Characterization of induced micro seismicity at the Gonghe geothermal project during the 2021 injection phase

The global development of Enhanced Geothermal Systems (EGS) and the increasing related occurrence of induced seismicity are topics of growing interest for the scientific community. One of the most recent EGS developments is in the Gonghe Basin, located in the northeastern Quinghai-Tibetan Plateau in China. The project is considered one of the most promising Hot Dry Rock (HDR) resources in the country due to the high temperature detected while drilling the first well in 2017 (236°C at a depth of 3705 m).

A surface seismic network of 20 three-component seismometers monitored the area around the wells GH01, GH02, and GH03 during the June – October 2021 injection and circulation phases. We used a beamforming method to detect and locate earthquakes. The beamforming process significantly improved the number of events detected, with over 10,000 detections (whereas previous research had identified roughly 2,600 events using an automated phase-picker for detections). The largest event had a magnitude of M_L 3.2, with the smallest events having magnitudes smaller than M_L -1. The increased event detection produced by the beamforming is fundamental for enhanced imaging of the faults and fractures activated by the geothermal stimulations. We compare the beamforming locations with those produced by manual phase picking for the largest events with M_L > 0.7.

Further seismological analysis has included analysis of shear wave splitting (SWS) phenomena, to understand the development of anisotropy in the reservoir during and after the injection procedures. Our results show that the fast S-wave polarization have a NE-SW orientation, congruent with the orientation of the maximum horizontal principal stress, characterized by a direction of NE55°. However, variations into the orientation are present at some stations, which could indicate additional geological complexity within the area.