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Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 4, pp. 523-784

Copyright © 2015, IFP Energies nouvelles

523 > Editorial – Characterization of European CO2 Storage – European Project SiteCharÉditorial – Caractérisation de sites européens de stockage de CO2 – Projeteuropéen SiteCharF. Kalaydjian

531 > SiteChar – Methodology for a Fit-for-Purpose Assessment of CO2 Storage Sites inEuropeSiteChar – Une méthodologie pour une caractérisation appropriée des sites de stockage de CO2F. Delprat-Jannaud, J. Pearce, M. Akhurst, C.M. Nielsen, F. Neele, A. Lothe, V. Volpi, S. Brunsting and O. Vincké

555 > CO2 Storage Feasibility: A Workflow for Site CharacterisationFaisabilité du stockage géologique du CO2 : une méthodologie pour lacaractérisation des sites de stockageM. Nepveu, F. Neele, F. Delprat-Jannaud, M. Akhurst, O. Vincké, V. Volpi, A. Lothe, S. Brunsting, J. Pearce, A. Battani, A. Baroni, B. Garcia, C. Hofstee and J. Wollenweber

567 > Risk Assessment-Led Characterisation of the SiteChar UK North Sea Site for theGeological Storage of CO2Caractérisation d’un site de stockage géologique de CO2 situé en Mer du Nord(Royaume-Uni) sur la base d’une analyse de risquesM. Akhurst, S.D. Hannis, M.F. Quinn, J.-Q. Shi, M. Koenen, F. Delprat-Jannaud, J.-C. Lecomte, D. Bossie-Codreanu, S. Nagy, Ł. Klimkowski, D. Gei, M. Pluymaekers and D. Long

587 > How to Characterize a Potential Site for CO2 Storage with Sparse Data Coverage – a Danish Onshore Site CaseComment caractériser un site potentiel pour le stockage de CO2 avec unecouverture de données éparses – le cas d’un site côtier danoisC.M. Nielsen, P. Frykman and F. Dalhoff

599 > Coupled Hydro-Mechanical Simulations of CO2 Storage Supported by PressureManagement Demonstrate Synergy Benefits from Simultaneous Formation FluidExtractionDes simulations du comportement hydromécanique d’un réservoir géologique de stockage de CO2 dans un contexte de gestion de la pression démontrent lesavantages de l’extraction de fluide de la formation au cours de l’injection du CO2T. Kempka, C.M. Nielsen, P. Frykman, J.-Q. Shi, G. Bacci and F. Dalhoff

615 > The Importance of Baseline Surveys of Near-Surface Gas Geochemistry for CCSMonitoring, as Shown from Onshore Case Studies in Northern and Southern EuropeImportance des lignes de base pour le suivi géochimique des gaz près de la surfacepour le stockage géologique du CO2, illustration sur des pilotes situés à terre enEurope du Nord et du SudS.E. Beaubien, L. Ruggiero, A. Annunziatellis, S. Bigi, G. Ciotoli, P. Deiana,S. Graziani, S. Lombardi and M.C. Tartarello

635 > Structural and Parametric Models of the Załęcze and Żuchlów Gas Field Region,Fore-sSudetic Monocline, Poland – An Example of a General Static ModelingWorkflow in Mature Petroleum Areas for CCS, EGR or EOR PurposesModèles structurels et paramétriques de la région des gisements de gaz de Załęcze et Żuchlów, Région monoclinale des Sudètes, Pologne – un exemple du déroulement d’une modélisation statique générale dans des zones de pétrole matures dans un but de CCS, EGR ou d’EORB. Papiernik, B. Doligez and Ł. Klimkowski

655 > Numerical Simulations of Enhanced Gas Recovery at the Załęcze Gas Field inPoland Confirm High CO2 Storage Capacity and Mechanical IntegrityDes simulations numériques de récupération assistée de gaz sur un gisement degaz de Załęcze en Pologne confirment les capacités de stockage de CO2 élevéeset l’intégrité mécanique dudit gisementŁ. Klimkowski, S. Nagy, B. Papiernik, B. Orlic and T. Kempka

681 > Pore to Core Scale Simulation of the Mass Transfer with Mineral Reaction in Porous MediaModélisation des phénomènes de transferts de masse dans les milieux poreuxsoumis à une réaction de surface : de l’échelle du pore à l’échelle de la carotteS. Bekri, S. Renard and F. Delprat-Jannaud

