Abstract
Abstract. In July 2013, the city of St. Gallen conducted a deep geothermal project that aimed to exploit energy for district heating and generating power. A few days after an injection test and two acid stimulations that caused only minor seismicity, a gas kick forced the operators to inject drilling mud to combat the kick. Subsequently, multiple earthquakes were induced on a fault several hundred meters away from the well, including a ML 3.5 event that was felt throughout the nearby population centers. Given the occurrence of a gas kick and a felt seismic sequence with low total injected fluid volumes (∼1200 m3), the St. Gallen deep geothermal project represents a particularly interesting case study of induced seismicity. Here, we first present a conceptual model based on seismic, borehole, and seismological data suggesting a hydraulic connection between the well and the fault. The overpressurized gas, which is assumed to be initially sealed by the fault, may have been released due to the stimulations before entering the well via the hydraulic connection. We test this hypothesis with a numerical model calibrated against the borehole pressure of the injection test. We successfully reproduce the gas kick and spatiotemporal characteristics of the main seismicity sequence following the well control operation. The results indicate that the gas may have destabilized the fault during and after the injection operations and could have enhanced the resulting seismicity. This study may have implications for future deep hydrothermal projects conducted in similar geological conditions with potentially overpressurized in-place gas.
Highlights
Industrial injection and extraction projects causing anthropogenic earthquakes have increased during recent years and have been conducted closer to densely populated areas (Foulger et al, 2018)
The seismic module of TOUGH2-seed is switched off during the simulation of the injection test and the gas kick to simplify the calibration of the model; we use the full model during the simulation of the main seismic sequence
The results showed that the induced events of the injection test all lay in regions of positive Coulomb stress change, the magnitude of the stress changes was on the order of 10−3 MPa or lower, which is about 3 orders of magnitude smaller than in the case of a hydraulic connection (Fig. 7)
Summary
Industrial injection and extraction projects causing anthropogenic earthquakes have increased during recent years and have been conducted closer to densely populated areas (Foulger et al, 2018). Several geothermal projects globally have induced seismicity (e.g., Baisch et al, 2015; Evans et al, 2012; Grigoli et al, 2018; Jeanne et al, 2015); recently, a Mw 5.5 earthquake struck the city of Pohang (South Korea) (Ellsworth et al, 2019; Grigoli et al, 2018), the largest earthquake recorded at an EGS site to date (Kim et al, 2018) This earthquake has challenged recently proposed models that relate the maximum expected seismic magnitude to the total injected fluid volume (Galis et al, 2017; McGarr, 2014). In comparison to the local stress field, SH intersects the fault with an angle of about 50◦
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