Eva Schill

Geothermal energy offers a growing opportunity to meet the future energy demand. High-temperature reservoir thermal energy storage (HT-RTES) can decrease seasonal mismatches between heat generation and usage while supporting the stability of the power network. In Upper Rhine Graben, HT-RTES is favored from suitable geological conditions, permeable reservoirs and formation temperatures which makes these reservoirs appropriate for injection temperatures above 100 °C.The study incorporates data from wells within the Leopoldshafen depleted field as well as two additional wells Eggenstein 1 and Stutensee 1 (EG 1 and ST 1) located approximately 2 and 4 km from the main Leopoldshafen study area. These wells target two Oligocene stratigraphic formations: the Niederrödern formation, deposited in a meandering system, and the Karlsruhe subformation, characterized by marine depositional environments.Through integration of the existing borehole logging data, we developed an approach to identify potential sandstone reservoir horizons and to evaluate regional heat storage capacity. This methodology relies primarily on the interpretation of self-potential (SP) logs, which enables the identification of sandstone bodies, their internal architecture, and lateral continuity. Based on the newly acquired core- and plug porosity and permeability measurements, the observed reservoir heterogeneity reflects significant variability associated with the distribution of different sedimentary facies.As a result, a three-dimensional facies-based geological model was constructed to identify the most suitable storage horizons and their associated channel geometries, which also enabled estimation of the potentially storable heat volumes.

The essentially non‐magmatic continental rift system of the Upper Rhine Graben (URG) is segmented and characterized by intra‐rift sub‐basins that exhibit NE to NW trends suggesting complex basin development during rifting. Structural modeling and analyses of an about 40 km² 3D seismic survey from the northern part of the central URG segment show deep rooted NNE‐striking faults controlling early, syn‐rift 1, extensional basin evolution from about 47 Ma to 27 Ma. The change in stress field at about 27 Ma and subsequent reactivation of these deep rooted basement faults resulted in a new set of shallow rooted en‐echelon faults. On a larger scale this change in stress field was accompanied by a shift of regional depocenters. The shallow rooted en‐echelon faults nucleated in 32–28 Ma old strata and are linked by conjugated relay ramps that control the development of latest Oligocene to early Miocene syn‐rift 2 basins trending N to NNW. Although we only studied a small part of the >11,000 km² large URG, we hypothesize that this type of intra‐rift basin development in the hanging wall of major deep rooted normal faults is more widespread in the graben and may represent a general style for the development of other late Oligocene‐early Miocene intra‐rift basins, such as for example, the distinct Heidelberg Basin. We suggest increasing mechanical coupling between the European lower plate and Adriatic upper plate at c. 30 Ma to 28 Ma as a possible cause for the Oligocene stress field change.

