Induced microseismicity and tremor signatures illuminate different slip behaviors in a natural shale fault reactivated by a fluid pressure stimulation (Mont Terri)

Summary Fault slip induced by fluid perturbation in shale formations may only lead to as sparse seismicity. However, fault strongly impact the integrity of that serve caprocks for geological reservoirs holding buoyant fluids such CO2, natural gas, or hydrogen. A better understanding reactivation processes and seismic triggering process is therefore critical reservoir monitoring stability. Here we analyze responses a exposed pressurization during an situ field-scale injection experiment at ∼340 m depth Mont Terri underground research laboratory (Switzerland). Two main types signals are observed was activated started slowly slip. After aseismic phase, tremor signatures increase noise amplitude, which were directly associated with propagating response injection. These later followed micro-earthquakes seem occur further away from fluid-pressurized area. We interpret these be triggered stress perturbations growth. two classes highlight different processes. Tremors more direct observation fluid-induced propagation than micro-earthquakes. Even hidden noise, they precede earthquake failures, thus providing useful tool leakage slow deformation on low permeable faults, applications sealing caprocks.