THE 2015 MESOSCALE FAULT SLIP EXPERIMENT AT MONT TERRI: MAIN FINDINGS, LESSONS LEARNED, AND NEXT STEPS (Invited Presentation)

Understanding fault reactivation as a result of subsurface fluid injection is critical for a variety of subsurface engineering applications, including but not limited to geologic carbon capture and sequestration (CCS) projects where fault slip may cause enhanced fault permeability and potentially CO₂ leakage from the injection zone through the overlying caprock layer. Our presentation introduces a controlled fault injection experiment conducted in 2015 in a clay formation in the Mont Terri Underground Research Laboratory (Switzerland) which attempts to address the following key questions: (1) what are the critical processes and parameters affecting reactivation of a fault in a clay formation typical of a caprock layer for CCS projects, (2) will such reactivation create a permeable and extensive flow path in the previously low-permeability rock, and (3) how will the permeability of this path evolve as a function of time. The Mont Terri fault zone, an analog to a minor fault zone that would hardly be detectable from surface seismic surveys during the initial characterization of a CCS site, is a few meters thick and, under the ambient stress state, has a very low static permeability (on the order of 10^-13 m/s). To estimate the potential of permeability variation of the fault upon reactivation, we conducted high-pressure fluid injections into compartments of the fault, about 30 minutes long intercalated with rest periods of the same duration. Fault slip was triggered between two packed-off sections of two vertical boreholes intersecting the fault. In each section, the three-component displacement of the fault, the pore pressure, and the injection flowrate were continuously monitored at high temporal resolution. A third borehole was used to synchronously monitor induced seismicity with two three components accelerometers respectively set in the hanging wall and in the footwall of the fault, while three more boreholes were used to monitor the pore pressures in the intact hanging wall close to the fault zone. Here, we discuss the main findings and lessons learned from the mesoscale experiment at Mont Terri, we explore what research gaps still exist, and we introduce a planned follow-up experiment at Mont Terri that specifically looks into long-term leakage behavior in the reactivated fault.