695 > Evaluation and Characterization of a Potential CO2 Storage Site in the SouthAdriatic OffshoreÉvaluation et caractérisation d’un site de stockage potentiel de CO2 au sud de la Mer AdriatiqueV. Volpi, E. Forlin, A. Baroni, A. Estublier, F. Donda, D. Civile, M. Caffau, S. Kuczynsky, O. Vincké and F. Delprat-Jannaud

713 > Southern Adriatic Sea as a Potential Area for CO2 Geological StorageLe sud de l’Adriatique, un secteur potentiel pour le stockage du CO2V. Volpi, F. Forlin, F. Donda, D. Civile, L. Facchin, S. Sauli, B. Merson, K. Sinza-Mendieta and A. Shams

729> Dynamic Fluid Flow and Geomechanical Coupling to Assess the CO2 StorageIntegrity in Faulted StructuresCouplage des modélisations hydrodynamique et géomécanique pour évaluer l’intégrité d’un stockage de CO2 dans des structures failléesA. Baroni, A. Estublier, O. Vincké, F. Delprat-Jannaud and J.-F. Nauroy

753> Techno-Economic Assessment of Four CO2 Storage SitesÉvaluation technico-économique de quatre sites de stockage de CO2J.-F. Gruson, S. Serbutoviez, F. Delprat-Jannaud, M. Akhurst, C. Nielsen, F. Dalhoff,P. Bergmo, C. Bos, V. Volpi and S. Iacobellis

767> CCS Acceptability: Social Site Characterization and Advancing Awareness at Prospective Storage Sites in Poland and ScotlandAcceptabilité du CCS : caractérisation sociétale du site et sensibilisation du publicautour de sites de stockage potentiels en Pologne et en ÉcosseS. Brunsting, J. Mastop, M. Kaiser, R. Zimmer,S. Shackley, L. Mabon and R. Howell

DOSSIER Edited by/Sous la direction de : F. Delprat-Jannaud

Characterization of European CO2 Storage — European Project SiteCharCaractérisation de sites européens de stockage de CO2 — Projet européen SiteChar

D o s s i e rCharacterization of European CO2 Storage – European Project SiteChar

Caractérisation de sites européens de stockage de CO2 – Projet européen SiteChar

How to Characterize a Potential Site for CO2 Storage

with Sparse Data Coverage – a Danish Onshore

Site Case

Carsten Møller Nielsen1*, Peter Frykman

1and Finn Dalhoff

2

1 Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen - Denmark2 COWI A/S, Parallelvej 2, 2800 Kongens Lyngby - Denmark

e-mail: cmn@geus.dk

* Corresponding author

Abstract — The paper demonstrates how a potential site for CO2 storage can be evaluated up to asufficient level of characterization for compiling a storage permit application, even if the site is onlysparsely explored. The focus of the paper is on a risk driven characterization procedure. In theinitial state of a site characterization process with sparse data coverage, the regional geological andstratigraphic understanding of the area of interest can help strengthen a first model construction forpredictive modeling. Static and dynamic modeling in combination with a comprehensive riskassessment can guide the different elements needed to be evaluated for fulfilling a permitapplication. Several essential parameters must be evaluated; the storage capacity for the site mustbe acceptable for the project life of the operation, the trap configuration must be efficient to securelong term containment, the injectivity must be sufficient to secure a longstanding stable operationand finally a satisfactory and operational measuring strategy must be designed. The characterizationprocedure is demonstrated for a deep onshore aquifer in the northern part of Denmark, the Vedstedsite. The site is an anticlinal structural closure in an Upper Triassic – Lower Jurassic sandstoneformation at 1 800-1 900 m depth.

Résumé — Comment caractériser un site potentiel pour le stockage de CO2 avec une couverturede données éparses – le cas d’un site côtier danois — L’article montre comment évaluer un sitepotentiel pour le stockage de CO2 à un niveau de caractérisation suffisant pour une demande depermis de stockage, même si le site n’est que partiellement exploré. Il met l’accent sur une procédurede caractérisation basée sur l’analyse de risques.Dans l’étape initiale d’un processus de caractérisation de site sur la base de données éparses, lacompréhension géologique et stratigraphique régionale de la zone d’intérêt aide à renforcer laconstruction d’un premier modèle de sous-sol en vue d’une modélisation prédictive.Une modélisation statique et dynamique, combinée à une évaluation des risques globale permet alorsd’appréhender les différents éléments requis pour l’évaluation d’une demande de permis. Plusieursparamètres essentiels doivent être évalués : la capacité de stockage du site doit être en adéquationavec la durée de vie du projet, le système de piégeage du CO2 doit être suffisamment efficace pourpermettre un confinement long terme, l’injectivité doit être suffisante pour garantir unfonctionnement stable et durable et enfin une stratégie de mesure satisfaisante et opérationnelle doitêtre prévue. La procédure de caractérisation est démontrée pour un aquifère salin côtier profond

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 4, pp. 587-598� C.M. Nielsen et al., published by IFP Energies nouvelles, 2015DOI: 10.2516/ogst/2015008

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

localisé dans la partie nord du Danemark, sur le site de Vedsted. Le site présente une fermeturestructurelle anticlinale dans une formation de grès du Trias Supérieur-Jurassique inférieur située à1 800-1 900 m de profondeur.

