In large parts of Germany and other countries, sedimentary rock dominates the subsoil. Since these rock layers are hydraulically less permeable, it was not possible until now to exploit the heat that lies in the layers below – this is also the case with the so-called North German Basin, where the test site Horstberg Z1 is located around 75 km northeast of Hanover.

This borehole is a former natural gas well, in which, in a previous project, an artificial vertical fracture was created between two layers of rock at a depth between 3600 and 3800 metres. Current research is now making use of this fracture. If the BGR scientists can prove that thermal water can circulate over such a fracture, then they will have geothermally exploited previously unusable rock formations.

The concept, which envisages the pumping of cold water through the fracture opening into the underlying geothermal reservoirs, is to be explored in numerous tests. The 140 °C hot-rock layer heats up the water, which then rises up to the earth’s surface without the need for a pump. The fracture surfaces act like a heat exchanger.

What is special about this project is that both the injection of water and the extraction take place via a single well, which is why this system is called the “single-well circulation test”. The aim of the tests, funded by the Federal Ministry for Economic Affairs and Energy until the end of 2019, is to develop a technique whereby the flow of water for injection and extraction is hydraulically separated. The longer-term circulation tests with tracers (tracing substances) are accompanied by comprehensive monitoring, which among other factors will also record alterations in the groundwater and shifts in the soil. In laboratory tests and numerical simulations, the hydromechanical properties of the facture are to be analysed.