THE MISSING INTERMEDIATE-DEPTH MEQs IN THE NEWBERRY GEOTHERMAL RESERVOIR STIMULATION: THE ROLE OF FRICTIONAL INSTABILITY

Abstract:

Stimulation of enhanced geothermal system (EGS) reservoirs by fluid injection enhances reservoir permeability by inducing microearthquakes (MEQs). Observations during the 2012 stimulation of the reservoir showed a bimodal depth distribution of seismicity. During seven weeks of hydraulic stimulation of well NWG 55-29, ~90% of the MEQs occurred in the shallow reservoir (~500 m to ~1800 m), ~10% occurred adjacent to the bottom half of the open borehole (~2500 to ~3000 m), while no seismicity was observed in the intervening interval (~2000 m to ~2500 m). Analyses of frictional stability using spatial models for fluid pressure diffusion of injected fluids shows that the distribution of MEQs is consistent with an observed distressing-of and leak-through the casing. However, this is incapable of explaining the anomalous absence of seismicity at intermediate depths. Hence, we performed rate-state friction (RSF) experiments on five samples of drill cuttings that represent the discrete depths of the MEQs zones. The results, with the effects of applied normal stresses, mineral contents and calcite dissolution on frictional parameter a-b of each sample, indicate that the observation of MEQ depths is indeed consistent with experimentally determined distributions of frictional strength and stability posited to transition from stability at intermediate depth to instability both below and above.