Thermo-hydro-mechanical coupling in a 3D stochastic fracture network at the site of Rosemanowes: Relationship between induced seismicity and thermal stress

Abstract : The injection of water in a deep reservoir can induce micro-seismic activities. The study of this process and more particularly the relationship between induced seismicity and stress due to the cooling of the rocks is very important for the economic performance of an HDR reservoir. To understand the physical processes involved form the cooling of the rocks and the increasing of the pressure in the reservoir we set up a 3D stochastic model with geometric properties (radius, length and orientation of fractures) and petrophysics parameters are derived from field data conducted at the site of Rosemanowes (Cornwall, UK) between August 1985 and December 1988. The main objective of this simulation is to show that rock cooling does create seismic activity within a couple of years at this site and that high magnitude failures may be mitigated using pulses of injection of short duration that would trigger phenomena earlier. To better simulate the triggering of shear failure due to thermal stress in the longer term, we introduced a new stick-slip algorithm with the objective of better simulating the effects of shear failure due to a pressure increase or due to thermal stresses during long term rock cooling. When the classic Coulomb stress criterion is met, we have introduced a coefficient of dynamic friction for calculating the amount of stress drop and the corresponding fracture slip (and attached seismic moment).