Strain localization in sandstone and its implications for CO2 storage

Geological storage of CO2 is a key technical solution to the climate-energy challenge, but it has a number of technological constraints (Baines and Worden 2004; Halland et al., 2011), broadly under the themes of assuring adequate storage capacity and long-term storage integrity. A suitable CO2-storage reservoir should consist of rock formations with sufficient porosity, permeability and connectivity in order to provide an adequate storage volume. The role of faults and their associated deformation structures (such as deformation bands and fractures) in controlling both storage capacity and long-term storage integrity is thus a key factor in achieving globally significant CO2 storage (Figure 1). Although some sedimentary basins on the Norwegian continental shelf already harbour operational CO2- injection and storage projects such as Sleipner (Zweigel et al., 2004) and Snohvit (Hansen et al., 2013), our understanding of reservoir fluid communication due to compartmentalization is far from complete and will be important for further use of the offshore basins for CO2 storage. In addition to the inherited structural features, elevated injection pressures may cause hydraulic fractures or stimulate fault reactivation which both point to the need to characterize the geomechanical response of the rock system to CO2 injection (Rutqvist, 2012; Iding and Ringrose, 2010). In the present work, we investigate the effects of faults and their related structures on the geomechanical and petrophysical properties of sandstone reservoirs. Important components of fault systems include fractures and deformation bands in the damage zone and fault core (Caine et al., 1996; Shipton and Cowie, 2003; Fossen et al., 2007). Fault systems may enhance or suppress fluid communication, which in turn may affect the storage capacity and conductivity of the candidate reservoirs (Figure 1). As a case study, a reservoir model of the Tubaen Formation at the Snohvit CO2 injection site in the Barents Sea (Grude et al., 2013; Hansen et al., 2013) was investigated using 4D seismic data and fault attribute analysis. The characteristics of deformation structures (e.g. sub-seismic faults, deformation bands and fractures) were investigated by field studies of outcrop analogues and by triaxial laboratory experiments to provide a basis for numerical modelling. Fault architecture within reactivated fault systems was studied by the use of analogue modelling. Key questions addressed in the work include: a) Where and when might strain localize in the reservoir? b) How does rock strain influence fluid communication? c) How might structural architecture affect CO2 storage effectiveness?