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The crystalline basement is considered a ubiquitous and almost inexhaustible source of geothermal energy in the Upper Rhine Graben (URG) and other regions worldwide. The hydraulic properties of the basement, which are one of the key factors in the productivity of geothermal power plants, are primarily controlled by hydraulically active faults and fractures. While the most accurate in situ information about the general fracture network is obtained from image logs of deep boreholes, such data are generally sparse and costly and thus often not openly accessible. To circumvent this problem, an outcrop analogue study was conducted with interdisciplinary geoscientific methods in the Tromm Granite, located in the southern Odenwald at the northeastern margin of the URG. Using light detection and ranging (lidar) scanning, the key characteristics of the fracture network were extracted in a total of five outcrops; these were additionally complemented by lineament analysis of two different digital elevation models (DEMs). Based on this, discrete fracture network (DFN) models were developed to calculate equivalent permeability tensors under assumed reservoir conditions. The influences of different parameters, such as fracture orientation, density, aperture and mineralization, were investigated. In addition, extensive gravity and radon measurements were carried out in the study area, allowing fault zones with naturally increased porosity and permeability to be mapped. Gravity anomalies served as input data for a stochastic density inversion, through which areas of potentially increased open porosity were identified. A laterally heterogeneous fracture network characterizes the Tromm Granite, with the highest natural permeabilities expected at the pluton margin, due to the influence of large shear and fault zones.

DeepStor-1 is the exploration well to the Helmholtz research infrastructure "DeepStor". DeepStor focuses on the investigation of high-temperature heat storage at the rim of the fromer oil-field „Leopoldshafen“. It is located about 10 km north of the city of Karlsruhe (Germany). The DeepStor-1 well is planned to reach the Pechelbronn group at 1‘460 m, i.e. it includes nearly the entire Oligocene sediments at the site. Seismic investigation reveal a structurally undisturbed section that below 200 m depth covers the Landau, Bruchsal, Niederrödern and Froidefontaine Formations. Cores will be taken from the entire section below 820 m. In addition to coring, the logging program is planned to include besides technical logging, a caliper-, self-potential-, temperature-, dual latero-, natural gamma spectrometry-, neutron-gamma porosity-, sonic-, elemental capture spectroscopy-, as well as image-logs in the sections 215-820 m as well as 820-1460 m. Drilling of DeepStor-1 is planned between 2022 and 2023.

Thermal energy storage is one of the key challenges of the energy transition, since a large portion of the final energy consumption is used for heating purposes and industrial processes and heat is subject to large seasonal fluctuations in supply and demand. To overcome this seasonal mismatch, geothermal storage offers a large, mostly unexploited potential with capacities that are unequalled by other technologies. The technology of high-temperature thermal storage in aquifers has a crucial role to play, since it could serve the need for large storage capacities. We highlight two aquifer types to which we attach a high importance as future technology.

The Cenozoic graben filling is currently investigated for heat storage potentials (DeepStor-project). The structural model based on 3D-seismic and borehole data constrains the geometry and sedimentary thicknesses of potential reservoirs of the DeepStor area north of Karlsruhe. Beside these site specific addressed questions the structural model was used to improve understanding of the development of structures and sedimentary units during Upper Rhine Graben development. The c. 44 km2 3D-seismic area is dominated by two major (N)NE-(S)SW-striking, W-dipping Leopoldshafen and Stutensee growth faults showing normal displacements of several hundred meters, most likely displacing crystalline basement rocks. Fault shadowing effects reduce imaging quality, identification and resolution of seismic reflectors with increasing depth mainly in the footwall. Footwall strata of the Leopoldshafen dip at very shallow angles to the E. Major growth strata along the Leopoldshafen fault comprise the early Miocene Hydrobia beds implying major fault activity during early Miocene times. Fault growth strata display migration of fault activity towards rift interior with time. NNW-striking en echelon normal faults, linked with relay ramps and antithetic faults, are limited to Cenozoic strata unconformably overlaying middle Jurassic rocks. Fault length – displacement analysis of individual faults indicate complex concomitant major and minor faulting activity, tip migration and subsequent fault linkage of minor fault segments that have evolved during progressive transtension.

