Can the risks associated with deep geothermal energy be controlled?

It is not certain. The industry will still have to demonstrate that it can control financial and technological risks, such as induced seismicity, through pilot and demonstration projects. However, deep geothermal energy remains a potentially very interesting source of energy in the Swiss context, and research and development efforts should continue in order to manage these risks.

Deep geothermal energy is a renewable form of energy with a low impact on the climate and the environment, and a limited footprint on the ground. It should therefore enjoy a good level of acceptance among the general public and nature conservation organisations. However, deep geothermal energy presents financial and technological risks that have so far prevented its breakthrough in Switzerland. To get an idea of this, a distinction must be made between two technologies for extracting geothermal heat: hydrothermal and petrothermal.

Hydrothermal systems extract high-temperature water (100-300°C) that is naturally present underground. This technology, used at shallow depths, is mature and widespread worldwide in areas of high tectonic or volcanic activity, which benefit from high temperatures near the surface [→ Q61]. But the challenge is on a completely different scale in Switzerland, where you have to drill at depths of 3 to 5 km to find sufficiently high temperatures for electricity production. There is a great deal of uncertainty about such drilling, as the geological conditions that allow large quantities of water to be found in the subsoil diminish sharply with depth. A project in the canton of St. Gallen had to be abandoned in 2013 for economic and technical reasons, as the water flow found at a depth of 4 km proved to be 10 times lower than expected, making it uneconomical to exploit. In addition, the drilling work had induced a small earthquake of magnitude 3.5 on the Richter scale. As a result of this failure, several hydrothermal projects in Switzerland were put on hold.

In the petrothermal approach, it is not necessary to find water at depth, since water is injected from the surface into a first wellbore and then pumped back to the surface through a second well once it has been heated. This greatly reduces the financial risk associated with underground exploration. On the other hand, this technology has not yet reached industrial maturity and presents the risk of generating micro-earthquakes when water, injected under pressure during initial drilling, artificially enlarges cracks in the rock. It was precisely such an earthquake, of magnitude 3.4, that led three years later to the final shutdown of the Basel geothermal energy project in 2009.

Petrothermal technology is the most promising in the Swiss geological context and has the greatest potential. Several pilot projects will probably be necessary to demonstrate its technical feasibility and to confirm public opinion that the induced seismic risks can be controlled before hoping for industrial development on a larger scale. At present, no petrothermal power plants are in operation in the world. Only research and development projects have been completed, such as the one in Soultz-sous-Forêts in Alsace.