INTRODUCTION

The present paper comprises the outline for producing a stor-age permit application for geological storage of CO2 in theVedsted structure, an anticlinal closure in the GassumFormation, onshore in the northern part of Denmark(Fig. 1). The permit application procedure is developed inthe SiteChar project funded by the EU 7th frameworkprogram. It is assumed that any exploration obligations aresuccessfully fulfilled for the license.

Even though the site is sparsely exploited and with mini-mal data coverage the reservoir characterization methodol-ogy can provide a sound base for a storage application to

be submitted to the competent authority. The methodologycan re-assure the authority that a planned way forward existsfor further reservoir characterization, when new data areacquired, that can be agreed upon for both operator andregulator.

A risk driven characterization procedure ensures that themost crucial elements of the planned storage operation areidentified and thoroughly evaluated during the process lead-ing up to the final application. The risk driven proceduresecures that a safe and secure storage operation for the siteis properly evaluated and described.

The Vedsted site is an onshore aquifer in sandstones ofUpper Triassic – Lower Jurassic age at 1 800-1 900 m depth,situated in the northern part of Denmark. The target reservoiris situated in a small graben bounded by northwest-southeasttrending faults and is part of a larger graben structure, theTriassic rift system forming the deep Fjerritslev Trough(Fig. 2). The reservoir is marine to fluviatile sandstonessealed by a thick package of marine claystones of theJurassic Fjerritslev Formation (Nielsen, 2003).

The storage concept is a four way dip closure for the res-ervoir (Larsen et al., 2003). The bounding faults for the gra-ben structure are outside the delineation of the lowest closingcontour. Further it is assessed that no major faults will con-strain the plume development inside the structure. Minorfaults exist on the structure but the fault properties are diffi-cult to evaluate.

Hydrocarbon exploration campaigns during the 1950’sdiscovered the closure, and a single exploration well wasdrilled on the structure, the Vedsted-1 well. The well wasdry and abandoned shortly after with only a sparse logsuite acquired. The sparse well data in combinationwith only a few 2D seismic lines challenge the sitecharacterization.

The closure includes several sandstone reservoirs of goodquality at depths of 1 200-2 000 m. Several hundred metersthick claystone intervals provide an excellent cap-rockabove the reservoirs. The Gassum Formation forms the pri-mary reservoir whereas the more shallow Haldager Sand-stone Formation can be used as a secondary reservoir(Fig. 3). Additionally a several hundred meters thick chalksection provides a secondary seal.

The storage capacity of the Vedsted structure was prelim-inarily estimated to approximately 160 Mt of CO2 based on areview of the existing data and preliminary reservoirsimulations (Dalhoff et al., 2011). In order to verify the clo-sure of the anticline structure, the existence and location of

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Top Triassic depth map covering an area of 160 km 9 160 kmin the northern part of Denmark. The purple square (17 km 9

20 km) in the centre delineates the Vedsted site model. Theinjection well is marked with the red pipe down flank to the eastof the structure.

588 Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 4

the bounding faults and the storage capacity a new 2D seis-mic survey was acquired in 2008.

A 400 MW coal-fired power plant situated 30 kilome-tres away from the site is the main CO2 source.The annual CO2 emission is approximately 1.8 Mt andthe CO2 can be transported by pipeline from the powerplant to the storage site (Klinkby, 2011). If the injectivityand capacity of the site prove to be as high as previouslypredicted, the CO2 stream can be increased by approxi-mately 1 Mt/year from a close by cement industry. A pro-ject lifetime of 40 years of constant injection is assumed.The number of wells and the well configuration must beoptimized together with pressure control management.Active pressure control might be necessary by waterproduction as studied by Kempka et al. (2015).

Due to the sparse data coverage for the Vedsted structure,it seems reasonable to plan for a phased development of thestorage operation so early reservoir response data can beused to history match the model and update the predictivereservoir modeling. For the full potential for the storage pro-ject incorporating both the power plant and the cementindustry, i.e. an injection of approximately 3 Mt/year,

it may be practical to have three or more injections wells.With three or more injection wells the injection rate can bekept below 1 Mt/year/well, which is assessed to be a realisticinjection rate due to the relatively low permeability sand-stone formation in order to keep the flowing bottom holepressure below a fracture safety limit of 85% of the litho-static pressure (Springer, 2009).

