveryefficiencyofadoubletcouldbe obtaineddependingontheexpectedsandstonevolume,required productionrateandrequiredlifetime.Finally,Fig.11confirmsthat reducingwellspacingreductiontoimproveNPVismorerelevant formarginallyeconomicdoubletswithlowproductionrates.Inthe

45%and70%N/Gmodelswith100m3_/h_production_rate,_an

opti-muminNPVismostclearlyrecognized.NopeakintheNPVcurves isobservedwhenthelifetimeistoolowbecauseofhighproduction rateorlowsandstonevolume.

7. Discussion

7.1. WellspacingreductionandtheadvantageonNPV

OurresultsshowapossibleimprovementoftheNPVof approx-imately15%,whenwellspacingwasreducedfrom1400to1000m. Evenasmallreductionof 100mcouldalready improveNPVby upto4%.Thesensitivityanalysisindicatesthatvariation ofT (Tprod−Tinj)andproductionratehaveamoresigniﬁcantimpacton

NPV.However,theseparametersaretoalargeextentdependent on(1)geologicaluncertaintiesand(2)technologicalconstraints. Geologicaluncertainties include aquifer temperatureof several degrees(Bontéetal.,2012)andaquiferproductivity.Alsoall finan-cialparametersareinfluencedbyexternalfactorssuchaseconomy, governmentpolicyandregionalratesofequipmentandtechnology. Wellspacingisoneoftheparametersinouranalysisthatcouldbe designedpriortodrilling.Sofar,wellspacingisprimarilydesigned tocreatesufficientlifetime(e.g.,Motthagyetal.,2011;Daniilidis etal.,2016).Thereforeourstudytakesamorecomprehensivelook attheeffectofwellspacing.Thisbecomesincreasinglyimportant withmorewidespreaddeploymentofgeothermaldoublets.The currentspacingstandardsignificantlylimitsthepossiblenumber ofdoubletsintheWNB(e.g.,MijnlieffandVanWees,2009).Well spacingreductionwouldthereforenotonlyimprovethefinancial

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Fig.11.ComparisonofNPV(blueline)andmaximalrecoverableenergy(redline)in(A)the70%N/Gand(B)15%N/Ghomogeneousmodels.OptimisingNPVbydecreasing wellspacingalsolimitspossiblerecoverableheat.NotthechangeinscaleoftheNPVaxisinthetwocolumns.(Forinterpretationofthereferencestocolourinthisﬁgure legend,thereaderisreferredtothewebversionofthearticle.)

competitivenessofindividualdoubletsbutalsoenhance exploita-tiononaregionalscale.

Furthermore,ourresultsindicatethatwellspacingreduction hasa strongerimpactin doubletswitha lowerproductivityor netenergyproductionrate.ThisisbecausetheNPVresultsfrom a combination of initial investment and the height of the net income.Theseparametersdeterminethestartingpointandslope ofthediscountedcumulativecashﬂow(Fig.5).Optimisationof NPVbyreducingwellspacingisthereforeofparticularinterestfor marginallyeconomicHSAdoublets.

Our results are based on the assumption that the minimal requiredlifetimeis15years.ThisappliestoHSAdoubletsinthe Netherlands,becausethisisthemaximumdurationoftheDutch feed-intariffscheme.Asaresultthewellspacingforanoptimal NPVisthesmallestwelldistancewithaminimalproduction tem-peraturereductioninthisperiodoftime.Thisisofcourserelatedto aspeciﬁcproductionrate.Mostgeothermalsystemshowever,are designedtoproduceforamuchlongertimesuchas30–50years, whichrequiresalargerwellspacing.

TheNPVmodelbyVanWeesetal.(2010)assumesanidealHSA exploitationscenario.ItdoesnotaccuratelyreflectcurrentWNB investmentcostsandNPV.Firstly,thisisaresultofneglectingthe costswhichareassociatedtoyearsofpreparationpriortodrilling andconsultants.Inaddition,oftenunexpectedmaintenanceand workovercostsaddtotheinitialinvestmentsinthefirstyears.HSA exploitationstartedapproximately10yearsagointheWNBand currentlypassesthroughalearningcurve.Therefore,ourestimated investmentcostsapplymoretofutureDutchdoubletsthatareable totakeadvantageoftheexploitationexperienceofthefirstdecade ofHSAexploitation.Furthermore,theNPVisincreasedinthebase casescenarioofthesensitivityanalysis(Fig.5A)becauseofthe rel-ativelyhighporosityandpermeabilityvaluesof28%and1000mD, respectively.In WNBdoubletstheNPV mightbelowerbecause oftheimpactofgeologicalheterogeneityonnetenergy produc-tion(e.g.,Fig.10A).Nevertheless,wellspacingreductionwouldbe especiallyrelevantformoremarginallyeconomicdoubletsaswas showninFig.8.