Energy storage on a large scale is one of the key challenges of the energy transition. Eight international partners from Germany, Switzerland and the USA address this challenge in the joint project VESTA. The goal of VESTA is the generic development and demonstration of high-temperature aquifer thermal energy storage (HT-ATES). The involvement of four pilot sites in the DACH (Germany-Austria Switzerland) region in various geologies and project phases allow feedback loops between generic scientific investigations and application of new geothermal technologies. Specifically, the pilot sites shall: 1) demonstrate HT-ATES technology, 2) evaluate technical and non-technical barriers, 3) support development and implementation by providing techniques and optimized component design, 4) provide field data to support coupled process modeling to assess HT-ATES system performance, and 5) support agencies with scientific and technical knowledge as a basis for advancing regulatory provisions. With this scientific program, VESTA shall form a technical-economic basis for future operational concepts.

At the Äspö hard rock underground laboratory in Sweden, six in situ hydraulic fracturing experiments took place at 410 m depth. A multistage hydraulic fracturing approach is tested with a low environmental impact, e.g., induced seismicity. The idea is to mitigate induced seismicity and preserve the permeability enhancement process under safe conditions. The fractures are initiated by two different injection systems (conventional and progressive). An extensive sensor array is installed at level 410 m, including simultaneous measurements of acoustic emissions, electric self-potential, and electromagnetic radiation sensors. The monitoring catalog includes more than 4300 acoustic emission events with estimated magnitudes from the continuous monitoring setup (in-situ sensors between 1-100 kHz). The experiment borehole F1 is drilled in the direction of Shmin, perpendicular to the expected fracture plane. Two electromagnetic radiation sensors are installed and aligned to (i) Shmin and (ii) the expected fracture plane with a sampling rate of 1 Hz and a frequency range between 35-50 kHz. The self-potential sensors are installed at level 410 with a distance of 50-75 m from the borehole F1, including nine measuring probes and one base probe. A second self-potential setup is deployed at level 280 m in the far-field with a distance of 150-200 m from F1. The self-potential data were measured with a sampling rate of 1 Hz. For the first time (to our knowledge), the electric and electromagnetic monitoring results of two hydraulic stimulation at mine-scale are presented. The results are discussed, including the different injection types (one conventional and one progressive experiment) and the acoustic emission events. The self-potential results reveal increases in amplitude during both hydraulic fracturing experiments at both depth levels. A second increase in the self-potential was observed only during the conventional injection and only at level 280 m. This is consistent with the results of the acoustic emission catalog, which show a larger number and larger magnitude of events during conventional injection experiments. The changes in the electromagnetic field are predominantly in the direction of Shmin during both the conventional and the progressive injection experiments.

DeepStor is an experimental facility with the goal to investigate High Temperature Aquifer Thermal Energy Storage (HT-ATES) systems at KIT Campus North. The operational seismic monitoring of DeepStor includes a network of five broadband and one borehole seismometer. In addition, we plan to install a scientific monitoring network with low-cost seismometers (such as the Raspberry Shake and the Quakesaver Hidra) to test innovative monitoring methods and for a Citizen Science project. Ambient noise tomography and coda wave interferometry are being used increasingly to image and monitor geothermal reservoirs. Especially in locations with a high anthropogenic noise level, such as the Oberrheingraben, these methods could potentially provide valuable insights in the evolution of the storage acquifer during injection/production cycles. Our monitoring approach focuses on the use of a larger number of these low-cost sensors instead of fewer and more expensive broadband instruments (Large-N approach). With the broadband network and established monitoring methods as a benchmark, DeepStor provides the ideal testing ground to explore the benefits of a dense network of low-cost sensors. The Citizen Science project will build on the successful Gecko project, which involved the public in the conceptualization of geothermal energy usage. A major conclusion from the Gecko workshops was the importance of transparent monitoring processes for the acceptance of geothermal energy usage. Consequently, we plan to involve the communities around KIT Campus North in the monitoring of DeepStor by distributing sensors for radon and seismicity. To ensure trust in the monitoring process, we will follow open data practice and investigate options to make the data easily accessible and understandable.