The Swedish energy company Vattenfall organized a ser-ies of studies to evaluate the potential for using the Vedstedstructure for geological CO2 storage in the period 2007 anduntil the company abandoned the project in 2011 due tocompany strategy re-organization. Many of the studies areused in the present work in combination with new studies.

1 METHODOLOGY

An essential finding from the SiteChar project, is to use arisk driven site characterization methodology sufficient toproduce a storage permit application. For the onshoreVedsted site a comprehensive risk assessment based on aninitial reservoir modeling study guided the nature of

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Map showing the structural elements of the Norwegian-Danish basin. Modified from Nielsen (2003).

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the studies undertaken to produce an adequate reservoircharacterization.

1.1 Risk Assessment

An early stage risk assessment workshop for the Vedsted sitewas performed in 2008 organised and facilitated by DNV.The outcome from the workshop was analysed and rankedby DNV and the project group (DNV, 2008). Risks wereorganized in risk matrices before and after any safeguardswere applied. A separate risk assessment workshop wasdedicated to the Vedsted-1 legacy well (DNV, 2010).

Four major risk areas were identified from a risk registerof 22 individual risks: a need for more detailed characteriza-tion of the Gassum reservoir, risk of leakage at the

abandoned legacy well, need for a better understanding offault properties and risk of pressure propagation outsidethe storage complex. The storage complex for the Vedstedsite was defined following the guidelines in the EU storagedirective (EC, 2009, 2011) and is illustrated in Figure 4.

The safe containment of the CO2 plume within the storagecomplex is well defined for a structural closure whereas theextension of pressure propagation can be more problematicalto delineate and impacts on the areal extent of the monitoringobligation. For the Vedsted site no direct neighbouring sub-surface activity is planned for and the pressure can beallowed to increase outside the storage complex, but thelevel of pressure footprint outside the complex is not definedand has to be discussed with the national competentauthority.

A number of studies were undertaken as result of the riskassessment: 3D static and dynamic modeling studies, cou-pled dynamic and geomechanical modeling, mitigation ofany unwanted pressure developments from the injectionoperation by introducing water production wells, assessmentof the state of the Vedsted-1 legacy well on the structure andfinally monitoring strategy evaluation combined withcomprehensive baseline studies.

Part of the study work could be performed in paralleli.e. the state of the legacy well could be assessed as a more

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Lithostratigraphic column for the Vested-1 well. In yellow, thetarget reservoirs; the Gassum Formation and the HaldagerSandstone Formation. In light brown, the sealing claystonesand in greenish colours marine sandstone to mudstone withlow sealing capacity, in blue, low permeability limestone withgood sealing capacity.

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Principal sketch of the Vedsted site. Primary containment: Gas-sum Fm. with the Fjerritslev Fm. acting as seal. Secondary con-tainment: Haldager Sandstone Fm. Three cement plugs areplaced in the P&Awell (Vedsted-1 well). The green and yellowdotted squares define the storage complex; the larger green stor-age complex definition must be used if the state of the faultscannot be determined and if the secondary containment alsois planned for storage and the state of the first plug in theP&A well is questionable. The red dotted square defines themonitoring area, i.e. following the definition in the EC Guid-ance Document (EC, 2011).

590 Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 4

or less independent study only dependent on some predictivemodeling of pressure and plume extension in to the wellarea. Whereas the modeling work was more sequential andto some part also dependent on an iterative procedure,i.e. statical, dynamical and geomechanical modeling aredependent and may be improved by critical re-modelingwith respect to peer reviews of modeling results.

1.2 Assessment of the Vedsted-1 Legacy Well

The Vedsted-1 well was drilled and plugged and abandonedin 1958, when no detailed requirements for proper abandon-ment was described in the Danish Subsoil Act. The final wellreport does not document a safe abandonment with respectto present day regulations; although three cement plugs wereplaced in the well bore (Fig. 4). No description of the amountand quality of the cement is documented and no pressure testwas conducted.

An assessment study of the state of the Vedsted-1 well per-formed in the SiteChar project recommended to re-enter thewell and to plug and abandon it according to the present dayregulations. Three sets of cement plugs must be set acrossthe Gassum and Haldager formations, and from the base ofthe chalk to the surface. It could be 5-6 individual plugs insteadof the three sets of plugs as both the top and base of eachreservoir interval should be cemented at least 50 m aboveand below each boundary to fulfil Danish regulation.

