title: Integrated Exploration, Geothermal Modelling and Techno-Economic Resource Assessment of the Crystalline Basement in the Northern Upper Rhine Graben
Creators: name: Frey, Matthis
Publisher: name: UNSPECIFIED
Year: 2023

description: The climate crisis is already causing significant humanitarian and economic impacts that will intensify in the future if global greenhouse gas emissions are not immediately reduced. Under the Climate Protection Act, Germany is therefore obliged to achieve net carbon neutrality by 2045. To meet this ambitious target, a far-reaching transformation of the energy sector is necessary, with imports of fossil fuels being replaced by domestic renewable energy production. In addition to established energy sources, deep geothermal energy, as a low-emission, base-load capable, local and scalable solution, will likely become a cornerstone of energy supply in the upcoming decades. The crystalline basement offers the greatest geothermal potential, which could be exploited through so-called enhanced geothermal systems (EGS). Particularly favourable conditions for geothermal utilization exist in the Upper Rhine Graben (URG), where compared to other regions in Germany higher reservoir temperatures and permeabilities are observed. To date, however, deep geothermal energy occupies only a small niche due to the comparatively high costs and risks associated with drilling, development, and operation of geothermal power plants. In addition, geological uncertainties in the basement are particularly large, as it has been insufficiently explored by the hydrocarbon industry and previous geothermal research projects. This thesis aims to quantify and reduce these uncertainties to promote geothermal development in the northern URG. A comprehensive lithological, petrophysical and structural reservoir characterization is carried out by combining geological and geophysical techniques on multiple scales. All relevant data are integrated into a 3D geothermal model that enables a regional resource assessment for the basement. In the northern ORG, geologic modelling of the basement faces significant challenges because well data from the basement are very sparse and 3D seismic data are often not openly available. Therefore, gravity and magnetic data were additionally considered in a stochastic joint inversion that provided new insights into the structure and composition of the basement while also quantifying model uncertainties. The inversion demonstrates that the geologic units of the graben shoulders can be traced below the sedimentary filling. Comparison of the inverted petrophysical properties with existing databases and newly collected susceptibility measurements yielded a map of the predicted basement lithology in the northern URG. Accordingly, most areas are dominated by granitoids, which tend to have higher permeability than shales and gneisses and thus are preferred targets of geothermal drilling. In contrast, a predominantly metamorphic basement can be assumed in the Saxothuringian Zone and at the northwestern rift margin. The developed 3D basement model and inversion results were key input for a techno-economic resource assessment, which furthermore incorporated data from thermal and geomechanical models, operating geothermal power plants, and financial aspects of geothermal utilization. Calculation of resources at the regional scale was based on the widely used volumetric 'heat in place' method, whereby model uncertainties were quantified by means of Monte Carlo simulation. The recoverable heat along large-scale fault zones, considered as preferential fluid pathways, was estimated as a function of the slip and dilation tendency in the recent stress field. The economically exploitable part of the resources (reserves) was subsequently investigated by a sensitivity analysis of relevant parameters. The assessment reveals that the basement in the URG is characterized by a vast resource base, of which between 8 and 16 PWh are potentially recoverable with current EGS technologies. This could sustainably provide a significant fraction of the heat and power supply in the northern URG. About 65% of the resources were economically recoverable at market conditions in January 2022. In view of the enormous increases in energy prices resulting from the war in Ukraine, the share is now likely higher. A comparison of the calculated resources with the socio-economic-environmental conditions for geothermal utilization at the surface shows a high level of correlation, especially in the densely populated areas around Mannheim and Darmstadt. As groundwater flow in the crystalline basement is mainly controlled by open fractures, accurate knowledge of the natural fracture network is essential for the planning, development and operation of geothermal power plants. Image logs from deep boreholes provide the most meaningful information on fracture properties, but these are very rare and often inaccessible in the URG. A comprehensive structural outcrop analog study was conducted to compensate for the lack of borehole data. The Tromm Granite in the southern Odenwald was selected as the study area as it is both a suitable analog for the granitoid reservoirs in the northern URG and a potential site for the upcoming GeoLaB project. Here, lineament analyses and lidar surveys in abandoned quarries were combined, resulting in a multiscale description of the basement's fracture network. Discrete fracture network (DFN) models were then developed based on the obtained properties to estimate the permeability under assumed reservoir conditions. While the Tromm Granite is overall intensely fractured and the network is well connected, the density and orientation of fractures is strongly influenced by nearby fault zones. Fractures cluster roughly in the N-S direction, parallel to σHmax, resulting in an order of magnitude higher permeability than in the E-W direction. The structural investigations were complemented by geophysical surveys, designed to map and characterize the buried faults in the Tromm Granite. As in the regional modelling, potential field methods (terrestrial gravimetry and aeromagnetics) were applied and additionally the radon activity concentration was measured along one profile. The gravity data show rather broad anomalies, which cannot be assigned to single faults, but rather to zones of increased fault and fracture density. Inversion of the gravity data indicated a fracture related porosity of up to 9% along the pluton margins. The drone-based aeromagnetic survey, conversely, allows a more detailed mapping of the fault network. After filtering, the dataset revealed a complex network of linear anomalies that are interpreted as altered fault zones with increased reactivation potential, thus representing preferred fluid pathways. In conclusion, the crystalline basement is an attractive target for deep geothermal exploitation in the northern URG due to the vast resource base. As part of the dissertation, a new detailed geothermal 3D model and a regional map of the resources have been developed, providing politicians, investors, and project engineers with a more reliable basis for decision-making. Furthermore, the understanding of the fracture network properties and thus of the hydraulic properties in the northern URG was improved. Nevertheless, significant uncertainties remain at the local scale that can only be eliminated through targeted exploration measures and coupled numerical modelling. Besides, the risk of noticeable induced seismicity persists, which is a major obstacle to the exploitation of deep geothermal energy. Great hope therefore lies in the development of new safe stimulation techniques for EGS reservoirs, which will be advanced in particular within the framework of the upcoming GeoLaB project.