Energy from the depths of the Earth: Collaborative research project studies temperature-induced change of rocks in geothermal reservoirs

(Press-News.org) Geothermal energy can make a decisive contribution towards the energy revolution in Germany while supporting the changeover from the use of fossil to climate-neutral energy resources. However, to ensure widespread public acceptance, it is essential to reduce associated risks, such as the triggering of earthquakes, as far as possible. A new research project led by Johannes Gutenberg University Mainz (JGU) will investigate the impact of changes in deep geothermal reservoirs caused by contact between hot rock and cold water. "We want to understand the effect of thermally induced fracture formation on mineral rock properties, such as permeability and strength, which may have an influence on the amount of thermal water that can be extracted and also on seismicity – although we are mostly unaware of the minor induced earthquake activity," explained Professor Miriam Christina Reiss of Mainz University, who coordinates the new joint project on "Formation of fractures and changes in permeability in geothermal reservoirs caused by thermally induced stress changes" (TRIGGER). The project is being funded by the German Federal Ministry for Economic Affairs and Energy and started in April 2025.

At Mainz University, a total of four research groups are involved in TRIGGER. Professor Reiss's Volcano Seismology group collaborates with the Tectonics and Structural Geology team headed by Professor Virginia Toy, Professor Boris Kaus' Geodynamics group, and the Metamorphic Processes group led by Professor Evangelos Moulas. Other partners in the research collaboration include the Institute for Geothermal Resource Management (igem) of the Institute for Innovation, Transfer, and Consulting (ITB) in Bingen, Ruhr University Bochum (RUB), and the Microstructure and Pores GmbH (MaP) in Aachen. The German Federal Ministry for Economic Affairs and Energy will provide a total of roughly EUR 2 million, with some EUR 1 million earmarked to fund the groups at JGU. The Mainz-based researchers will mainly contribute to establishing the microstructural and microchemical composition of samples, to the analysis of rock deformation, and to the mathematical modeling of fluid flow and fracture processes.

Understanding fundamental processes to discover options for a more efficient exploitation of natural hot water reservoirs

Deep geothermal systems exploit natural hot water reservoirs at depths of more than 1,500 meters that can be used to generate electricity and heat. Moving down towards the interior of the Earth, the temperature increases by an average of 3 degrees Celsius for each 100 meters. In order to make use of this geothermal heat, hot water has to be extracted. Once the heat has been used, the cooled water is then returned back into the depths. The difference in temperature can be used to generate energy. In the Upper Rhine Graben rift system, the temperature gradient variation is even as much as 5 degrees Celsius for each 100 meters so that exploitable geothermal layers lie at a depth of 3 to 5 kilometers. This presents cost-effective resources as the layers are readily accessible and can supply large amounts of heat. "Geothermal energy is an important option here in Rhineland-Palatinate, particularly in view of the fact that the Upper Rhine Graben rift system is close by," said Reiss. At present, the typical temperatures of injected water are in a range around 70 degrees Celsius. Further reduction of this temperature could result in a greater yield of heat and improved productivity. However, the new TRIGGER project will first investigate the effects of such a reduction on subsurface rocks.

Laboratory trials followed by computer-generated modeling

The contact between cold water and much hotter rock can cause the rock to fracture and change its porosity and permeability. This could influence potentially induced seismicity. Such small-scale processes result in local changes to the properties of rock that, in turn, can influence the large-scale characteristics of thermal reservoirs. The TRIGGER research project thus aims to ascertain exactly what happens during these small-scale processes. "With this in mind, we will be examining drill core samples obtained at depths of up to 3 kilometers. We will analyze their thermal, mechanical, structural, and chemical properties," explained Reiss. In order to simulate the corresponding processes for better understanding, the samples will be subjected to various experiments, such as deformation by the injection of cold water into heated samples. The researchers will use sensors to detect whether and at what point the material fractures.

Moreover, the lab trials will be replicated in the form of computer models to gain greater insight into the occurring processes over a more extensive range of rock characteristics and temperatures. "We will be able to perform experiments in the lab that would not be possible in the field", said Reiss. "Our approach will enable us to understand the ongoing processes in more detail and also to uncover options for a more efficient exploitation." One of the goals of the joint research project is thus to determine the long-term effects of temperature fluctuations of at least 100 degrees Celsius on fracture formation and on the interaction between injected liquid and rock.

Research is being conducted to determine with more precision the risk of induced seismicity, i.e., the risk of earthquakes caused by human activity. "The public in general is more open to the use of geothermal energy today than in previous decades. It is our goal to acquire a better scientific understanding of the associated natural processes to be able to reduce the potential risks of geothermal heat exploitation," concluded Reiss. The geothermal power plant at Insheim in the Vorderpfalz region of Rhineland-Palatinate has been supplying green electricity for 10 years now. Furthermore, the state of Rhineland-Palatinate is currently planning other geothermal-related projects designed to obtain heat, for example in Speyer and Wörth am Rhein.

New insights into subsurface structures

Miriam Christina Reiss was appointed Junior Professor for Volcano Seismology at Johannes Gutenberg University Mainz in August 2023. It was during her education degree in English and Physics that she discovered her interest in geophysics during an academic exchange in New Zealand. She was awarded a doctorate in Seismology / Geosciences by Goethe University Frankfurt in 2017, subsequently held a postdoc position at Yale University, and was entrusted with a research project funded by the German Research Foundation at Goethe University. Through her research, Professor Miriam Christina Reiss aims to extend the knowledge of volcanoes and their subsurface structures. Her investigations in the field of geothermal energy are intended to enable more effective assessment of the framework conditions for this form of power generation and, in particular, of the associated risk of earthquake activity.

 

Related links:

https://www.geosciences.uni-mainz.de/seismology/ – Volcano Seismology group at the JGU Institute of Geosciences https://www.geosciences.uni-mainz.de/ – Institute of Geosciences at JGU END