The seasonal mismatch of the thermal energy demand can be addressed by thermal energy storage systems of high capacity (e.g. Lee 2013). In this scenario, High-temperatures aquifer thermal energy storage systems (HT-ATES), which commonly supply domestic needs could expand to meet heating or industrial processes demands by storing excess heat. DeepStor is a planned scientific infrastructure that address the demonstration the concept of HT-ATES in former hydrocarbon reservoirs of deep sedimentary rocks. Specifically, the concept development of the use of deep geothermal energy at Campus North (CN) of KIT. The latter is located on the largest known thermal anomaly in Germany (up to 140°C at 2 km depth), and in the central part of the Upper Rhine Graben (URG). In order to improve the understanding of the depleted oil reservoir conditions, a gravity survey is being carried out to support the modeling of geological structures. The gravity data in and around the CN is being acquired in an area of ~10 km2 using a CG-6 Autograv Gravity Meter (Scintrex Ltd) has a measurement range of over 8000 mGals and a resolution of 0.0001 mGal. This enable to study in both detailed local and large scale regional structures. Previous works in the URG have shown that temperatures above 100°C located in the central part of the graben superpose with areas of low values of Bouguer anomaly (Baillieux et al., 2013). On the other hand, the gravity observations on the URG have been interpreted in terms of subsurface density variation due to lithological heterogeneities. The results of the new gravity data at a local scale will improve the understanding of the local lithological heterogeneities and fracture porosity, giving feedback for the improvement of the new geological model in this area.

In Central Europe, the largest geothermal potential resides in the crystalline basement rock with important hotspots in tectonically stressed areas. To better harvest this energy form under sustainable, predictable and efficient conditions, new focused, scientific driven strategies are needed. Similar to other geo-technologies, the complex processes in the subsurface need to be investigated in large-scale facilities to ensure environmental sustainability. The proposed new underground research laboratory GeoLaB (Geothermal Laboratory in the Crystalline Basement) will address the fundamental challenges of reservoir technology and borehole safety. The specific objectives of GeoLaB are 1) to perform controlled high flow rate experiments, CHFE, in fractured rock, 2) to integrate multi-disciplinary research to solve key questions related to flow regime under high flow rates, or higher efficiency in reservoir engineering, 3) risk mitigation by developing and calibrating smart stimulation technologies without creating seismic hazard, and 4) to develop save and efficient borehole installations using innovative monitoring concepts. Planned experiments will significantly contribute to our understanding of processes associated with increased flow rates in crystalline rock. The application and development of cutting-edge tools for monitoring and analyzing will yield fundamental findings, which are of major importance for safe and ecologically-sustainable usage of geothermal energy and further subsurface resources. As an interdisciplinary and international research platform, GeoLaB will cooperate with the German Research Foundation (DFG), universities, industrial partners, and professional organizations to foster synergies and technological and scientific innovations. GeoLaB is designed as a generic underground research laboratory in the crystalline rock adjacent to the Rhine Graben, one of the most prominent geothermal hotspots in Germany. GeoLaB is an analogue site representative of the world‘s most widespread geothermal reservoir rock, the crystalline basement. In an initial phase, the suitability of a site for GeoLaB located either in the Black Forest or the Odenwald, will be proven by geological, geophysical, and geochemical drilling exploration. At the selected site, a two km long gallery will be excavated, tapping individual caverns, from which controlled, high flow rate experiments will be conducted. The experiments will be continuously monitored from multiple wells, drilled from the underground laboratory or from the surface. This will create a unique 4D-benchmark dataset of thermal, hydraulic, chemical and mechanical parameters. A virtual reality concept accompanies the development of the complex infrastructure concept from the very beginning, supporting the infrastructure set-up and the scientific experiments in planning, documentation and analysis. GeoLaB will become a cornerstone for the target-oriented development of the enormous geothermal resource. With its worldwide unique geothermal laboratory setting, GeoLaB allows for cutting-edge research, associating fundamental to applied research for reservoir technology and borehole safety, bridging laboratory to field scale experiments and connecting renewable energy research to social perception. GeoLaB comprises a novel approach that will shape research in earth science for the next generations of students and scientists.