It is crucial to convey to the appropriate authorities toassure that the well is not a show stopper if correct measuresare taken.

1.3 Static Model

Static modeling is challenged by the sparse data coverage forthe storage site area and regional geological studies are usedto support the site modeling.

The Vedsted structure was first identified as apossible CO2 storage site in 2003 (Larsen et al., 2003).The structure was investigated by two old oil explorationwells (Haldager-1, 1950 and Vedsted-1, 1958), old regionalseismic lines (1967 and 1983) and a new 2D seismic surveyacquired in 2008 further strengthened the model construc-tion (Frykman et al., 2009, 2011).

From the seismic interpretation five horizons wereexported to the Petrel software for static modeling(Schlumberger, 2011): Base Chalk Group, Top HaldagerSandstone Fm., Base Haldager Sandstone Fm., Near TopGassum Fm. and near Top Skagerrak Fm. (Fig. 5). In thepresent study, the focus is on the primary reservoir, i.e. onlyCO2 injection in to the Gassum Formation, the other forma-tions are included in the model to secure proper boundaryconditions for pressure and geomechanical modeling.

The entire Gassum Formation is penetrated by theVedsted-1 well drilled centrally on the Vedsted structure.In this area, the mainly coarse clastic Gassum Formation isinterfingering with the mudstones of the Fjerritslev Forma-tion (Fig. 6). The Gassum Formation consists of a lower unit,140 m thick which consists of fluvial sandstones interbeddedwith lacustrine mudstones in the lower part, whereas shallowmarine sandstones interbedded with thin marine mudstonestake over in the middle and upper parts of the unit. Thislower reservoir unit is overlain by a tongue, 75 m thick ofmarine mudstones belonging to the Fjerritslev Formation.Within the lower part of the mudstones occurs a 2-3 m thickmarine sandstone bed. The mudstones are topped by 5 m ofmarine sandstones included in the Gassum Formation.Further up section follows 19 m of Fjerritslev Formationmudstones overlain by 50 m of marine shoreface sandstones,which forms the upper reservoir unit of the Gassum

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Seismic section east/west across northern part of Vedsted Struc-ture. Colour code: dark green: Base Chalk Group; light blue:Top Frederikshavn Formation; yellow: Top Haldager Sand For-mation; light green: Base Haldager Sand Formation (MCU);purple: Intra Fjerritslev F-III Member (sequence boundarySB15 defined by Nielsen, 2003); blue: Near Top Gassum For-mation; red: Near Top Skagerrak Formation; orange: Intra Tri-assic marker. Note: minor extensional faults cut Base Chalk,but terminate in Lower Cretaceous. From Andersen (2009).

C.M. Nielsen et al. / How to Characterize a Potential Site for CO2 Storage with Sparse Data Coverage – a DanishOnshore Site Case

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Formation (Nielsen and Japsen, 1991). This reservoir unit isinterpreted to gradually thin and change to mudstonestoward the deeper parts of the basin (e.g. Hyllebjerg-1).The two thick mudstone tongues splitting the Gassum For-mation into the two principal parts in the Vedsted-1 wellare absent in the Børglum-1 and Flyvbjerg-1 wells, wheresandstones of the Gassum Formation occupy this strati-graphic level (Fig. 6). The mudstones are thus expected togradually disappear toward the northeast due to lateral facieschanges from mudstones to sandstones. It is thus likely thatthe upper Gassum reservoir unit thickens in the eastern partof the Vedsted structure and thins in the western part.

The lower principal part of the Gassum reservoir is over-lain by a relatively uniform and 75 m thick succession ofmarine mudstones. The upper principal reservoir unit isoverlain by 525 m of relatively uniform marine mudstones.Log-correlations supported by detailed biostratigraphy indi-cate that the mudstones are laterally uniform and widely dis-tributed (Michelsen, 1989; Nielsen, 2003). Both seals arethus expected to be laterally well-developed with a goodintegrity and sealing capacity. As discussed above itis expected that the lower seal is gradually replaced by

sandstones toward the northeast. The sealing capacity ofthe lower seal may thus be slightly reduced in the eastern-most part of the structure. On the contrary, the upper sealis interpreted to be very well-developed in the entire struc-ture and further beyond as the marine flooding that causedthe change in deposition from marine sandstones to mud-stones in the Early Jurassic influenced the entire basin andthe Fennoscandian Border Zone (Nielsen, 2003) (Fig. 7).

