eprintid: 2276 rev_number: 9 eprint_status: archive userid: 6 dir: disk0/00/00/22/76 datestamp: 2020-07-13 08:37:55 lastmod: 2020-07-13 08:37:55 status_changed: 2020-07-13 08:37:55 type: article metadata_visibility: show creators_name: Zbinden, Dominik creators_name: Rinaldi, Antonio Pio creators_name: Diehl, Tobias creators_name: Wiemer, Stefan corp_creators: Swiss Seismological Service, ETH Zurich, Switzerland corp_creators: Swiss Seismological Service, ETH Zurich, Switzerland corp_creators: Swiss Seismological Service, ETH Zurich, Switzerland corp_creators: Swiss Seismological Service, ETH Zurich, Switzerland title: Potential influence of overpressurized gas on the induced seismicity in the St. Gallen deep geothermal project (Switzerland) subjects: MP4 subjects: RS2 subjects: SG divisions: S4CE-1 full_text_status: none 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. date: 2020 date_type: published publication: Solid Earth volume: 11 number: 3 publisher: EGU pagerange: 909-933 id_number: doi:10.5194/se-11-909-2020 issn: 1869-9529 official_url: https://doi.org/10.5194/se-11-909-2020 access_IS-EPOS: limited owner: Publisher citation: Zbinden, Dominik and Rinaldi, Antonio Pio and Diehl, Tobias and Wiemer, Stefan (2020) Potential influence of overpressurized gas on the induced seismicity in the St. Gallen deep geothermal project (Switzerland). Solid Earth, 11 (3). pp. 909-933. DOI: https://doi.org/10.5194/se-11-909-2020