Modelling of Strasbourg 2019-2020 seismicity crisis induced by geothermal operations

The risk of induced and triggered seismicity is often present in deep geothermal resources exploitation. These facilities allow exploiting practical and green energy resources.There are many regions with high geothermal capacity, such as Alsace, France. In Vendenheim, north of Strasbourg, the Geoven plant project was expected to extract geothermal energy from the Robertsau fault by circulating fluid at depth.Two clusters of humanly perceivable seismicity occurred in 2019-2020, one of them close to wells and other one at the Robertsau area at 5km to the south. A question was raised about a possible link between these seismic events and wells activities of Geoven site. A large distance with no earthquakes between the injection wells and the southern cluster was reason for disagreement between scientific experts and the company in charge about the link between the injection and the seismicity on this cluster.Our objective is studying such a possible connection with numerical modeling through a simple methodology based on fluid/solid deformation and mechanical coupling. In addition, we aim at modelling the pressure perturbation during the time resulting from the history of the injection flux and comparing it with measured data.The methodology has 3 steps: 1. Structural plan: extracting the geometry of the fault and tectonic stresses 2. Mechanical stability: the stresses on the fault are evaluated and the risk of earthquake triggering is analyzed based on Mohr-Coulomb criterion. 3. Pore Pressure: a quasi 2D pressure diffusion equation with the proper injection parameters is solved for modeling pressure perturbation in the area due to water injection/extraction.According to the results, the fault is strong enough in the northern cluster area and slip can happen only by high activation pressure. However, slip and micro-earthquakes resulted from large pressure increase near the wells, which is necessary for permeability increase to improve the transmissivity of the reservoir. On the other hand, the fault is in the weakest state around the southern cluster, because the pressure required for sliding drops sharply. Indeed, with low amount of pressure increase, slip occurs. Our simulation shows that, the triggered earthquake is expected at this point, due to large enough pressure increase. But between these two clusters, not only is the fault resistant based on its orientation, but also the pore pressure increase is notlarge enough for slip. This explains, the distance of 5km between the two clusters, and absence of earthquakes in between.Also, we simulated the pressure perturbation in the wells resulting from real injection regime data, during 85 days of operation in Geoven site in 2020, and compare it with real pressure change.In conclusion, the lowest activation pressure in comparison to other parts can be observed around the southern cluster, which is coherent with the fault and stress tensor geometry implying a weak state in this location. Also, we identify a physical mechanism showing that earthquakes in that zone were possibly triggered by pore pressure perturbation resulting from the Geoven operation.