Active‐Source Seismic Imaging of Fault Re‐Activation and Leakage: An Injection Experiment at the Mt Terri Rock Laboratory, Switzerland

We conducted a time‐lapse seismic experiment utilizing automated active seismic source and sensor arrays to monitor a reactivated fault within the Opalinus clay formation at the Mont Terri Rock Laboratory (Switzerland), an analog caprock for geologic carbon storage. A series of six brine injections were conducted into the so‐called Main Fault to reactivate it. Seismic instrumentation in five monitoring boreholes on either side of the fault was used to continuously probe changes in P‐wave travel‐times associated with fault displacement and leakage. We performed time‐lapse travel‐time tomography on five hundred sequential data sets; this revealed a zone of decreased P‐wave velocity, up to 16 m/s, during each injection cycle, followed by a velocity increase during shut‐in. These results demonstrate varying elastic property perturbations, both spatially and temporally, along the fault plane during reactivation. We then interpreted these velocity changes in terms of fault dilation induced by pressurized fluids along the fault. Plain Language Summary Faults within clay formation caprocks for CO2 storage reservoirs are possible pathways for leakage and loss of containment. Understanding how these faults in clay‐rich rocks reactivate and leak fluids is important for predicting, detecting, and preventing CO2 movement. Passive seismic monitoring is challenging because of the lack of observable seismic events in such clay‐rich fault rupture. In this study, we measure changes in P‐wave velocity to monitor a fault reactivated by brine injections directly into the Main Fault at the Mont Terri Rock Laboratory, Switzerland. We use a recently developed time‐lapse seismic technique called Continuous Active‐Source Seismic Monitoring (CASSM), which allows us to make these measurements within a few minutes and observe small changes on the same timescale. We relate the measured changes in P‐wave velocity to the opening of that fault damage zone by using a rock physics model, which helps explain changes in permeability within the fault zone. Key Points measuring p‐wave velocity changes during fault reactivation monitoring fault reactivation in an analog caprock for geologic carbon storage fracture damage zone modeling from p‐wave velocities