The Swiss Seismological Service (SED) at ETH Zurich performs an independent baseline seismic monitoring of the planned deep geothermal project in Haute-Sorne (JU) on behalf of the Canton of Jura. Baseline seismic monitoring can improve the knowledge of the natural, local seismicity already in the planning phase of the geothermal projects. In the exploration phase and the as yet unapproved stimulation phase of the geothermal project, it aims to detect earthquakes quickly and accurately helping to clarify whether they are associated with the geothermal project’s activities or are instead of natural origin.
On request of the Jurassian authorities the SED, additionally, supports them, by offering its expertise in scientific and applied aspects of induced seismicity. This typically involves reviewing induced-seismicity-related aspects of operator documents and studies, and actively participating in the project-accompanying expert group set up by the canton.
The Canton of Jura and the SED have been collaborating in this manner since September 2022 in the framework of the GEOBEST2020+ project funded by the Swiss Federal Office of Energy (SFOE).
Planned procedure by the operator
Geo-Energie Jura SA (GEJ), a subcompany of Geo-Energie Suisse AG (GES), is planning to build a geothermal power plant in Haute-Sorne, using the "enhanced geothermal system" (EGS) technology (exploiting a so-called petrothermal system). The approach involves creating a heat exchanger at a depth of approximately 4.5 km by enhancing the permeability of the crystalline basement rocks.
Unlike the hydrothermal geothermal technology used already in other parts of Switzerland and Europe, EGS does not rely or only little on accessing pre-existing permeability found, for example, in major fault zones or karstic geological structures. EGS, therefore, can conceptually be implemented in many geological contexts. However, EGS involves deliberately increasing the permeability of existing underground fracture networks using many small earthquakes triggered by the injection of water under high pressure. While EGS projects are already contributing to heat and energy generation in some places, they have also triggered felt earthquakes in others. In fact, there was one case where it has been suggested that these earthquakes were strong enough to cause substantial damage (see news article on the earthquake in Pohang, South Korea).
While EGS projects are already contributing to sustainable heat and energy generation in some places, they have, in few cases, caused felt earthquakes. In rare cases these earthquakes were strong enough to cause damage. The largest earthquake related to a deep geothermal project occurred in 2017 in Pohang, South Korea. This event prompted the canton of Jura to ask GES to reassess the earthquake risk as part of the planned geothermal project in Haute-Sorne. The SED reviewed this assessment on behalf of the canton (see the associated report).
Consequently, high-resolution seismic monitoring, risk assessment and appropriate mitigation methods are required throughout the EGS process to ensure safety and minimize potential risks. Although the probability of inducing felt larger earthquakes is small in both petrothermal and deep hydrothermal projects, it is not negligible.
Researchers and industry experts have applied and refined the multi-stage stimulation technique that aims to better control the effects of hydraulic stimulation on the crystalline basement. This should reduce the risk of ineffective permeability enhancement and induced seismicity. This approach attempts to stimulate limited rock volumes at a time, gradually establishing a sufficiently large geothermal reservoir in multiple, separated stimulation stages. It differs from the single-stage, massive-stimulation strategy applied, for example, in the 2006 Basel or 2017 Pohang EGS projects. The multi-stage method shall improve the control over the stimulation process, making it more manageable and safer. It has been tested on a smaller scale in the BedrettoLab operated by ETH Zurich. GES plans to use the multi-stage stimulation technique, for which they hold the patent, at the Haute-Sorne project.