The subsurface conditions of the Upper Rhine Graben are favorable for the development of novel geothermal utilization concepts. In particular, they allow optimization of energy use with flexible heat production and storage scenarios. A first potential analysis revealed an enormous storage potential of formerly used and well-explored oil fields. The involvement of former hydrocarbon reservoirs as components of geothermal concepts perfectly symbolizes the transition from the fossil-fuel age to the use of carbon-neutral renewable energies. The proposed DeepStor concept takes advantage of these preconditions. The comprehensive geothermal concept is tailored to the Campus North of the Karlsruhe Institute of Technology (KIT) that is located in the central-eastern Upper Rhine Graben. It includes multi-level utilization with heat recovery from the deep Mesozoic reservoirs (associated GeoHeat project) and seasonal high-temperature heat storage in the Tertiary Sandstones above (DeepStor project). The KIT Campus North Campus offers good prerequisites for the concept implementation with extraction, seasonal storage and distribution of heat from deep geothermal energy: The underground of the campus is characterized by the largest known heat anomaly in Germany, with temperatures exceeding 100 °C at a depth of 2 km. An existing area-wide local heating network allows for heat distribution. In the long term, the concept provides for the coverage of a significant part of the basic heat load of the KIT Campus North in a climate-neutral way. The scientific DeepStor storage project represents the first stage in the step-by-step development of deep geothermal energy utilization at the KIT Campus North. The targeted reservoirs involve the same Tertiary strata from which hydrocarbons have been extracted until the 1990s. Initially, the high-temperature thermal storage reservoir will be fed from cogeneration as well as current renewable waste heat from scientific infrastructures such as the biomass pilot facility "bioliq". The overarching scientific goal of the first DeepStor phase is the establishment of a scientific demonstrator to validate the technical feasibility of high-temperature heat storage in the deep underground. In the associated GECKO project, a transdisciplinary approach with natural and social sciences is pursued to develop concepts for deep geothermal energy usage on KIT Campus North in a co-design process with the local population.

In the discussion about the future role of geothermal in the energy transition policy, the topic of underground heat storage became recently more and more prominent. High Temperature Aquifer Storage (HTAS) may make geothermal more efficient by extending it beyond its traditional usage as a base load with also covering middle and even peak load. Depleted oil reservoirs can provide this underground storage capacity and Stricker et. al (2021) have numerically described the thermal storage potential in depleted oil fields from examples of the Upper Rhine Graben. Hydrocarbon exploration and production in the Northern Alpine Foreland Basin accelerated after 1950. It reached its peak in the 1980s, and then decreased mainly due to the low oil prices. Numerous separated reservoir units were successfully developed and exploited. The related extensive exploration campaigns provide exhaustive seismic profiles and borehole data for delineation of geometric underground features and reservoir properties. Since the outgoing 1990, parts of this data were already applied for the successful hydro-geothermal exploration of the Upper Jurassic Malm, especially in the greater Area of Munich and the Eastern part of the Molasse. The present study focusses on the geological and hydrogeological potential of high temperature storage in the surrounding of the existing oil fields in the South German Molasse basin. Reservoir information and data as e.g. thickness, porosity and depth of the reservoir rock as well as overlying barrier properties are compiled from two meta-studies, the Geothermal Atlas of Bavaria (STMWi, 2004) and Storage Catalogue of Germany (BGR, 2011). As a result, about one third of the area of the Bavarian Molasse shows a potential underground storage with a reservoir thickness of 10 m and more in depths between about 500 and 1700 m. In the Western part, the potential storage units are the “Bausteinschichten” of the Lower Oligocene with a porosity ranging from 5 – 31 %, and the Middle Jurassic Dogger “Eisensandstein” with an average of 15%. In the Eastern part, Chattian sandstones of the Upper Oligocene with porosities of 20% are present. In a next step, oil field information with the borehole data and its exploitation history has to be investigated, to gather more details on local reservoir characteristics as e.g. temperature, pore pressure and to develop an exploration and exploitation strategy to better determine the uncertainties and risks.

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.