Seismic efficiency: From hydraulic fracturing-acoustic emission laboratory experiments of shale based on energy budget

Abstract

Fluid injection-triggered earthquakes have been documented worldwide and quite a number of events have significant moment magnitudes (M_w ≥ 3). Seismic efficiency (η), defined as the ratio of injection volume to net seismic moment release in hydraulic fracturing operations, is a crucial parameter to evaluate seismic hazard. However, a quantitative assessment of seismic and non-seismic (aseismic) energy release is a key aspect of understanding the intrinsic properties of cracking rocks. Therefore, we develop a novel η model based on the hydraulic-fracturing-propagation energy budget and performed laboratory experiments on hydraulic fracturing in shale by injecting distilled water at different rates under pseudo-triaxial stress conditions with simultaneous monitoring of acoustic emission (AE). We estimate AE energy accurately with absolute value correction of sensors using a laser Doppler vibrometer, and the dissipation of the potential energy using displacement and pressure sensors. The results show that the proposed η model can evaluate the induced seismic characteristics effectively compared with field data and the injection rate controls the change of η to some extent. Moreover, there is a log-linear relationship between seismic efficiency and injection efficiency (ratio of AE energy and injection energy), which may provide an experiential method for evaluating seismicity during the early phase of hydraulic fracturing.