Experimental determination of the fracture toughness of rocks from deep geothermal reservoirs in Bavaria, Germany

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.