Leakage assessment and identification of fluid leakage scenarios at CO2 storage sites (Invited)

One of the major objectives of the ECO2 project (EU FP7) is to assess the risks associated with the storage of CO2 below the seabed. Within this frame, work package (WP) 1 investigates the sedimentary cover at currently active and potential storage sites using novel geophysical baseline studies, monitoring and modeling techniques in order to better understand CO2 migration mechanisms and its spatial and temporal evolution. A proper risk assessment of CO2 storage hinges on a thorough understanding of the geological evolution of an area and a sound comprehension of subsurface anomalies associated with the flow of fluids and their governing geological controls. To this end, WP 1 has analyzed a wealth of seafloor imaging and seismic data from the industrial storage sites at Sleipner and Snøhvit on the Norwegian Margin. In addition to conventional seafloor and seismic data, several novel high-resolution acquisition technologies have been used during offshore expeditions in 2011 and 2012, and their data integrated into this study. A solid background on the geological development and the stratigraphic framework has been developed including an interpretation of subsurface structure and structures related to the presence of fluids and possible fluid pathways on the basis of conventional 3D seismic data and by integrating several additional high-resolution data sets. The data revealed a number of fluid-flow features, as for example gas chimneys, pipes, shallow gas accumulations, leaking faults, fractures along the seafloor as well as gas hydrates. Each of these structures or set of structures has been evaluated with respect to their occurrence, distribution, origin and as a means for providing a potential pathway for CO2 if it would leak out of the storage formation. On the basis of this evaluation and the assumptions that paleo fluid-flow structures may be reactivated by CO2 injection and that the caprock of the storage formation may breach, a number of potential leakage scenarios have been formulated for both the Sleipner and Snøhvit CO2 storage sites. The leakage scenarios largely include leakage along a chimney (blow-out structure) or along a fault but are adapted to the specific geological background at each storage site and hence, depending on its exact subsurface location and context may yield a complex migration pathway for CO2 from the storage formation to the seafloor. Initial modeling work shows that the leakage scenarios can be successfully implemented into the simulations of fluid flow. This modeling work is integrated with seismic modeling work on detection thresholds of CO2 in the overburden in order to develop simulation-assisted monitoring strategies for seabed and sub-seabed leakage detection of CO2. Within ECO2, the modeling of leakage scenarios provides important constraints on flux rates at the seafloor interface for associated activities in other work packages.