Sustainable Earth Sciences 2013 30 September - 4 October 2013, Pau, France

P006High-resolution Fracture Characterization of aSiliciclastic Aquifer Targeted for CO2Sequestration, Svalbard, NorwayK. Ogata* (University Centre in Svalbard), K. Senger (Uni CIPR), A.Braathen (University Centre in Svalbard), S. Olaussen (University Centrein Svalbard) & J. Tveranger (Uni CIPR)

SUMMARYThe target siliciclastic aquifer investigated by the Longyearbyen CO2 Lab as a possible test-scale CO2storage unit is a dual-permeability reservoir characterized by fractured, tight lithologies. By integratingborehole and outcrop data, the reservoir section has been subdivided in intervals defined by 5 litho-structural units (LSUs), each one characterized by different lithologies and fracture sets interpreted torepresent pseudo-geomechanical units. Due to their contrasting features, these LSUs are believed to have acrucial influence on subsurface fluid migration. Our results indicate that fractured shale intervals controllateral fluid flow (predominance of low-angle fracture) whereas sandy and coarser intervals seem tocontrol vertical fluid flow (predominance of high-angle fractures), locally enhancing the contribution ofthe matrix porosity. Horizontal and vertical high permeability conduits can be found at the LSUs’interfaces, along the chilled margins of igneous sills and dykes, and along the damage zone of mesoscopicfaults, due to the localized enhanced fracturing (fracture corridors). A large database containing structuraldata on fractures has been acquired and analyzed in order to extrapolate calibrated parameters fornumerical modeling and flow simulations. These in turn allow reservoir volumetric calculations,assessment of seal integrity and forecasting of vertical/lateral connectivity of the reservoir.

Sustainable Earth Sciences 2013 30 September - 4 October 2013, Pau, France

Introduction

The Longyearbyen CO2 Lab is developing an onshore, test-scale (ca. 60.000 CO2 tons/year) site for geologic sequestration of carbon dioxide in a siliciclastic aquifer located at 700-1000 m depth in Spitsbergen, Svalbard (Braathen et al. 2012; Fig. 1). Eight slimline boreholes have been drilled and fully cored at two drill sites. Four of these penetrate the planned tidally influenced, shallow marine storage formations (Fig. 1). The target aquifer conforms to a gentle regional monocline and reaches the surface ca. 15 km NE of the drill site, but sub-hydrostatic pressure recorded at 853 m in the Dh4 borehole (Braathen et al. 2012) shows that the reservoir is compartmentalized by structural and/or stratigraphic seals, although the target formation is vertically sandwiched between two main detachment zones related to the development of the Paleogene, West Svalbard fold-and-thrust belt (see Fig. 1) (Braathen et al. 1999). Tectonic-sedimentary burial down to ca. 4.5 km during the Eocene caused mechanical compaction and quartz cementation, with consequent lowering of the matrix permeability. Drill core plugs permeability measurements show values less than 2 mD, with porosity varying from 6 to 18% (Braathen et al. 2012). The bulk of the present matrix porosity consists of non-connected secondary pores. Effective porosity is likely to be below 10% (Mork, 2012). Despite this, water injection tests show an average flow capacity of 45 mD·m in the lowermost part of the reservoir, thought to be primarily due to the natural fracture network (Ogata et al. 2012). In this contribution we investigate the aquifer section, presenting the results of an integrated study carried out on both borehole datasets and outcrops. The processed data are used as input parameters in the development of a static geological model of the target aquifer (Senger et al., this conference).

Figure 1 A: Simplified geological map of central Spitsbergen with relative cross section and explanation. Locations of the drill sites and the fieldwork area (B) are labeled. B: study area at Deltaneset and approximate path of the composite stratigraphic log presented in Fig. 2. Redrawn and modified after Major et al. (2001). Shapefiles of the geological units from the NPI-Geonet project are used.

