Permeability Enhancement Process by Shear Slip on Existing Fractures from the Microseismic Perspective

Abstract

As a key physical phenomenon of Enhanced Geothermal System (EGS) development, shear slip on existing fracture enhances its permeability. Previous laboratory and numerical works presented that permeability enhancement occurs in a direction perpendicular to the shear slip direction by dilation on the fracture surface (e.g., Yeo et al., 1998). This is attributed to the accumulation of distributed contact areas, which results in the development of channeling flow on fracture (e.g., Watanabe et al., 2008). We relied on these phenomena in the interpretation of microseismic observations, such as hypocenter migration in EGS fields (Evans et al., 2005; Mukuhira et al., 2016). However, a recent laboratory study suggests permeability enhancement by shear slip is not significantly visible (Ishibashi. et al., 2021). Therefore, this study analyzed the permeability enhancement on a field scale using the microseismicity information at hydraulic stimulation. In this study, we picked up the microseismic data of EGS development in Basel, Switzerland, and analyzed the direction of shear slip from the orientation of fault and in-situ stress information. We chose the group of events that occurred from one macroscopic fracture. We evaluate the shear slip direction from the geomechanical analysis and compare it with the overall microseismic evolution direction to investigate the directional effect of shear slip on permeability enhancement. Then, we analyze inter-event time and inter-event distance as a proxy of pore pressure migration velocity. Based on the observations from these analyses, we discuss the effect of permeability enhancement by shear slip and its orientation in comparison with the shear slip direction.