Earthquakes and very deep groundwater perturbation mutually induced

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.

Seismic Evidence for Fluid‐Driven Pore Pressure Increase and Its Links With Induced Seismicity in the Xinfengjiang Reservoir, South China

“…The reservoir water may have probably penetrated to the depth and laterally from the surface to the preexisting faults via a series of shallow faults (and fractures), such as DYF and RZSF (Figure 12). Water infiltration would increase pore pressure in rocks and reduce the effective stress (e.g., Cesca et al., 2021; Goebel et al., 2017; Huang et al., 2018; Kaown et al., 2021; Lu & Wang, 2015), forming the low‐velocity channel underlying the high‐velocity belt in a regional scale (Figure 12). In this sense, the preexisting fault could possibly reactivate at depths below ∼6 km by decreasing the effective stress and it could play a critical role in inducing earthquakes.…”

Need for Seismic Hydrology Research with a Geomicrobiological Focus

“…Among the related literature, there have been few studies correlating community structures of deep and shallow groundwater microbes with the age of groundwater, or correlating changes in hydrogeochemical characteristics with changes in microbial community structures [16]. Recently, there has been a gradual increase in studies that monitor changes in the ecological environment of the groundwater system in response to hydrogeochemical reactions in aquifers after an earthquake [2,[17][18][19]. Hydrochemical data indicated changes and transitions related to immediate responses of indigenous microorganisms to an earthquake's influence on the groundwater environment [18,19].…”

“…Recently, there has been a gradual increase in studies that monitor changes in the ecological environment of the groundwater system in response to hydrogeochemical reactions in aquifers after an earthquake [2,[17][18][19]. Hydrochemical data indicated changes and transitions related to immediate responses of indigenous microorganisms to an earthquake's influence on the groundwater environment [18,19]. According to Yang and Lou [19], Flavobacterium is a representative organism that appears immediately after an earthquake.…”

“…Hydrochemical data indicated changes and transitions related to immediate responses of indigenous microorganisms to an earthquake's influence on the groundwater environment [18,19]. According to Yang and Lou [19], Flavobacterium is a representative organism that appears immediately after an earthquake. The results of microbial community studies on the groundwater environment after the Gyeongju earthquake in 2016 in South Korea revealed that the hydrochemical data changed along with the appearance of Flavobacterium [18].…”

“…It is believed that using data from microbial groundwater communities together with hydrochemical data in earthquake-related studies will enable a more comprehensive understanding of seismic hydrology. Furthermore, correlating the geological characteristics of rocks in the fault areas with the changes in hydrogeochemistry [24] and the ecology of the deep crustal fluids following stress changes can aid in the development of earthquake risk assessment methods for each characteristic [18,19,24] of the fault zone aquifer.…”