Challenges for geosciences with societal relevance include the use of crystalline geothermal reservoirs for the energy transition as well as final disposal. Crystalline rocks, along with clay and salt, are being considered as host rocks in the ongoing search for a suitable site for a nuclear waste repository in Germany. Currently, there is no existing experience in terms of excavating a repository within crystalline rocks in Germany. In the case of geothermal energy, the greatest potential – as yet untapped – lies in the crystalline basement. New targeted, science-based strategies are the key to harness this energy under safe, sustainable, predictable, and efficient conditions. The research infrastructure GeoLaB (“Geothermal Laboratory in the crystalline Basement”) is the first underground laboratory to be built in Germany for many years and offers unique opportunities for the geoscientific community. In the following, GeoLaB will be described, and the synergies it offers for geothermal research and also for nuclear waste disposal will be elaborated. GeoLaB aims to scientifically address and overcome the key challenges of deep geothermal exploitation of fractured crystalline reservoirs. To reach these goals of (1) sustainable reservoir technology, (2) wellbore safety, and (3) productivity enhancement in deep crystalline reservoirs, GeoLaB is designed as an underground laboratory consisting of a main gallery starting at the surface, providing access to several subsurface openings. These are typically small caverns in which or from which the planned scientific experiments will be conducted and observed. The first phase of the project began in January 2023 and will primarily consist of an exploration campaign to identify and characterize a suitable site. The Black Forest and Odenwald are considered suitable locations for the planned underground laboratory related to specific scientific and technical criteria. Here, the rocks of the crystalline basement, which are an important geothermal reservoir in the Upper Rhine Graben, have been lifted up and are thus accessible for science. In the Upper Rhine Graben and Black Forest–Odenwald complex, a good database and a high level of expertise has been built-up over decades by science, the petroleum industry, and geothermal activities. The technical measures in the development phase until 2029 include intensive monitoring as well as underground mining development and the construction of a control and visitor center above ground. A digital twin of the GeoLaB, named “Virtual GeoLaB”, will accompany the development of the research infrastructure already during the construction phase and support the scientific investigations, including planning, analysis, and documentation, in the operation phase. It is used for visual analysis and data storage and will play an important role in communication with stakeholders, decision-makers, and the public. In its dialog and participation concept, GeoLaB focuses on transparency and offers citizens opportunities for active participation. It takes action in communication and knowledge transfer as well as education. GeoLaB will be developed as a blueprint for environmentally sound technology developments in the underground, and it will incorporate multiple research perspectives. With BGE (Bundesgesellschaft für Endlagerung) joining GeoLaB, the exploration and building phases, in particular, have become a mutual effort. From the perspective of safe nuclear waste disposal, several topics of the GeoLaB project are of interest even though the target rock characteristics differ to some extent. The collaboration starts with the planning phase for the underground laboratory. The geophysical exploration together with the required drilling, logging, testing, and monitoring of exploration wells is comparable – to a large extent – regardless of the purpose of the mine. Several topics with relevance to Germany's current nuclear waste disposal site selection process are part of the scientific work within GeoLaB, for instance the characterization and quantification of the subsurface stress regime. Furthermore, regulatory topics and outreach intersect greatly. Finally, the excavation of the mine is of utmost interest from the perspective of nuclear waste disposal. Recent experience of mining in Germany is rare, especially with respect to crystalline rocks.

A number of six in situ hydraulic fracturing experiments were carried out at the Äspö Hard Rock Laboratory (Sweden) in 2017 in a depth of 410 m. Here we present electric self-potential monitoring during the conventional and the step-wise cyclic injection experiments HF2 and HF3. Electric self-potential data were acquired through a two-sensor array, each including nine measuring probes and one base probe, that were installed at the 410 m and 280 m levels. The experimental borehole F1 is drilled in the direction of Shmin, perpendicular to the expected fracture plane. The self-potential sensors are installed sub-parallel to Shmin at level 410 at a distance of 50-75 m to the borehole F1 and sub-perpendicular to Shmin at level 280 m at a distance of 150-200 m to F1. The self-potential data were measured with a sampling rate of 1 Hz. Here, we propose a 1-D modelling of the streaming potential that approximates the measured self-potential data. These streaming potential gradients ∆V are estimated from the simulated pressure signals and the coupling coefficient.

