Mitigating Post-injection Induced Seismicity in Enhanced Geothermal Systems (EGS): the Role of Pressure-dependent Hydraulic Diffusivity in Stimulated Fractures

Abstract

Delayed induced seismicity remains one of the major challenges for Enhanced Geothermal Systems (EGS), particularly when large earthquakes occur weeks to months after prolonged mitigation efforts. At the Pohang EGS site, a Mw 5.5 earthquake occurred approximately two months after injection stopped, following extended post-injection flowback that recovered less than half of the injected fluid. Although fluid-pressure diffusion has been identified as a dominant mechanism driving this delayed earthquake, the physical reasons why flowback failed to sufficiently depressurize the reservoir remain unclear. Motivated by field observations showing strongly pressure-dependent and largely reversible hydraulic diffusivity within the stimulated fracture zone around the injection well, we investigated the role of pressure-sensitive stimulated fractures in post-injection mitigation failure. We developed a pressure-diffusion model that explicitly incorporates pressure-dependent hydraulic diffusivity and evaluated post-injection pressure evolution under different mitigation strategies. Our results show that rapid flowback in unpropped fractures, as implemented at Pohang, can be ineffective despite generating large reverse pressure gradients toward the well. Rapid depressurization near the well induces fracture closure and a sharp collapse in near-well permeability, transforming stimulated fractures from high-transmissivity conduits into hydraulic barriers. As a result, excess pressure remains trapped away from the well and continues to diffuse toward the fault, producing substantial delayed fault pressurization consistent with the timing of the Mw 5.5 event. Modifying well operation strategies by adjusting the rate of wellhead pressure depressurization alone provides only limited improvement. In contrast, preserving fracture diffusivity through addition of proppant substantially enhances pressure reduction and slows fault pressurization, with rapid flowback combined with proppant providing the most effective mitigation tested in this study. These findings highlight the importance of fracture hydromechanical behavior in post-injection seismic risk mitigation.