Two seismicity patterns

Characterizing seismic responses to hydraulic fracturing (HF) in shale gas development is crucial for seismic hazard assessment and mitigation strategy design. Although intensive HF operations have led to severe induced seismic hazards in the Changning shale gas field (CSF) in China for over a decade, the detailed correlation between fluid injection and seismicity is still unclear. Using a 336-nodal-sensor dense array of 70 days in 2019, we develop an enhanced CSF seismicity catalog and combine it with focal mechanism solutions, fluid injection time series, seismic reflection profiles and geomechanical models to identify the distinct shallow and deep seismicity responses to HF. The first pattern consists of deep earthquake clusters that migrate along strike-slip faults in the limestone formation at ~1 km below the treatment depth. These clusters contain frequent M > 2 earthquakes, including the largest M 3.3 event, and exhibit transient seismicity rate changes in rapid response to HF. In contrast, the second pattern consists of shallow clusters in the target shale formation that persist for over a year following HF. The shallow clusters include smaller earthquakes and exhibit thrust-style faulting with no discernible spatial migration. Our geomechanical simulations suggest the deep fault reactivation is best explained by the combined effects of poroelastic stress loading and pore-pressure increases. Stable seismicity rate, frequent casing deformation in conjunction with low-frequency waveform characteristics indicate post-HF long-term aseismic deformation may drive the shallow seismicity. These distinct seismic responses during and after HF operations underscore the need for a spatiotemporally adaptive hazard mitigation strategy for the CSF.

Under review in Journal of Geophysical Research:Solid Earth