Anomalous distribution of microearthquakes in the Newberry Geothermal Reservoir: Mechanisms and implications

Stimulation of enhanced geothermal system (EGS) reservoirs by fluid injection can enhance the reservoir permeability but may also result in undesired microearthquakes (MEQs). A bimodal depth distribution of fluid-injection-induced MEQs was observed in the 2012 stimulation phase of the Newberry Volcano EGS Demonstration project in Oregon, US. During 7 weeks of hydraulic stimulation of well NWG 55-29, 90% of MEQs occurred in the shallow reservoir (∼500 m to ∼1800 m), only a few occurred adjacent to the bottom of the open borehole (∼2500 to ∼3000 m) while almost no seismicity was observed in the intervening interval (∼1800 m to ∼2500 m). Our analysis of frictional stability using spatial models for fluid pressure diffusion of injected fluids show that the distribution of MEQs is consistent with observed casing damage, and a possible leak at ∼700 m, and is inconsistent with migration of fluids from the casing shoe. The role of fluid injection through the ruptured casing is further supported by the analyses of shear failure and pore-pressure diffusion. Finally, the absence of seismicity at intermediate depths is consistent with our laboratory determinations of frictional stability, showing velocity strengthening frictional behavior for samples from intermediate depths, bracketed by velocity neutral and weakening behavior for samples from shallower and greater depths.