From pore pressure modeling to seismic risk assessment - a fully-integrated modeling approach

Increased subsurface fluid injection accompanying oil, gas, and geothermal exploitation and carbon dioxide sequestration, among other fluid injection applications, has increased the potential impact of injection-induced seismicity over the last few years, escalating the need to model the underlying physical processes and to assess potential seismic hazard and risks. We are developing a method to assess the seismic impact of proposed fluid injection projects that combines fluid flow modeling and earthquake simulation with probabilistic seismic hazard analysis. We use the multi-phase, multi-component flow and transport model TOUGH2 code, which enables complex pore-pressure time histories on specific faults and fractures resulting from injection to be calculated. A recent addition to RSQSim, the code used for physics-based earthquake simulations, enables such pressure histories to be treated as external stressing factors added to stresses due to tectonic loading. Earthquake catalogs generated by RSQSim are used as input to the RiskCat code, which performs probabilistic seismic hazard and risk calculations based on time- and space-dependent seismicity frequency-magnitude distributions. We present an application of this integrated approach to hypothetical CO2 sequestration injection scenarios based on a simplified model of a faulted reservoir at King Island in the northern Sacramento Valley, California. Multiple pore-pressure distributions calculated by TOUGH2 for different injection scenarios are used as external stress inputs to RSQsim. By varying fault and other properties, we generate the multiple realizations of the seismic catalogs required by RiskCat to carry out probabilistic seismic hazard assessments that fully incorporate both epistemic and aleatory uncertainty distributions. We propose that simulation-based methods like this can be used during the pre-injection design phase of planned projects.