Inoursimulations,verticalwellsareused.Deviatedwellscould improvetheinjectivityandproductivityincreasingthenetenergy

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productionandimproveNPV.Hence,foroptimisationofdoublet design,thepreferredangleofthedeviatedwellshouldbe deter-minedforaspeciﬁcaquiferasitdependsonthegeologicalmodel, netsandstonevolumeandaquiferthickness.

7.2. Lifetime

Themainreason forthelargecurrentwellspacingstandard of1000–1500mistoavoidearlycoldwaterbreakthrough. How-ever,ourproductionsimulationsindicatethatthisspacingmight beovercautious.Thelifetimeinourproductionsimulationsvaries between60and100yearsinlifetimescenarioA,and15–35years inlifetimescenarioBbothwith150m3_/h_production_rate_and_a

1000mwellspacing(Fig.9).Thisshowsthatdespiteour conserva-tiveaquiferthicknessof50m,stillaconsiderablelifetimecould beexpected.Mostcurrent WNBdoubletsactuallyhave alarger wellspacing.However,wellspacingoptimisationrequiresabetter understandingoftheuncertaintiesinlifetime.Doubletlifetime issigniﬁcantlyaffectedbyfaciesarchitecture(HammandLopez,

2012;Crooijmansetal.,2016).Ourproductionsimulationswith

detailedfaciesarchitecturerealisationsindicatethathomogeneous modelsunderestimatedoubletlifetime.Thisisbecause imperme-ableclaystonebodiesprovidethermalrechargetothecoldwater plumeandflowbafflesincreaseflowpathlengthbetweenthewells. Secondly,homogeneousmodelsoverestimatethenetenergy pro-ductionofthedoublet(Fig.10B)becauseflowbafflesorbarriers coulddecreaseinjectivityorproductivity.Especiallyinlow N/G aquifers,wellsmightintersectdifferentsandstonebodiesthatdo notformflowpathsbetweenthewellswhichincreasestherequired pumpenergylosses.Doubletwellpairswereorientedparallelto thepaleoflow directionandtheorientationofthefluvial sand-stone bodies. Perpendiculardoublet orientation would increase therequiredpumpenergylosses(e.g.,Willemsetal.,2016)and thereforereduceNPV.Inaddition,perpendiculardoublet orienta-tionwouldhaveincreasedlifetimeuncertainty.Thisisaresultof ahigherprobabilitythatimpermeableclaystonebodiesformflow bafflesorbarriers.Flow-barrierscanreducelifetimewhenthey cut-offpermeablepartsoftheaquiferreducingthenetreservoir volumewhiletheflowbafflesincreaselifetimewhentheyincrease flowpathlength(e.g.,Crooijmansetal.,2016).

Ourresultsshowthatastatisticalapproachwithmultiple reali-sationsisrequiredtocaptureuncertaintiesinlifetimeanddoublet capacity.Thelargespreadinlifetime(Fig.9)indicatesthatmore realisationswould berequiredtoaccuratelycapturethis uncer-taintyinlifetimeandcapacityaswereusedinourstudy.Thegoal ofourstudywas,however,toexaminehowfaciesheterogeneities inﬂuencethetheoreticaladvantageofwellspacingreductionon NPV.Ourresultssupportthepossibilitytoreducethewell spac-ing,especiallybecausethenetsandstonevolumeinourmodelsis veryconservative.Forexample,noWNBdoubletonlyencounters a15–20%N/Gaquiferof50mthickness.Ingeneral,WNBaquifers havealargerthicknessandhigherN/Gpercentage.Our conserva-tivesandstonevolumesresultsinlowdoubletcapacities(Fig.10B) andthereforelowerNPVcomparedtothebasecasescenario.In reality,doubletswithverylowinjectivityornetsandstone vol-umewouldnotbetakeninproductionwithoutanymeasures(e.g.,

Blöcheretal.,2015).Examplesofsuchameasuresaretocontinue

drillingintoahigherN/Gintervalortoincreaseheatexchange sur-facebyhydraulicstimulation.Ifwewouldcompensateforthis,life timeandNPV inourresultswouldincrease.Thedetailed facies architecturerealisations in this current studyare simpliﬁed by neglectingsmallscalepermeabilityheterogeneitiesandassuming isotropicaquiferpropertiesingridblocks.Examplesofsuchsmall scaleheterogeneitiesareshaledrapes,accretionsurfacesand bed-dingplanes(e.g.,Pranteretal.,2007).Thesefeaturesdecreaseon averagethepermeabilityperpendiculartothepaleoﬂowdirection.

Fig.12.Productiontemperaturedevelopmentovertimefora600mwellspacing andaproductionrateof100m3_/h,_for_all_facies_realisations_(thin_coloured_lines)_and

thetwohomogeneousmodelsasindicatedinthelegend.(Forinterpretationofthe referencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversion ofthearticle.)

