Experimental Determination of the Fracture Toughness of Rocks from Deep Geothermal Reservoirs in Bavaria, Germany
Authors: Drexl Catharina , Mattheis Justin , Thuro Kurosch 
Published: August 2024

ABSTRACT:
For a comprehensive understanding of deep geothermal reservoirs, precise knowledge of geomechanical parameters is essential. These include fracture toughness for both tensile and shear fractures, assessed through semi-circular bend and double-edge-notched Brazilian disc tests. Due to their complexity regarding specimen preparation and setup, empirical correlations based on literature values are often utilized. However, this paper directly determined the fracture toughness in Mode I and Mode II for potential Bavarian geothermal sites using analog rocks in semi-circular bend test (SCB test) and double-edge notched Brazilian disk tests (DNBD test). High-speed cameras recorded the experiments, allowing precise differentiation between valid and invalid fracture patterns for the DNBD test. Comparison with derivations based on the tensile strength revealed deviations of up to 35% for the better-fitting literature correlations. If derived values for fracture toughness are used instead of experimentally determined parameters, it can lead to significant discrepancies from the natural conditions when used, for example, as input parameters in a numerical model. Achieving a realistic numerical representation of geothermal reservoirs necessitates meticulous consideration of these parameters. Improving the input parameters for numerical modeling in geothermal systems can lead to more efficient and reliable exploitation of this renewable energy source.

1. INTRODUCTION
To mitigate the catastrophic consequences of adverse climate change, the coalition agreement between the governing parties in Germany stipulates that renewable energies must cover at least 50% of the heating requirements by 2030 (German Government, 2021). Although geothermal energy is the only renewable energy source, apart from hydropower, that can meet the base load energy requirements, its development is still complicated by several factors that must be reduced. One of these factors is problematic borehole stability caused by propagating fracture patterns due to rock excavation. These fractures can also spread in the long term during the use of geothermal systems due to load changes or hydrogeological processes.
The most detailed knowledge of the geotechnical and hydrogeological conditions, which can be generated by numerical modeling, is required to reduce these risks for economic exploitation. However, the results of such modeling are highly dependent on correct and realistic input parameters. Fracture toughness is one of the necessary input parameters for visualizing fracture processes in deep boreholes. Previous research also highlights the importance of this parameter to the results obtained on borehole stability (Mattheis et al., 2023). The fracture toughness is often estimated based on literature values or calculated using empirical derivations based on other rock mechanical parameters. Since it is generally recognized that a model can only be as good as its input parameters, such an approach makes it questionable whether derived values represent realistic conditions.