Numerical modelling of fluid-induced fault slip reactivation,application to Geo-Energy systems

Geothermal energy is one of the most promising techniques to exploit renewable energy resources from the Earth and to limit emissions of greenhouse gas. Deep geothermal exploitations are associated with long term fluid circulation and pressure perturbations at great depth, in fractured and faulted zones and are likely associated with a risk of triggering earthquakes. Such earthquakes are usually interpreted as the reactivation of rapid (m/s) shear slip on critically stressed faults caused by fluid flow and poroelastic stress changes. In some cases however, slow aseismic slip (m/d) can take place on faults in response to fluid flow. How fluid pressure perturbations reactivate aseimic or rapid slip still remains poorly understood. A better understanding of the hydromechanical processes controlling fault slip is therefore crucial to mitigate seismic hazards associated with geothermal exploitation.

In this framework, our study aims at constraining the influence of stress state, fluid injection rate, diffusivity and frictional failure criterion on the reactivation of slip on pre-existing faults through mechanical modelling of a set of laboratory experiments. The experiments consist of a fluid injection into a saw-cut rock sample loaded in a triaxial cell. Fault reactivation is triggered by injecting fluids through a borehole directly connected to the fault. This experimental setup is modelled by a 3D Finite Element Method (FEM) coupled with a solver of the fluid diffusion. The sample fault is modelled as a contact surface obeying slip-weakening Mohr-Coulomb friction law. This approach allows to compute slip and stress evolutions, as observed during the laboratory experiment. The FEM model is calibrated and is able to reproduce the experimental results. We show that fluid injection triggers a shear crack that propagates varying from 1 to 300 m/d along the fault. This approach can be used to investigate the relationship between fluid front and slip front during reactivation, which is an important issue to control the effects of fluid injections at depth.