Inferring Permeability Enhancement During Fluid-Induced Fault Slip Reactivation In The Laboratory

Earthquakes that occur during geothermal exploitation or any other fluid-injection activity (hydraulic fracturing, CO2 waste disposals…) are attributed to the reactivation of rapid slip along critical faults. This highlights the urgent need of comprehensive monitoring and mitigation strategies to ensure both energy production and environmental safety.

Our main objective is to develop numerical methods to infer the permeability enhancement during fault reactivation induced by fluid injection in the laboratory. To this end, we model an experimental protocol conducted on a rock sample with a saw-cut fault, led by F.X. Passelègue and collaborators from the EPFL and GéoAzur rock mechanics laboratories. At the beginning of the injection experiments, pore pressure was uniformly set to 10 MPa along the fault plane. The injection experiment was preceded by a loading phase, during which shear stress was increased to approximately 90% of the peak shear stress to bring the fault to a critically pre-slip state. Fluid was then injected along the fault at a constant rate of 1 Mpa/minute, with simultaneous measurements of pore pressure, slip, and shear stress recorded continuously.

To simulate the experimental process, we established a system of coupled partial and ordinary differential equations that describe the evolution of key variables, including permeability, porosity, fault slip, and pore pressure. These equations are solved numerically. The general behavior of our key variables generated by the model reproduces the trend observed in the data recorded in the laboratory. For an advanced data match we develop a deterministic inversion approach, specifically the adjoint state method, to infer the permeability model. We are currently focused on enhancing this inverse model.