Recent occurrence of moderate to large seismic events (Mw ≥ 3) after terminating well operations is unlikely to be caused only by pore-pressure diffusion into conductive faults; it is necessary to address additional mechanisms in the earthquake nucleation. Our coupled fluid flow and geomechanical model describes the processes inducing seismicity corresponding to the sequential stimulation operations in Pohang, South Korea. Simulation results show that the combined effect of poroelastic shearing and delayed pore-pressure accumulation can cause slip on a fault, potentially inducing the post shut-in large earthquakes. Alternate injection-extraction operations through multiple wells can enhance the efficacy of pore-pressure diffusion and subsequent stress transfer through rigid and low-permeability basement rocks to the fault. This mechanistic study addresses that comprehensive characterization of the faulting system and optimal injection-extraction strategies are critical to mitigate unexpected seismic hazards associated with the site-specific uncertainty in operational and geological factors.

1. INTRODUCTION

Over the past decade a number of induced seismic events have been increasingly observed due to extensive subsurface energy activities such as wastewater injection [e.g., Kim, 2013, Hornbach et al., 2016], geothermal stimulation [e.g., Diehl et al., 2017], or geological carbon storage [e.g., Bauer et al., 2016]. Numerical models provide a critical link between field observations and theory of mechanisms inducing earthquakes by quantifying transient perturbations in pore pressure and stresses throughout a domain of interest.

At the Pohang site in South Korea (Fig. 1), the first EGS (Enhanced Geothermal System) stimulation began on 29 January 2016 and total of five phases of injectionproduction operations had taken place at ∼4.3 km of depth through PX-1 and PX-2 wells until September 2017 with a net injected volume of 6,000 m3 (total injected volume of 12,800 m3 and total produced volume of 6,800 m3). The spatial footprint of detected seismic events delineates the geometry of the fault plane (strike/dip = N214°/43°NW), separating PX-1 and PX-2 [GSK, 2019], which was not found prior to the EGS stimulation. The focal mechanisms indicate that the Korean Peninsula is under tectonic compression, and the local stress field reveals that the 2017 Pohang earthquake was induced by the oblique reverse slip of a previously extensional fault at optimal orientation. This fault was critically stressed, implying that a fault slips with a small stress perturbation, and drilling or fluid injection-production initiated seismic activities along the fault.

This content is only available via PDF.
You can access this article if you purchase or spend a download.