Reliability of earthquake-size distribution and stress regime relationships for fluid-injection-induced seismicity

Abstract

Induced seismicity caused by fluid injection associated with unconventional gas and geothermal projects, poses a significant hazard that requires effective mitigation and management. This issue is of concern to local and indigenous communities, regulatory bodies, operators and their insurers, as it poses risks to health, safety, and project economics and success. To assess the likelihood and potential magnitude (i.e., severity) of fluid-injection-induced seismicity events, empirical relationships for natural earthquakes have been co-opted that relate event size distributions (b-value) to focal mechanisms and inferred in-situ stress regimes. However, it remains unclear whether these associations can be generalized and applied to fluid-injection- induced seismicity. Analyses presented here using a global dataset of induced seismicity events suggest they can't. Unlike natural earthquakes where tectonic forces generally activate the faults most critically aligned to the far-field in-situ stresses, fluid-injection-induced seismicity is driven mainly by localized pressure perturbations that may activate a fault with a non-critically aligned orientation. These findings are further investigated and supported using 3-D Mohr-circle slip tendency analyses and 3-D numerical models employing a fully-coupled hydro-mechanical bonded-lattice methodology via the commercial hydraulic fracturing numerical simulation program XSite. The combined empirical, analytical and numerical simulation results indicate that under a transform stress regime (SH > SV > Sh), for typical reservoir depths (i.e., 1–6 km), a larger area of the fault affected by the injected fluid pressures is critically stressed compared to a compressional stress regime (SH > Sh > SV), resulting in a larger slip area and an increased likelihood of larger magnitude fluid-injection-induced seismicity events. These findings have important implications for the development of effective hazard mitigation strategies and risk management protocols for fluid-injection-induced seismicity.