High-resolution fracture characterization of a siliciclastic aquifer targeted for CO2 sequestration, Svalbard, Norway

Abstract

The target siliciclastic aquifer investigated by the Longyearbyen CO2 Lab as a possible test-scale CO2 storage unit is a dual-permeability reservoir characterized by fractured, tight lithologies. By integrating borehole and outcrop data, the reservoir section has been subdivided in intervals defined by 5 lithostructural units (LSUs), each one characterized by different lithologies and fracture sets interpreted to represent pseudo-geomechanical units. Due to their contrasting features, these LSUs are believed to have a crucial influence on subsurface fluid migration. Our results indicate that fractured shale intervals control lateral fluid flow (predominance of low-angle fracture) whereas sandy and coarser intervals seem to control vertical fluid flow (predominance of high-angle fractures), locally enhancing the contribution of the 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 mesoscopic faults, due to the localized enhanced fracturing (fracture corridors). A large database containing structural data on fractures has been acquired and analyzed in order to extrapolate calibrated parameters for numerical modeling and flow simulations. These in turn allow reservoir volumetric calculations, assessment of seal integrity and forecasting of vertical/lateral connectivity of the reservoir.