Investigating fracture and stress controls on induced seismicity in geothermal reservoirs with a coupled THM model

Enhanced Geothermal Systems (EGS) aim to provide sustainable energy by increasing the permeability of deep, low-productivity reservoirs through hydraulic stimulation. While micro-seismicity is an expected outcome of stimulation, larger induced earthquakes such as those recorded in Basel (2006) and Pohang (2017) remain a major challenge for the safe deployment of deep geothermal projects. This highlights the need for physics-based models capable of resolving the coupled processes and fault behavior that control induced seismicity, and of assessing how reservoir properties and stimulation strategies influence seismicity rates and maximum magnitudes.

We present a numerical framework designed to investigate how coupled thermo-hydro-mechanical (THM) processes govern fault reactivation and induced seismicity in EGS. The model explicitly couples fluid flow, heat transfer, and stress evolution, and incorporates stress-dependent deformation, fault reactivation, and a built-in earthquake detection algorithm based on deviatoric strain rate. This approach enables consistent identification of induced events within simulations and quantification of their magnitudes, providing a process-based framework to explore the spatio-temporal evolution of seismicity. To resolve fault complexity and process coupling at high spatial and temporal resolution, the model is implemented using high-performance computing tools, enabling efficient exploration of a wide range of scenarios.

Preliminary simulations of the 2006 Basel project reproduce key seismic characteristics, including b-values and maximum magnitudes consistent with observations. Early tests on different fracture networks indicate that fracture size strongly influences the resulting seismicity. Ongoing work systematically investigates the roles of fracture size, fracture criticality, and stress ratio in controlling induced seismic behavior. Overall, this modelling framework provides a flexible tool to explore the physical mechanisms driving induced seismicity in EGS and to support the development of safer stimulation strategies.