Hydromechanical modeling of the hydraulic stimulations in borehole PX2 (Pohang, South Korea)

Abstract

A Mw 5.5 earthquake struck Pohang (South Korea) on November 2017, following a sequence of five hydraulic stimulations of an Enhanced Geothermal System (EGS). The processes that led to this earthquake, which nucleated two months after the end of the last stimulation in borehole PX2, are not well understood yet. We propose a hydromechanical model that integrates available data to understand the potential relationship between the earthquake and the EGS. Data scarcity is translated into model uncertainties, which we address with sensitivity analyses. Results show that the Mw 5.5 earthquake is linked to the high-injection overpressures of up to 90 MPa induced during the stimulations in borehole PX2 and highlight the usefulness of hydromechanical modeling to forecast the seismicity of an EGS and, more specifically, the need to integrate the low permeability fault core that hindered fluid pressure dissipation at Pohang, which explains the long delay of the mainshock.