Modeling caprock bending stresses and their potential for induced seismicity during CO2 injection

Recent experiences with large-scale injection of fluids into geological formations within the Oil & Gas, Geothermal and Waste Disposal industries have demonstrated a risk of induced seismicity. In the case of geological sequestration of CO2, reactivation of faults may result in leakage pathways for the buoyant plume and thus compromise the integrity of seal formations. In this study, we investigate the potential for an overpressured reservoir formation to cause deformation and mechanical failure in an overlying, low-permeability caprock, thereby compromising seal integrity. In particular, we show that uplift and associated extensional strain in the caprock lead to a reduction in the minimum horizontal principal stress that reinforces the ambient extensional tectonic stress. Changes in the Coulomb failure stress (ΔCFS) characterize the tendency for fault failure. We use normalized and ΔCFS-weighted frequency distributions as an integrated measure of the 3-D distribution of ΔCFS. These measures quantify the magnitude and nature of the risk of induced seismicity. Using the example of the Springerville-St. Johns CO2 reservoir as an analogue site, we explore the sensitivity of the induced seismic risk to caprock stiffness, reservoir overpressure and well configuration. Over a range of these parameters, we calculate the geomechanical response of a large reservoir over a ten-year period of injection. The magnitude of induced stresses within the caprock is approximately 1-2 MPa for typical overpressures of 5-10 MPa, even in regions where the low-permeability caprock prevents appreciable increases in pore pressure. These stresses would be sufficient to cause reactivation of an undetected, well-oriented, critically stressed structure present above or near the injection location. Importantly, we show that this occurs outside a sphere of influence delineated by sub-surface pressure increase.