Vertical variations in reservoir porosity are interpreted fromthe Vedsted-1 log and upscaled to average values to populatethe model grid. For the reservoir section (Gassum Fm.)an average layer thickness of approximate 2 m was used andthe log porosity values averaged arithmetically. The initialmodels are constructed by extrapolating the upscaled porosityvalues along the layers. Permeability values are calculatedfrom a porosity-permeability relation determined from coreanalysis. To differentiate non-reservoir from reservoir rock(lithology subdivision in to sand and shale) a cut-off valuefor porosity of 0.05 was used.

Even though a regional geological description exists, asimple “layer-cake” approach with averaged values for res-ervoir properties in each layer was adopted, since a lack of

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SW-NEwell-log panel across the Danish Basin, the Sorgenfrei-Tornquist Zone and the Skagerrak-Kattegat Platform showing the upper part of thelower Gassum reservoir and the upper part of the Gassum Reservoir. Modified from Nielsen (2003).

592 Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 4

high resolution data made it difficult to derive any meaning-ful lateral property distribution. It is assessed that anapproach with low and high case scenarios in the dynamicmodeling is the way forward for the present cases withsparse data coverage.

Models with different areal extent and grid resolutionwere constructed for different modeling purposes. A regio-nal model covering an area of 160 km 9 160 km (Fig. 1),a site specific model of 17 km 9 20 km for modeling thedevelopment and migration of the CO2 plume (Fig. 1) andan intermediate model size of 50 km 9 50 km for coupleddynamical and geomechanical modeling (Fig. 8) were cre-ated. The large regional model was used to guide the setupof boundary conditions for both the site specific model andthe geomechanical model so the pressure level is modeledcorrectly.

1.4 Dynamic Model

The static models were exported to the Eclipse 100 black oilreservoir simulator (Schlumberger, 2007) for dynamic mod-eling. Details for modeling the CO2 injection process for theVedsted site can be found in Frykman et al. (2009, 2011),gridding, saturation functions, PVT and fluid descriptionand boundary conditions.

For an onshore structural closure like the Vedsted site, theobjectives for setting up dynamic modeling are, besides thedynamic capacity estimation, to predict the pressure devel-opment, both around faults and the regional pressure foot-print outside the closure. Several essential issues must beevaluated: first, the construction of a regional model largeenough to capture the full overpressure development andwith a sufficiently detailed level to be as realistic as possible

but still computationally manageable. Secondly, for moredetailed studies on injection and filling strategies, with lim-ited size site models, boundary conditions must be managedwith proper and correct procedures to capture the pressuredevelopment. Finally, the dynamic simulation must be cou-pled to geomechanical simulations in order to assess thestresses in the reservoir and overburden and possible faultre-activations.

For all the dynamic modeling, the injection well wasplaced down flank and to the east of the structure (Fig. 1).The well location was chosen to obtain a large amount ofresidual trapping when the CO2 migrates to the top of thestructure and leaving a residual CO2 tail behind and to obtaina distance to the Vedsted-1 legacy well.

Someminor faults interpreted from seismicmay exist on theGassum level of the structure that could impact on the CO2

plume distribution (Fig. 5, 9), but at present no knowledgesof fault properties are available. The wedge shape part of thesouthern flank of the structure, delineated by Faults 3 and 4(Fig. 9) could be a challenge to exploit. Dynamic simulationsconfirm that injection of 1Mt/y of CO2 for a 40 years injectionperiod does not fill the structure to spill point.

The pressure development at the fault near the top of theanticline is of major concern for a potential CO2 leakage asCO2 will be present there after around 8 years of simulatedtime (Fig. 10). A maximum difference of about 18 bar canbe observed after 10 years of injection in the closed fault sce-nario, while a difference of about 2 bar occurs for the openfault scenario (Fig. 11). Consequently, Fault 4 has to betaken especially into account with regard to potential reacti-vation resulting from the significant pressure difference after10 years of injection. This may induce geomechanicaleffects impacting fault integrity, and hence has to beadressed by coupling of a geomechanical simulator to thedynamic flow simulator.

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Model for coupled dynamical and geomechanical modeling.A 50 km9 50 kmmodel with over- and underburden enclosingthe Gassum reservoir.

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A regional SW-NE geosection through the Danish Basin andthe Fennoscandian Border Zone. From Nielsen (2003).

C.M. Nielsen et al. / How to Characterize a Potential Site for CO2 Storage with Sparse Data Coverage – a DanishOnshore Site Case

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1.5 Coupled Dynamical and Geomechanical Modeling

Dynamic simulations have been coupled to geomechanicalsimulations in order to assess the stresses in the reservoirand overburden and possible fault re-activations.The 50 km9 50 km model (Fig. 8) is used and the five faultsindicated in Figure 9 are implemented in to the model.

