Fault slip potential induced by fluid injection in the Matouying enhanced geothermal system (EGS) field, Tangshan seismic region, North China

Abstract

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 28 July near the Tangshan fault zone. The Matouying enhanced geothermal system (EGS) field is located ∼90 km away from the city of Tangshan. Since late 2020, preliminary hydraulic stimulation tests have been conducted at depths of ∼3965–4000 m. Fluid injection into geothermal reservoir facilitates a 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 increases with continuing fluid injection over time, especially on the faults with well distances of ∼6–10 km. The probabilistic fault slip potential (fsp) 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 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.

Current Tectonic Stress State in an Iron Mine District, North China, Based on Overcoring, Hydraulic Fracturing, and Acoustic Emission Stress Measurements

“…The iron mine district is surrounded by several large faults, such as the Lengkou fault, Yejituo fault, and Luanxian-Laoting fault (Figure 1(a)) [7]. The fault structures in this district are well developed and numerous, and faults of various scales and directions formed during different periods exhibit crisscross patterns.…”

“…Figure1: Geologic setting map of the distribution of seismic tectonics around the iron mine (a) (after Feng et al[7]) and geological structure of the iron mine (b) (after Tan et al[1]). …”

Experimental Investigation of the Effect of Fault Reactivation Induced by Water Injection

“…At present, enhanced geothermal systems (EGS) use hydraulic stimulation to create highly permeable flow paths to retrograde reservoirs and to extract geothermal energy by injecting cold water into injection wells to produce hot water or steam from production wells [5,6]. However, a series of earthquakes has occurred during geothermal exploration projects in many countries, which has become the primary obstacle to further geothermal energy exploration [7][8][9]. For example, in 1997, hundreds of earthquakes with a moment magnitude (M W ) of 1.5 were induced by injection beneath The Geysers, a geothermal field in the United States.…”