A Small CO₂ Leakage May Induce Seismicity on a Sub-Seismic Fault in a Good-Porosity Clastic Saline Aquifer

Despite public concerns, only a few CO₂ injections into saline aquifers have reported microseismicity. We analyze passive seismic monitoring of a small (15,000 tonnes and 0.15 MPa pressure) injection of supercritical CO₂-rich mixture for Stage 2C of the CO2CRC Otway Project (Victoria, Australia), which induced 19 detectable events with maximum moment magnitude M_W-0.5. The locations and dynamic parameters of the triggered events indicate a reactivation of a small fault patch where CO₂ flowed through the fault. Time-lapse seismic images of the plume and reservoir simulations show that the reactivation occurred when the CO₂ plume reached this fault. This might be indicative of a fault weakening by the plume that enabled subsequent reactivation by pressure variations. Our observations suggest that a leakage from a commercial-scale storage may trigger felt seismicity in the overburden without strong overpressure, thus, the de-risking workflows should involve a detailed study of small faults.