Coupled poromechanical and quasi-dynamic earthquake simulation

ABSTRACT: Many geo-engineering applications, such as hydrocarbon extraction, geothermal energy production, and geologic carbon storage, involve the injection and/or extraction of fluids from the subsurface. A key hazard associated with these activities is induced seismicity. Fluid injection or extraction can perturb the stress state of the surrounding rock formation, potentially triggering the sliding of critically oriented faults and resulting in seismic events. Therefore, it is crucial to develop numerical models that can accurately capture reservoir behavior, including complex fluid migration, the mechanical response of the surrounding rock mass, and the physics of earthquake nucleation. In this work, we present a novel strategy, implemented in the GEOS simulation framework, for coupled poromechanical and quasi-dynamic earthquake simulations, incorporating fault frictional behavior described by a rate- and state-dependent friction law. The poromechanical equations are discretized using a low-order finite element method for the mechanical response, coupled with a finite volume method for fluid flow. Faults are modeled as lowerdimensional manifolds and discretized using surface elements at the interfaces between 3D matrix cells. Contact constraints are enforced using face-based, piecewise-constant Lagrange multipliers to represent fault tractions, and stabilization is achieved through face-based bubble functions that enrich the displacement space. Slip velocities, slip and the state evolution variable are considered to be piecewise-constant on fault elements. The quasi-static poromechanical equations are coupled with a quasi-dynamic earthquake model using a split-operator approach. At each timestep, the poromechanical equations are first solved under the assumption of fixed fault slip, after which the resulting fault tractions are used as input for the quasi-dynamic problem. The earthquake model is then solved locally for each fault element. We validate our approach against benchmark problems developed by the Sequences of Earthquakes and Aseismic Slip (SEAS) working group, supported by the Statewide California Earthquake Center (SCEC). Results demonstrate the robustness and accuracy of our method in capturing the complex interactions between fluid flow, fault mechanics, and earthquake nucleation processes. This work was partially performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Labo-ratory under Contract DE-AC52-07NA27344.