Abstract
The Tangshan region is one of the most seismically active areas in the North China, and the 1976 M 7.8 earthquake occurred on July 28th near the Tangshan fault zone. The Matouying Enhanced Geothermal Systems (EGS) field is located ~90 km away from Tangshan City. Since the late 2020, preliminary hydraulic stimulation tests have been conducted at depths of ~3965–4000 m. Fluid injection into geothermal reservoir facilitates heat exchanger system. However, fluid injection may also induce earthquakes. In anticipation of the EGS operation at the Matouying uplift, it is essential to assess how the fault slip potential of the nearby active and quiescent faults will change in the presence of fluid injection. In this study, we first characterize the ambient stress field in the Tangshan region by performing stress tensor inversions using 98 focal mechanism data (ML ≥ 2.5). Then, we estimate the principal stress magnitudes near the Matouying EGS field by analyzing in situ stress measurements at shallow depths (~600–1000 m). According to these data, we perform a quantitative risk assessment using the Mohr-Coulomb framework in order to evaluate how the main active faults might respond to hypothetical injected-related pore pressure increases due to the upcoming EGS production. Our results mainly show that most earthquakes in the Tangshan seismic region have occurred on the faults that have relatively high fault slip potential in the present ambient stress field. At well distances of less than 15 km, the probabilistic fault slip potential on most of the boundary faults increase with continuing fluid injection over time, especially on these faults with well distances of ~6–10 km. The probabilistic fault slip potential increases linearly with the fluid injection rate. However, the FSP values decrease exponentially with increased unit permeability. The case study of the Matouying EGS field has important implications for the deep geothermal exploitation in China, especially for Gonghe EGS (in Qinghai province) and Xiong’an New Area (in Hebei province) geothermal reservoirs that are close to the Quaternary active faults. Ongoing injection operations in the regions should be conducted with these understandings in mind.
Highlights
Enhanced geothermal systems (EGS) are a promising source of renewable energy for a decarbonizing world and can provide a valuable contribution to the production of renewable energy (Lee et al, 2019).The EGS technologies exploit geothermal resources through hydraulic stimulation, which involves the injection of high-pressure cold water into the target formation in order to increase the unit permeability by creating new fractures or causing preexisting fractures to widen (Terakawa et al, 2012; Grigoli et al., 2018; Lee et al, 2019)
We investigate the probabilistic fault slip potential (FSP) values for faults located within distances of ~15–20 km away from the hypothetical injection wells in the MTY EGS field
The Tangshan fault belt has FSP values ranging from 31% to 41%, and the Jiyunhe fault belt has FSP values ranging from 27% to 37%
Summary
The EGS technologies exploit geothermal resources through hydraulic stimulation, which involves the injection of high-pressure cold water into the target formation in order to increase the unit permeability by creating new fractures or causing preexisting fractures to widen (Terakawa et al, 2012; Grigoli et al., 2018; Lee et al, 2019). To economically produce electricity and heat with an EGS, it is necessary to employ an efficient hydraulic subsurface heat exchanger system that can circulate through the hot rock that hosts the permeable fracture network (Bromley et al, 1987; Häring et al, 2008). The industrial process of hydraulic stimulation involves creating tensile fractures and subsequently increasing the permeability of the target rock formations via the controlled injection of pressurized fluid (Ellsworth, 2013). Seismic events caused by fluid injection are a possible hazard faced by most engineering endeavors that result in changes to the ambient subsurface stress or pore pressure (Evans et al, 2012)
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