On the Importance of the Earthquake-Earthquake Triggering in Microseismicity Induced by Hydraulic Fracturing

Abstract:

Permeability-enhancing treatments such as hydraulic fracturing induce microseismic events with reported magnitudes in the range of -3.0 to -0.5, although significantly larger induced earthquakes up to M_L 4.4 have been reported. The primary controlling mechanism is thought to be diffusion of pore pressure away from the hydraulic fracture system, which leads to activation of fractures that are close to optimal orientation a critical state of stress. Understanding the specific primary and secondary triggering mechanisms during hydraulic fracturing is expected to furnish new insights regarding stress, strength of faults and rupture initiation and propagation. Here, we present evidence for the existence of earthquake-earthquake triggering cascades in microseismicity induced by hydraulic fracturing using novel methods from statistical seismology. While background seismicity dominates, we find that these triggering cascades exhibit features that also characterize tectonic aftershock sequences such as the empirical Omori-Utsu law and the productivity law. While this suggests that the underlying physical triggering mechanisms are similar in both cases, there are also pronounced differences. Specifically, we find that the magnitudes of earthquakes in microseismicity induced by hydraulic fracturing are not random but tend to cluster – independent of whether one considers background events or triggered events. We attribute this to the specific geometry associated with tight shale gas and oil reservoirs, which may lead to the occurrence of fracture arrest at bedding boundaries and gives rise to stratabound fracture networks. This has important implications for seismic hazard assessment and forecasting related to hydraulic fracturing.