It is unclear how the faults intersect the overburden andthereby the caprock. Additional four major faults aremapped on the Base Chalk level meaning that they intersect

the caprock from the top reservoir to the Base Chalk, butthey are outside the structural closure for the storage site.

The fault properties are not fully understood so in the sim-ulations they are treated as being either fully transmissive orfully closed. Only the five faults mapped on the top Gassumlevel are used for the coupled geomechanical and flowmodeling.

The TOUGH2-MP/ECO2N software was used to assessthe pressure elevation in the reservoir and CO2 plume migra-tion towards the top of the anticline. Two different scenarioswere addressed; the five faults were set to be either open orclosed for flow.

Detailed description together with results of the coupleddynamic and geomechanical modeling can be found inKempka et al. (2015). It is found that no shear and tensilefailures will occur under the injection operation and demon-strated that pressure control management can be achieved byactive water production. It was assumed that the major faultswere non-active at the initial stress regime and found that nore-activation of the faults occurs.

1.6 Pressure Management

For a CO2 injection operation any pressure propagationaway from the closure of the CO2 plume may be a potentialproblem if e.g. close to potential leakage pathways or neigh-bouring subsurface activities. It is not clear from the EC stor-age directive (EC, 2009) how the overpressure can developoutside the storage complex. This raises the question, onhow large does the storage complex need to be and howlarge an area do monitoring activities have to cover.The EC CCS guidance document (EC, 2011) addresses theissue with statements on competitions between subsurfaceoperations but returns no absolute values. The pressuredevelopment issue then becomes an issue that must beaddressed in dialog with the national regulator.

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Gaseous CO2 plume migration in the open fault a) and closedfault b) scenarios for simulation times of 10 years.

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Pressure development in the open fault a) and closed faultscenarios b) for simulation times of 10 years.

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Two fault systems mapped from seismics; solid lines indicatefaults mapped on the Base Chalk level and dotted lines indicatefaults mapped on the Top Gassum level all shown on the topGassum map contours for clarity. Faults on the Top Gassumlevel are used for the geomechanical study.

594 Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 4

The regional pressure distribution will develop muchfaster and wider than the distribution of the CO2 plume(Birkholzer et al., 2009). Figure 12 shows the simulatedpressure development during an injection period of 40 yearsin to the Vedsted site. Here the 160 km 9 160 km regionalmodel is used for simulations. The CO2 plume is capturedwithin the structural closure but the pressure footprint coversan area of almost the size of the regional model.

Figure 12 also shows the importance of choosing propermodeling boundary conditions when using detailed sitemodels for simulation. The model pressure developmenton the site delineation must be captured and reflect the truenature of the physical site delineation, i.e. open aquifer,semi-open aquifer etc. It is evident that constant pressureboundary conditions for a site model in an aquifer cannotcapture the pressure increase on the site boundary(Fig. 12). In the present application, when using detailed sitesimulations, the pore volume multiplication option was usedas boundary conditions, and the value of the multipliers wasadjusted through the simulation scheme so the “correct”pressure development is simulated, i.e. the multipliers arevaried with simulation time (Nielsen et al., 2013).

Pressure management through pressure release wellscould be an option to mitigate undesirable over-pressuredevelopments (Fig. 13). If the release wells are placed out-side the structural closure pressure development can be mit-igated without introducing a potential leakage by drilling

inside the trap. Pressure is released through a balanced waterproduction (Nielsen et al., 2013).

1.7 Baseline Survey and Monitoring Plan

Base line survey data are crucial to justify natural and sea-sonal variation in background fluxes of soil gases in relevantland use types and to capture any irregularities in back-ground fluxes not being a result of the CO2 storage opera-tion. A comprehensive base line study was performed inthe SiteChar project for the Vedsted site. The study isreported in Beaubien et al. (2015).

1.7.1 Monitoring Plan

In the CO2GeoNet project, an early but rather detailed mon-itoring plan for the Vedsted site has been produced. Further-more a comprehensive description of several differentmonitoring techniques applicable for the Vedsted site is dis-cussed (CO2GeoNet, 2010; Arts et al., 2010).

From the risk and performance assessment a number ofrisk-related issues have been identified which determinethe specific monitoring systems to be deployed in order tomeet the regulatory requirements. Additional reservoir mod-eling and well studies conducted in the SiteChar projecthelped to focus a list of specific aims for a monitoring plan

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Over-pressure (water potential) distribution after 40 years ofconstant CO2 injection with a single injection well in to theNE flank of the Vedsted structure. Insert picture displays thepressure profile to different times of injection. The orange ver-tical lines delineate the storage site.

