With the generic underground laboratory GeoLaB (Geothermal Laboratory in the Crystalline Basement), fundamental questions of reservoir technology and borehole safety of EGS are to be investigated. The planned experiments will significantly improve our understanding of the relevant processes in the fractured crystalline rock under increased flow rates.
The application and development of state-of-the-art observation and evaluation methods will lead to findings that are of great importance for a safe and ecologically sustainable use of geothermal energy and the underground. As an interdisciplinary and international research platform GeoLaB will generate synergies and technical-scientific innovations in cooperation with the German Research Foundation, universities as well as industrial partners and specialized authorities.
To limit global warming to 2 °C above pre-industrial levels, our society is confronted with the urgent need to make the transition to a globally sustainable energy system (1). Geothermal energy is available regardless of season or time and, unlike many other renewable energies, is therefore suitable for base-load sytems. Geothermal energy is regarded as renewable as heat flows back into the reservoir due to temperature conditions and transport processes. It uses the energy source from the earth’s interior, which is inexhaustible by human standards. Geothermal energy can play an important role in the decarbonization of the energy system in Germany.
In Central Europe, the greatest geothermal potential lies in the crystalline basement with important hotspots in areas under tectonic tension. These include the Upper Rhine Graben as a rift zone with hydrothermal fluid flows and exceptional temperature anomalies in the deep underground (2). The technology “Enhanced Geothermal Systems” (EGS) was developed to exploit the geothermal potential in the crystalline (3). EGS use the deep fractured subsoil as a natural heat exchanger. With at least two boreholes, a thermal water cycle is created that brings geothermal energy to the surface and makes it usable (4). However, since relatively high flow rates (> 10 l/s) are required for economic operation, the natural permeability of the rock in the crystalline – in contrast to hydrothermal systems – must be increased by hydraulic or chemical stimulation measures (reservoir engineering) to increase the flow rates.
A major challenge for EGS is to control and minimize the induced seismicity generated in this process, both in the reservoir engineering and operation phase and with a view to increasing public acceptance. A profound understanding of the multi-physical processes in the reservoir, such as the complex interactions of the fluid with the reservoir at high flow rates, is indispensable for this. New scientifically based strategies and technologies are urgently needed to exploit the geothermal potential economically and at the same time in an environmentally compatible way.
Geothermal laboratory in the crystalline basement
The underground laboratory GeoLaB (“Geothermal Laboratory in the Crystalline Basement”) scientifically addresses the central challenges of EGS reservoir management. The specific objectives of GeoLaB are
- efficient and safe management of fractured reservoirs;
- cutting-edge multi-disciplinary and multi-process research with visualization concepts;
- developing new benign environmental strategies for subsurface installations; and
- transparent interaction with public, shareholders and -decision makers.
GeoLaB is designed as a generic underground laboratory in the crystalline basement of the Black Forest-Odenwald Complex (Figure 1). Here, the crystalline reservoir rocks of the Upper Rhine Graben are exposed and accessible to the scientific community. The target rock is at the same time representative of the world’s largest reservoir rock, the crystalline basement, and allows a transfer to other regions. In an exploratory phase of the project, the suitability of the future laboratory location is first determined by means of a catalogue of previously defined criteria and confirmed by a research well. Subsequently, individual caverns will be developed via an approximately 1 km long access tunnel, in which controlled high flow rate experiments will be conducted at a depth of approximately 400 m.
The planned experiments will allow detailed in situ observations for the first time and will contribute significantly to the fundamental understanding of the processes associated with operating conditions in reservoir structures. GeoLaB has a high innovation potential in the creation of environmental standards and safety research as well as in the development of a communication standard for geothermal energy. The use and development of state-of-the-art observation and evaluation methods lead to findings that are of great importance for the safe and ecologically sustainable use of geothermal energy and the underground.
Controlled high flow rate experiments
Experiments related to fluid and mass transport in underground laboratories usually address questions of barrier properties of host rocks, often in connection with nuclear repository research, for which purpose about 30 generic and site-specific underground laboratories have been established worldwide (5). In experiments on fractures, sampling flow rates of < 1 l/min are typically used here, e. g., Hoehn et al. (6). With “controlled high flow rate experiments” (CHFE), GeoLaB aims at the opposite problem, namely the best possible permeability of the rock. A high fracture density and an extensive fracture network is supposed to make CHFE with flow rates > 10 l/s possible. Thus, it enables the understanding of the coupled processes of fluid injection or circulation with geomechanical processes at high flow rates. Due to the expected coupling of hydraulic and mechanical processes, the CHFE are controlled by advanced monitoring methods, comprehensive numerical modelling and innovative visualization techniques (Figure 2).
THMC processes and simulation
Geothermal reservoirs are a dynamic system of high complexity. Previous strong simplifications and linear approaches describe the system only insufficiently. For understanding and prediction purposes, more and more comprehensive numerical models are therefore being developed, which map and transiently simulate these THMC processes (THMC – thermal, hydraulic, mechanical and chemical). However, the necessary parameters for this are subject to great uncertainties due to the lack of a database. The experiments in GeoLaB are continuously monitored by measurements in fan-shaped boreholes. This creates a worldwide unique 4D benchmark data set of thermal, hydraulic, chemical and mechanical and additionally microbiological parameters. With the planned experiments, experimental determinations and the verification in 3D of hydrodynamics, e. g. Navier-Stokes laws, and hydromechanics, e. g., triggering and propagation of microseismicity, in the fractured crystalline basement are possible for the first time at high flow rates. Thus, dynamic and coupled processes such as variability of the stress field in space and time and THMC-processes can be experimentally recorded for the first time.
