Earthquakes and very deep groundwater perturbation mutually induced

Dugin Kaown,In-Wook Yeo,Jeong-Ung Woo,Kang-Kun Lee,Jaeyeon Kim,Sanghoon Lee,Jin-Yong Lee,Heejung Kim,Daeha Lee,Shemin Ge,In-Woo Park

doi:10.1038/s41598-021-92937-y

Abstract

We report unique observations from drilling and hydraulic stimulation at a depth of approximately 4.3 km in two Enhanced Geothermal System (EGS) wells at the Pohang EGS site, South Korea. We surveyed drilling logs and hydraulic stimulation data, simulated pore pressure diffusion around the fault delineated by seismic and drilling log analyses, conducted acoustic image logging through the EGS wells, observed significant water level drops (740 m) in one of the two EGS wells, and obtained hydrochemical and isotopic variation data in conjunction with the microbial community characteristics of the two EGS wells. We discuss the hydraulic and hydrochemical responses of formation pore water to a few key seismic events near the hypocenter. We focused on how the geochemistry of water that flowed back from the geothermal wells changed in association with key seismic events. These were (1) a swarm of small earthquakes that occurred when a significant circulation mud loss occurred during well drilling, (2) the MW 3.2 earthquake during hydraulic stimulation, and (3) the MW 5.5 main shock two months after the end of hydraulic stimulation. This study highlights the value of real-time monitoring and water chemistry analysis, in addition to seismic monitoring during EGS operation.

Highlights

We report unique observations from drilling and hydraulic stimulation at a depth of approximately 4.3 km in two Enhanced Geothermal System (EGS) wells at the Pohang EGS site, South Korea
Induced earthquakes at Enhanced Geothermal System (EGS) sites have been observed globally, but the magnitude of the M W 5.5 event that occurred owing to the rupture of the critically stressed fault at the Pohang EGS site in South Korea was unusual; this event corresponded to a “run-away” earthquake[1,2,3]
Until two months prior to the M W 5.5 mainshock on November 15, 2017, the fault that eventually ruptured during the mainshock had been continuously pre-stimulated by five hydraulic stimulations and mud circulation, which led to mud loss while drilling the two EGS wells (Fig. S1)

Summary

Introduction

We report unique observations from drilling and hydraulic stimulation at a depth of approximately 4.3 km in two Enhanced Geothermal System (EGS) wells at the Pohang EGS site, South Korea. During hydraulic stimulation at the Pohang EGS site, Westaway et al.[32] compared the chemical constituents of the injected water and flowback water, indicating the possible degradation of the fault strength through hydrochemical reactions. These observations, were mostly obtained from surface or subsurface environments shallower than several hundred meters in depth or for a short time before the earthquake. The objectives of this study were (1) to report unique observations of (i) high-pressure water injection and flowback through two deep EGS wells, (ii) two injection-induced earthquakes and huge water level drop at an EGS well, and (iii) subsequent earthquake-initiated deep pore water perturbations near the EGS wells; and (2) to explore co-seismic and post-seismic interactions between deep pore water and earthquakes using hydraulic, chemical, and microbial signatures

Objectives

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Conclusion