Induced seismicity due to natural gas production is observed at different sites worldwide. Common understanding states that the pressure drop caused by gas production leads to compaction, which affects the stress field in the reservoir and the surrounding rock formations and hence reactivates preexisting faults and induces earthquakes. In this study, we show that the multiphase fluid flow involved in natural gas extraction activities should be included. We use a fully coupled fluid flow and geomechanics simulator, which accounts for stress-dependent permeability and linear poroelasticity, to better determine the conditions leading to fault reactivation. In our model setup, gas is produced from a porous reservoir, divided into two compartments that are offset by a normal fault. Results show that fluid flow plays a major role in pore pressure and stress evolution within the fault. Fault strength is significantly reduced due to fluid flow into the fault zone from the neighboring reservoir compartment and other formations. We also analyze scenarios for minimizing seismicity after a period of production, such as (i) well shut-in and (ii) gas reinjection. In the case of well shut-in, a highly stressed fault zone can still be reactivated several decades after production has ceased, although on average the shut-in results in a reduction in seismicity. In the case of gas reinjection, fault reactivation can be avoided if gas is injected directly into the compartment under depletion. However, gas reinjection into a neighboring compartment does not stop the fault from being reactivated.

In 2013, fluid injection during the St. Gallen deep geothermal project, Switzerland, induced hundreds of seismic events, including a ML 3.5 earthquake on a fault hundreds of meters away from the well. Recent studies have suggested the direct pressure effect through permeable hydraulic connections and poroelastic effects as possible mechanisms for inducing seismicity on distant faults. In St. Gallen, operational, seismic, and earthquake data are available to investigate the underlying physical mechanisms using a numerical model. The results show that Coulomb stress changes at the fault can be 3 orders of magnitude greater when a hydraulic connection is present. Combining this with several field observations, we conclude that the direct pressure effect was more likely the predominant mechanism behind the seismicity induced in St. Gallen. The detection of hydraulic connections may be important for future projects as pressure can be driven far from the well and reactivate remote faults.

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 the temporal and spatial characteristics of the main seismicity sequence that followed 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 important implications for future deep hydrothermal projects conducted in similar geological conditions.