Hydromechanical Modeling of Fault Reactivation in the St. Gallen Deep Geothermal Project (Switzerland): Poroelasticity or Hydraulic Connection?

Abstract

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.

Potential influence of overpressurized gas on the induced seismicity in the St. Gallen deep geothermal project (Switzerland)

“…CC BY 4.0 License. supporting observations for a hydraulic connection and the results from previous simulations (Zbinden et al, 2019), a scenario without hydraulic connection is much less plausible than the fracture zone scenarios examined here.…”

“…The main shock (M L 3.5) was located at a depth of about 4.6 km, 0.2 km further south-west with respect to the top of the open well section. The spatial gap between the borehole and the seismicity suggests that the seismic events were either triggered remotely by poroelastic stress changes or by a hydraulic connection (e.g., a fracture zone or a damage zone of a fault; Zbinden et al, 2019). Temperature and gamma-ray anomalies from borehole logs support a connection to the lower Mesozoic sediments and possibly to the pre-Mesozoic basement (Wolfgramm et al, 2015).…”

“…From a geometrical analysis, Diehl et al (2017) suggested that at least one fault mapped by the seismic survey (Heuberger et al, 2016) might act as a hydraulic connection. The presence of a hydraulic connection would suggest that the injected fluid moved from the borehole toward the fault, resulting in an increase in pore pressure and a decrease in effective normal stress, eventually destabilizing the fault and leading to seismicity (Zbinden et al, 2019). Models have shown that the seismicity can be significantly stronger for hydraulically connected faults than for unconnected faults (Chang and Segall, 2016).…”

“…8). In order to explain the induced seismicity, Zbinden et al (2019) concluded that the reactivated fault in St. Gallen was most probably hydraulically connected to the well. This interpretation was in contrast to another study in which the St. Gallen deep geothermal project was classified as a site where poroelastic effects, rather than the direct influence of the injection and the associated increase in pore pressure, predominate at a greater distance from the well (Goebel and Brodsky, 2018).…”

“…In such a case, it was shown that stress changes on the fault are much smaller than in the case of a hydraulic connection (Zbinden et al, 2019), implying that the fault needs to be in a highly critical stress state (i.e., only a few 10 −3 MPa or less away from failure). In addition to the reasons already provided for the injection test, the following observations would be inconsistent with a poroelastic scenario:…”