Imaging the Long-term Loss of Faulted Caprock Integrity - In-situ Experiments in the Mont Terri Rock Laboratory, Switzerland

Understanding fault reactivation is crucial during CO2 injection and storage because they may result in enhanced fault permeability, potentially inducing fluid leakage from the injection zone through overlying caprock, and possibly triggering shallow earthquakes. We present results from a decametre-scale, controlled field stimulation experiment conducted in the Mont Terri underground rock laboratory (Switzerland) in a natural fault zone intersecting the Opalinus Clay, a shale formation considered as a reference low-permeability caprock-like formation. We demonstrated that a 0.4 mm fault slip could be triggered and continuously monitored while the pore pressure was increased by ca. 2.0 MPa above the initial pressure exhibited by the stimulated fault zone. We also observed that an exponential permeability increase from 10E-17 to 10E-13 m2 occurred in the fault and interpret this to be associated to creep, episodic slow slip of the fault, and a low magnitude earthquake swarm. Based on the encouraging results from the FS experiment, we propose a new experiment aiming at imaging long-term fluid flow, as well as permeability and stress variation through a ruptured minor fault, to assess CO2 storage safety and the integrity of reservoirs caprocks.