Thiscouldbeaccountedforadjustingthepermeabilityindifferent directionsineachgridblocklikeinBierkensandWeerts(1994). Furthermoreporositywasrandomlydistributedamongstthe sand-stonegridblocks.Inrealitytheporosityofchannellags,point-bars andsandplugsvariesmoresystematicallyacrosssandstonebodies asaresultofsedimentaryprocess(WillisandTang,2010),which couldinﬂuenceﬂowpathformationthroughthesandstonebodies

(LarueandHovadik,2008).Theseheterogeneitiescouldfurther

dif-fusethecoldwaterplume.Notethatinoursimulationsﬂowonly occursthroughtherockmatrixandnofracturesarepresent(e.g.

Hardeboletal.,2015;Bisdometal.,2016).Fractures,especially

par-alleltotheﬂowbetweenthedoubletscoulddecreasethethermal breakthroughtimeofthecoldwaterplume.

Finally,itcouldbearguedthatnotonlythesandstonevolumes inoursimulationswereconservative,alsotheassumptionsonthe minimalproductiontemperaturecouldbeconsideredassuch.The minimumrequiredproductiontemperatureforeconomic Dutch HSAexploitationforgreenhouseheatingisassumedtobe45◦C and65◦Cfordistrictheatingpurposes(Pluymaekersetal.,2012). Theseminimaltemperaturesarelowerthanthe67.5◦Cinourlife timescenarioAandmuchlowerthanthe74◦CofscenarioB. Fur-thermore,duetolikelytechnologicalimprovementsoninsulation andtechnologicalefﬁciencyin thenextdecades,lower produc-tiontemperatureswillmostlikelybesufﬁcientinthefuture.Our simulationsshowthatproductiontemperaturereducesby0.5–1◦C peryear,dependingonthenetsandstonevolumeandproduction rate.Thespeedofproductiontemperaturereductioninour simula-tionsisrelativelylowcomparedtopreviousstudies(e.g.,Mijnlieff

and Van Wees, 2009) becausethermal rechargefrom over and

underburdenistakenintoaccount(Poulsenetal.,2015).InFig.12

theproductiontemperaturedevelopmentover timein homoge-neousaquifermodelsanddetailedaquiferarchitecturemodelsare compared asanexample. Doubletsin thesesimulationshave a 600mwellspacingand100m3_/h_production_rate._In _the_lowest

N/Gaquiferrealisation,theproductiontemperaturedropsbelow 60◦Cafter60years.InthehighestN/Grealisationthisproduction temperatureisreachedafterapproximately140years.Therefore,

Fig.12underlinesconservativenatureofourminimaltemperature assumptions.

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8. Conclusion

Thispaperpresentsamodellingbasedstudyontheadvantages andimpactsofawellspacingreductioninHSAdoublets.Firstly,our resultsindicatethatareductionof400mwellspacingcouldlead toaNPVimprovementofupto15%.Theimpactofwellspacing reductiononNPVismoresignificantinmarginallyeconomic dou-blets.Secondly,productionsimulationswithdetailedfaciesaquifer architecturederivedfromWNBgeologicaldataareusedto eval-uatetheimpactofsmallerwellspacingonlifetime.Ourresults suggestthatsufficientlifetimecouldbeobtained,ifthewell spac-ingis reduced belowthecurrent 1000–1500mstandard. These current standards aim toavoid early cold water breakthrough. However,ourresultsshowpotentialoverdesignofthesestandards. Thisresultsinthenegativeeffectofdecreasingthefinancial com-petitivenessofgeothermalexploitationandreducingthepossible numberofdoubletsinaregion.Finally,thecomparisonof produc-tionsimulationswithdetailedfaciesarchitecturerealisationsand homogeneousmodelsshowsthathomogeneousmodels underesti-matetheuncertaintyinlifetimeanddoubletcapacity.Theaquifer heterogeneitieshaveasignificanteffectontheflowpathformation betweenthewells.Therefore,astatisticalapproachwithmultiple realisationisrequiredtoaccuratelyassessHSApotentialandto optimiseexploitationefficiency.

Acknowledgements

WethankIsabelleCojanandherteam(MinesParisTech,Paris, France)forthepermissiontousetheFlumyprocess-basedfacies modelling software. Harmen Mijnlieff (TNO Geological Survey oftheNetherlands)is thankedfor thediscussionsand support. GeothermaloperatorsintheNetherlandsarethankedforsharing theirgeologicalandproductiondata.Wekindlythankthe consor-tiumofshare-andstakeholdersoftheDelftGeothermalProject (DAP)fortheirsupport.WethanktheEditorandtheanonymous reviewerfortheirconstructivereviewsandfeedbackthat signiﬁ-cantlyimprovedtheinitialmanuscript.

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