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Pressure management by water production. Different totalvoidage replacements from four water production wells.The orange vertical lines delineate the storage site. A voidagereplacement of 100% corresponds to an equal amount of waterproduced and CO2 injected in reservoir volumes.

C.M. Nielsen et al. / How to Characterize a Potential Site for CO2 Storage with Sparse Data Coverage – a DanishOnshore Site Case

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for the Vedsted site: 4 for the deep subsurface and 3 for themore shallow or surface part.

For the deep subsurface part spreading of the CO2 plume,spreading across spill point(s), fault integrity and caprocksealing behaviour were found to be essential for a monitor-ing plan.

For the more shallow and surface part the CO2 (natural)fluxes together with the quality and movement of the ground-water are important elements to document to a regulator.

Several monitoring techniques can be applied to measurethe specific aims and at different time schedules during thelife-time of the project. Some of the monitoring techniquesare run before CO2 injection starts to provide proper baselinemeasurements and some continue after injection has ceased.It is anticipated that the injection period runs for 40 years.The full monitoring plan is illustrated in Figure 14.

Vedsted will primarily be regulated under the EC StorageDirective implemented in to the Danish Subsoil Act.The Directive requires that monitoring is carried outthroughout the lifetime of a project to verify that storage per-formance is as predicted, to detect leakage of CO2 and toensure that the storage operations do not lead to adverseenvironmental or safety impacts.

The monitoring plan should be designed to meet the reg-ulatory requirements of the Vedsted site assuming that site

behaviour is in conformance with predictions and no signif-icant irregularities develop.

Also verification of site performance, which involvescomparing predicted and observed behaviour with respectto selected site performance measures should be picked upby the monitoring scheme. The aim is to demonstrate thatsite processes become better-understood and increasinglypredictable with time, as more monitoring data becomeavailable through the operational phase. The key site perfor-mance parameters at Vedsted are migration of the CO2

plume and development of the pressure field in the reservoir.

CONCLUSIONS

The use of a risk driven reservoir characterization methodol-ogy is presented for the onshore Vedsted CO2 storage site.It is demonstrated that even though the site may be sparselyexploited, the reservoir characterization methodology canprovide a sound base for a storage application to submit tothe competent authority and to be processed to a storage per-mit. For a sparsely explored site, the use of all availableregional geological description and interpretation can helpstrengthen the reservoir characterization and guide the firstpredictive modeling. A compilation of previous studies in

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Figure 14

Proposed monitoring plan for the Vedsted site including both the pre-injection phase and part of the post-injection phase.

596 Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 4

combination with new reservoir modeling, well interpreta-tion and monitoring technique studies undertaken in theSiteChar project comprises an adequate reservoir character-ization and technical description of the storage project forsubmitting an application for geological storage of CO2 toa competent national authority. For the characterizationmethodology, it was found that some of the specific studiesmust be run in a sequential scheme, but with emphasis onintegration of expert disciplines and iterative modeling pro-cedures. For (onshore) structural closures, the pressuredevelopments outside the storage complex are crucial andmust be modeled as realistically as possible. Pressure prop-agation in the surroundings must be assessed by properregional modeling and correct handling of simulationboundary conditions. By coupled dynamical and geome-chanical modeling the use of water production to mitigateany undesired pressure development can be evaluated.The coupled modeling also shows that there is no risk offault re-activation during the injection period. To assessthe reservoir quality and inter-reservoir connectivity of thereservoir units, a 3D seismic survey with high resolutioncan help strengthen the interpretation of the reservoirarchitecture in combination with well tests.

ACKNOWLEDGMENTS

The research leading to the SiteChar results has receivedfunding from the European Union Seventh FrameworkProgramme (FP7/2007-2013) under grant agreementNo. 256705. Additional funding comes from industry part-ners (ENEL, PGNiG, STATOIL, Vattenfall, Veolia Environn-ement) and the Scottish Government. Vattenfall is furtheracknowledged for making previous study results for theVedsted structure available for the SiteChar project.

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Manuscript submitted in July 2014

Manuscript accepted in March 2015

Published online in June 2015

Cite this article as: C.M. Nielsen, P. Frykman and F. Dalhoff (2015). How to Characterize a Potential Site for CO2 Storagewith Sparse Data Coverage – a Danish Onshore Site Case, Oil Gas Sci. Technol 70, 4, 587-598.

598 Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 4