Induced and triggered seismicity by immediate stress transfer and delayed fluid migration in a fractured geothermal reservoir at Pohang, South Korea

This study reveals the important role of hydromechanical responses of large hydraulic fractures in the cause and mechanisms of induced seismicity associated with the Mw 5.5 earthquake at the Pohang geothermal site, South Korea. Fractures intersecting two stimulation wells, PX-1 and PX-2, were identified through a comprehensive analysis of various data produced from drilling and hydraulic stimulations, including injection flow-pressure and seismicity data. Coupled hydromechanical numerical modeling of five stimulations performed in the PX-1 and PX-2 wells, showed that injection-induced hydraulic jacking and fracture shearing induce immediate stress transfer that plays a significant role in understanding seismic response at the Mw 5.5 fault. The friction coefficient times fluid pressure change by the PX-2 stimulations accounts for only 20% of the total CFS change at the location of the Mw 5.5 earthquake. The absence of seismic events at the Mw 5.5 fault by the PX-1 stimulations is attributed to the transfer of reduced shear stress at the PX-1 fracture due to shear slip. Kaiser effects in the seismic response observed during the PX-1 stimulation is explained by the irreversible nature of shear slip of the PX-1 fracture for the first time. In addition, post-injection seismicity, observed during the PX-2 stimulations, is attributed to a low permeability of the PX-2 fracture. The current modeling study suggests that coupled hydromechanical analysis with appropriate consideration of fractures is a powerful approach in understanding the mechanism of injection-induced seismicity in fractured geological media.