Modelling of fault slip: beyond Mohr-Coulomb?

In this study, we carry out an evaluation of the potential for induced seismicity arising from hydraulic stimulation of low to intermediate enthalpy porous reservoirs, by taking the geothermal reservoir of Groß Schönebeck (northern Germany) as study case. The aim is to evaluate the spatial and temporal distribution of 26 events, which were triggered by hydraulic stimulations in the volcanic section of the reservoir. The results from the THM simulations of this hydraulic stimulation are in agreement with the calculated overpressure and indicate that an increase in the reservoir fluid pressure is most likely responsible for the recorded micro-seismicity. Our current evaluation shows an increase of slip and dilation during the treatment on the seismic plane, of magnitudes close to the failure level as based on Mohr-Coulomb friction concept which would have led to a reactivation of the fault plane and related seismic activity. We conclude this contribution by presenting a thermodynamically consistent framework to describe the deformation dynamics at the semi-brittle semi-ductile transition, thereby extending the classical mechanical analysis to faulting processes. We will focus on (i) the role of damage weakening and its impact on the evolution of localized deformation and (ii) the role of porosity evolution as precursor to dilatant brittle deformation. We will demonstrate how such a framework can bridge the gap between the microstructural evolution, expressed in terms of a damage intensity variable, and the macroscopic response of porous rocks subject to differential loading and in the presence of fluids.