Method and data

The characterization of structural discontinuities (Schultz and Fossen, 2008) has been performed using the following standard parameters (Singhal and Gupta, 2010): 1) orientation, 2) mid-point depth, 3) spacing, 4) length/persistence (e.g. bed-confined vs. through-going), 5) linear density/frequency, 6) connectivity, 7) relative aperture, 8) asperity, 9) wall coatings and infillings. Fieldwork was conducted on target reservoir outcrops (Fig. 1). Scanlines were measured to provide

Sustainable Earth Sciences 2013 30 September - 4 October 2013, Pau, France

fracture frequency plots along individual intervals and stratigraphic logging (1:50 scale) was carried out in order to correlate outcrop and borehole data. Various stratigraphic intervals within a range of different lithologies have been analyzed in order to capture potential stratigraphic variations in the fracture character. A total of 105 scanlines were collected, totaling about 1.400 m with 7.672 individual fractures measurements. In addition, fracture logging of the reservoir cores has been performed on the Dh4 Dh5R and Dh7A reservoir sections (Fig. 2).

Sustainable Earth Sciences 2013 30 September - 4 October 2013, Pau, France

Figure 2 Stratigraphic and structural correlation of the reservoir section between the drill sites and the fieldwork area. Interpreted LSUs columns and vertical fracture frequencies for the boreholes are indicated. A: pie-diagrams showing the relative volumetric amount of LSUs in the Dh 4 borehole and outcrops. B: rose diagrams of the fracture sets orientations (strike) for each LSU, subdivided between the reservoir and the caprock section. C: Fracture frequencies on unit thickness (log scale) for each LSU. Nonetheless the high data dispersion and the consequent poor fitting of the lines, it is possible to point out the apparent opposite trends for the LSU A and D.

Results and discussion

Based on our integration of stratigraphic and structural observations from boreholes and outcrop (Fig. 2), the reservoir was subdivided into 5 different litho-structural units (LSUs): A) massive to laminated shaly intervals, B) massive to thin-bedded, heterogeneous, mixed silty-shaly intervals, C) massive to laminated, medium to thick-bedded, fine to coarse-grained sandstones and conglomerates, D) igneous intrusions (i.e. dolerite sills) and E) carbonate beds (i.e. limestones, bioclastites, etc.). Further complexity is added to the LSU framework by the presence of fracture corridors related to mesoscopic (sub-seismic) normal faults and the chilled/sheared margins of the dolerite dyke intrusions. A summary of the collected database and extracted statistics is shown in Fig. 2. Each LSU is characterized by distinct systematic and non-systematic fracture sets, sedimentary facies, bed thicknesses and degree of cementation. The defined LSUs can be recognized both in the boreholes and outcrops (Fig. 2a), and reflect contrasting rheological/mechanical behavior. As such they are inferred to represent proxies of geomechanical units (Shackleton et al. 2005). Lateral and vertical changes in fracture set orientations and fracture frequencies are observed within the LSUs, especially at their boundaries. At larger scale, a striking contrast in fracture orientation between the reservoir and the cap rock interval can be recognized (Fig. 2b), suggesting a marked mechanical decoupling between the two at the level of the upper detachment. When investigating the relationships between thicknesses and fracture frequency of individual LSUs’ an apparent direct correlation for the LSUs B, C and E, and a possible inverse relationship for the LSUs A and D is revealed (Fig. 2c).

Figure 3 Conceptual diagram of the fracture network interconnectivity and the possible fluid flow pathways across the different LSUs (see text for details).

Sustainable Earth Sciences 2013 30 September - 4 October 2013, Pau, France

Conclusions

The observed moderate injectivity of the tight, naturally fractured reservoir shows that carbon dioxide may potentially be injected into it and stored. Fracture sets have been identified in both drill cores and outcrop data, and five major litho-structural units have been identified in the investigated section. Influencing lateral and vertical fluid migration, such fracture associations represent the key factors in controlling and forecasting the internal connectivity of the reservoir and the direction of the fluid flow (Fig. 3). The large amount of available data calibrated through different methods, coupled with the possibility of detailed studies on the reservoir and cap rock directly in the field, allow the compilation of an extensive database of reliable parameters for geology-based reservoir modeling, along with a comprehensive characterization of the potential storage framework (Senger et al., this conference).

Acknowledgements

This work is part of the “Geological input to Carbon Storage (GeC)” project funded by the CLIMIT program of the Research Council of Norway, with Kim Senger’s fieldwork supported by Arctic Field Grants from the Svalbard Science Forum. Andreas Rittersbacher, Dave Richey, Laura Farrell and Marie Marušková are thanked for assistance in the field. The GeC project team works in close co-operation with the UNIS CO2 Lab (http://co2-ccs.unis.no) and the SUCCESS center. Shapefiles from NPI-Geonet have been used to construct the geological maps.

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