An initial comparison of the induced seismic activity in St. Gallen with the activities at the geothermal borehole confirms the known correlation between the fluid pressure (pressure caused by fluids and gases) and the seismic activity: increased rates of earthquakes occur above all when pumping fluids into the subsoil (injection). Pumping fluids out of the subsoil (production), on the other hand, generally results in a decrease of the earthquake rate.
Read more...This observation was essentially proven correct in the late 1960s in a scientific experiment to monitor earthquakes in the Rangely oil field in Colorado (USA). It shaped the classic explanatory model of injection-induced seismic activity. This states that acting against the shear motion along a rupture surface in the bedrock is a frictional force that increases the stronger the rock blocks separated by the fault are pressed together. Only if the driving force of the shear motion is greater than the frictional force can abrupt shifts along the rupture surface and, thus, an earthquake occur. If fluids are present in the subsoil, the fluid pressure acts against the pressing together of the rock blocks, thereby reducing the frictional force on the rupture surface. If the fluid pressure increases, for example because fluid is pumped in, the frictional force is reduced further. In this case, the rupture surfaces can be more easily activated the higher the fluid pressure and the greater the tension of the rupture surfaces caused by tectonic processes. A decrease in pressure has the opposite effect: fewer earthquakes occur. Ongoing production (pumping out) of water can, however, also lead to an increase in earthquakes in the medium term, since the decrease in volume leads to redistributions of tension. For example, earthquakes have been triggered during the course of groundwater or oil and gas production.
During the experiment in the Rangely oil field, earthquake activity could be switched on and off by injecting and producing water. There are now also indications of a similar response in the St. Gallen geothermal energy project. Unfortunately, the classic model of injection-induced seismic activity does not provide a comprehensive explanation of the phenomenon. This is because, on the one hand, the frictional force is also dependent on the material properties of the rocks and the history of shifts in the bedrock. On the other hand, small-scale variations of these properties and the rock tension lead to a superposition of processes, which prevents reliable deterministic predictions. There is nevertheless hope that this deficit can be overcome through statistical real-time analysis and improved physical models of induced seismic activity. In collaboration with other working groups, the SED is currently developing such predictive models for induced earthquakes.
Due to the current state of scientific knowledge and the reasons mentioned above, it is difficult to make a reliable statement on the further development of induced seismic activity. It can be expected that, with the sealing of the borehole, the decreased reservoir pressure caused by the production tests will slowly increase to its natural level. During this phase, there is again an increased likelihood of induced earthquakes. It is not possible to exclude the possibility of earthquakes that can be felt. The probability for such earthquakes is, however, low.
"Production Tests Completed” – information from the St. Gallen public utilities company