Coupling of thermo-hydro-mechanical modeling with seismicity modeling in a faulted geothermal reservoir

Geothermal energy is seen as an effective factor in the global energy transition for the supply of heat and electricity. To produce large amount of geothermal energy, reservoirs with high temperature, usually deep, should be targeted. At depth, aquifers are not common and most of the geothermal fluid circulates within a fault network. Hence, such geothermal reservoirs can be exploited using production and injection wells drilled in faulted geological formations. The operation of such faulted geothermal system generates seismic events, which sometimes can be felt by humans. The events result from the complicated thermo-hydro-mechanical and chemical response of the system against operational and geo-reservoir parameters, e.g. production flow rate, fault permeability, stress field, etc. Consequently, numerical simulation of the operation of faulted geothermal systems can provide operators and the public with information on the likely occurrence of seismic events during and after the lifetime of the powerplant. Considering seismic event as an additional response of the system to the THMC response in the numerical model makes the simulation more complicated and involves more parameters.

In this study, to solve the simulation complexity, we propose to couple two finite element numerical codes, one based on the MOOSE framework and SeisSol. The former is used to simulate the THM response of the reservoir (chemical effects are not taken into account) to site operations. The latter is applied to simulate the dynamic seismic response of the fault(s). For the coupling, a bash script is written to call and execute each code, manage the feedback of the corresponding results and loop over time. Thus, the coupled modeling starts with the MOOSE-based code to simulate the THM behavior in relation to the operation until fault failure. For the time being, the Mohr-Coulomb failure criterion is used. Once failure has occurred, the bash calls SeisSol to simulate the seismic event based on the MOOSE outputs, e.g. stress, and propagates the rupture through the fault system. The outputs of the SeisSol, e.g. stress, will be imported into the MOOSE simulator to continue the simulation after the seismic event. Therefore, the outputs of each code are considered as initial counterpart conditions for the next step in the loop. Looping will continue for the predefined duration of the field operation.