Induced seismicity within geological carbon sequestration projects: Maximum earthquake magnitude and leakage potential from undetected faults

Abstract

With developing countries strongly relying on fossil fuels for energy generation, geological carbon sequestration (GCS) is seen as a candidate for large reductions in CO2 emissions during the next several decades. GCS does, however, raise some safety concerns. Specifically, it has been associated with induced seismicity, as a result of pressure buildup arising from prolonged CO2 injection in GCS projects. This seismicity is a delicate issue for two main reasons. First, over a short time scale, deformation of rock could release seismic energy, potentially affecting surface structures or simply alarming the population, with negative consequences for the social acceptance of this kind of projects. Second, over a longer time scale, activated faults may provide preferential paths for CO2 leakage out of reservoirs. While known major faults intersecting target aquifers can be identified and avoided during site screening, the same might not be true for faults that are not resolvable by geophysical surveys. In this study, we use geological observations and seismological theories to estimate the maximum magnitude of a seismic event that could be generated by a fault of limited dimensions. We then compare our estimate with results of geomechanical simulations that consider faults with different hydrodynamic and geomechanical characteristics. The coupled simulations confirm the notion that the tendency of faults to be reactivated by the pressure buildup is linked with the in situ stress field and its orientation relative to the fault. Small, active (critically stressed) faults are capable of generating sufficiently large events that could be felt on the surface, although they may not be the source of large earthquakes. Active, relatively permeable faults may be detrimental concerning the effectiveness of a storage project, meaning that they could be preferential pathway for upward CO2 leakage, although minor faults may not intersect both CO2 reservoirs and shallower potable aquifers.