In central Europe, the majority of the CO2 emissions in the energy sector are related to the provision of building and process heat. Due to seasonal fluctuations in demand, especially to provide heat for residential and industrial buildings, local storage of excess heat in summer for utilization in winter is becoming increasingly important. With the current state of technology and foreseeable developments, sufficient amounts of heat can only be stored underground, taking advantage of the large available storage volumes. In contrast to typical near-surface aquifer thermal energy storage (ATES), the utilization of deep reservoirs enables the storage of much larger thermal energies due to potentially significantly higher injection temperatures (e.g. > 80 °C).Previous studies demonstrated the high potential of deep reservoirs for high-temperature (HT) ATES, in particular for former hydrocarbon reservoirs in the Upper Rhine Graben. However, these studies focused on thermo-hydraulic processes, only rarely considering the impact of coupled mechanical processes. Using the case study of the DeepStor project, a demonstrator for HT-ATES under development in the north of Karlsruhe (Germany), the present study investigates the influence of coupled thermo-hydraulic-mechanical (THM) processes during the operation of HT-ATES systems.In particular, we investigate the impact of seasonal HT-ATES with biannual injection/production cycles on the stress distribution in the subsurface and subsequently caused displacements in the reservoir and the surface as well as the shear capacity at faults. This study further aims at improving the understanding of poro- and thermoelastic processes related to HT-ATES. Whereas the thermoelastic component dominates the vertical displacements at the top of the reservoir, the uplift at the surface is primarily controlled by the poroelastic component. Furthermore, an assessment of potential risks such as surface uplift or shear capacity at faults is performed. Our results show that surface uplift is primarily controlled by the reservoir depth, Young’s modulus, and the injection/production flow rate.

Zusammenfassung: Es werden mögliche Beiträge geologischer und mariner Kohlenstoffspeicher für die Vermeidung von CO₂-Emissionen in die Atmosphäre oder für die Entnahme von bereits emittiertem CO₂ aus der Atmosphäre vorgestellt. Neben der Einlagerung von CO₂ in geologischen Speichern unter Land und unter dem Meeresboden werden eine forcierte CO₂-Entnahme aus der Atmosphäre und Abgabe in den Ozean durch Erhöhung der Alkalinität, durch Ozeandüngung und durch das Management vegetationsreicher Küstenökosysteme untersucht. Alle Optionen können sowohl global als auch aus deutscher Perspektive eine Rolle für das Erreichen der Klimaziele spielen. Umweltverträglichkeit, Permanenz der Speicherung sowie infrastrukturelle und rechtliche Voraussetzungen, gesellschaftliche Akzeptanz und wirtschaftliche Realisierbarkeit bedürfen für alle Ansätze weiterer Klärung, bevor hieraus realisierbare Optionen werden können.

Zusammenfassung: Die schnelle und umfassende Emissionsminderung in der Energie- und Kreislaufwirtschaft ist der Schlüssel für die Begrenzung der Erderwärmung auf unter 2 °C. In Deutschland werden entsprechende Politiken seit den 1990er-Jahren verfolgt, allerdings erst in jüngster Vergangenheit mit dem notwendigen Nachdruck und der notwendigen Orientierung hin zu mehr Marktmechanismen. Wesentliche Handlungsfelder sind Energieeinsparung und erhöhte Energieeffizienz, Umstieg auf erneuerbare Energien, Bepreisung von Klimagasen sowie eine Reduzierung und Schließung der Stoffkreisläufe. In allen Handlungsfeldern sind Grundlagen geschaffen, es bleibt jedoch der Großteil des Weges noch zu gehen, um Klimaneutralität zu erreichen. Für einen schnellen Fortschritt spielen neben der Überwindung der technischen, ökonomischen und organisatorischen Herausforderungen auch Verteilungsfragen und die Einbettung in internationale Maßnahmen eine zunehmende Rolle.

The geoelectric properties of the geothermal system associated with the Tolhuaca volcano were investigated by three-dimensional (3D) inversion of magnetotelluric (MT) data. This study presents the first resistivity model of the Tolhuaca volcano derived from 3D MT inversion to have a better understanding of its magmatic and hydrothermal system. We selected data from 54 MT stations for 3D inversion. We performed a series of 3D MT inversion tests by changing the type of data to be inverted, as well as the starting model to obtain a model in agreement with the geology. The final 3D MT model presents a conductive body (<20 Ωm) located 2 km below the summit of Tolhuaca volcano, inferred as a shallow magmatic storage compartment. We also distinguish a ~300 m thick layer of high conductivity (1800 m from the surface) that had so far not been identified by previous resistivity models. The result of this study provides new insights into the complexity of the Tolhuaca geothermal system.