New machine-learning methods become necessary to cope with the expected amount of data and process complexity. The extremely high computational effort required for realistic simulations means that artificial intelligence and high-performance computing are essential for the simulation platform. Furthermore, new methods of scientific visualization (Virtual Reality) are necessary. Against this background, a “Virtual GeoLaB” concept will be introduced for visual analytics and as a visual data repository of GeoLaB. The visualization center VISLab (7) serves as a starting point for this. The digital twin of GeoLaB will accompany the development of the research infrastructure already during the construction phase and support scientific studies including planning, analysis and documentation during the operational phase. Virtual GeoLaB will also play an important role in the communication with stakeholders, especially decision makers and the public.
Training and teaching
GeoLaB as a research and innovation platform offers ideal conditions for teaching and training the next generation of scientists, geotechnology experts and decision makers. Therefore, a training center will provide access to the innovative GeoLaB environment for students and young professionals from various industries with geotechnological questions and enable them to establish contacts with leading scientists and industrial companies. Interdisciplinarity becomes a natural component of the teaching due to the variety of topics of GeoLaB. The sensitization of students and industry employees to social issues, the active exchange of experiences between scientists, project sponsors, industry and relevant interest groups can also stimulate an active dialogue and create a common level of communication. GeoLaB thus offers opportunities for highly qualified young professionals with a broad horizon.
GeoLaB has the potential to fundamentally change the understanding of complex reservoir processes. It offers international research groups and industry a unique platform for experimental geothermal energy research and technological development. It serves as an integrated platform for teaching, development and communication. It holds a high innovation potential in the creation of environmental standards and safety research as well as the development of a communication standard for geothermal energy. The use and development of state-of-the-art observation and evaluation methods lead to findings that are of great importance for the safe and ecologically sustainable use of geothermal energy and the underground.
GeoLaB offers an international and interdisciplinary research platform and enables research beyond geothermal energy. Among the points of reference are
- environmental system analysis;
- materials science, e. g., corrosion resistant materials;
- methods of numerical modelling;
- Artificial Intelligence and Virtual Reality;
- Industry 4.0;
- geotechnologies, mechanical engineering and engineering in general, e. g.
- drilling techniques and borehole safety;
- autonomous driving in the underground;
- sensor technology;
- intelligent pump systems;
- Artificial Intelligence in exploration technology;
- risk management.
The last major research infrastructure in geosciences in Germany, the Continental Deep Drilling Program (KTB), has shown in economic analyses that a geoscientific lighthouse project can generate an innovation boost and a long-term economic effect. GeoLaB has the potential to achieve a similar effect.
(1) IPCC, 2018: Global Warming of 1.5 °C. An IPCC Special Report on the impacts of global warming of 1.5 °C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [V. Masson-Delmotte, P. Zhai, H. O. Pörtner, D. Roberts, J. Skea, P. R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, T. Waterfield (eds.)].
(2) Kohl, T.; Signorelli, S.; Engelhardt, I.; Berthoud, N. Andenmatten; Sellami, S.; Rybach, L. (2005): Development of a regional geothermal resource atlas. In: J. Geophys. Eng. 2 (4), pp 372 – 385. DOI: 10.1088/1742-2132/2/4/S11.
(3) Genter, A.; Evans, K.; Cuenot, N.; Fritsch, D.; Sanjuan, B. (2010): Contribution of the exploration of deep crystalline fractured reservoir of Soultz to the knowledge of enhanced geothermal systems (EGS). In: Comptes Rendus Geoscience 342 (7 – 8), pp 502 – 516. DOI: 10.1016/j.crte.2010.01.006.
(4) Schill, E.; Genter, A.; Cuenot, N.; Kohl, T. (2017): Hydraulic performance history at the Soultz EGS reservoirs from stimulation and long-term circulation tests. DOI: 10.5445/IR/1000071274.
(5) NEA, OECD (Ed.) (2013): Underground Research Laboratories (URL). Nuclear Energy Agency Organisation for Economic Co-Operation and Development (Nea No. 78122). Available online at http://www.oecd-nea.org/.
(6) Hoehn, E.; Eikenberg, J.; Fierz, T.; Drost, W.; Reichlmayr, E. (1998): The Grimsel Migration Experiment: field injection–withdrawal experiments in fractured rock with sorbing tracers. In: Journal of Contaminant Hydrology 34 (1-2), pp 85 – 106. DOI: 10.1016/S0169-7722(98)00083-7.
(7) Bilke, L.; Fischer, T.; Helbig, C.; Krawczyk, C.; Nagel, T.; Naumov, D. et al. (2014): TESSIN VISLab — laboratory for scientific visualization. In: Environ Earth Sci 72 (10), pp 3.881 – 3.899. DOI: 10.1007/s12665-014-3785-5.