Despite the well-known geothermal potential in the North Alpine Foreland Basin, large scale exploration is still limited by the economic risk of well instabilities originating from inadequate prediction of the heterogeneous rock mass conditions in the reservoir. One decisive factor is the rocks toughness against fracture propagation. Hence, analogs to the reservoir rocks are subject to Semi-circular Bend tests to determine the required energy for tensile fractures (mode I) and accordingly Double-edge Notched Brazilian Disk tests for shear fractures (mode II). The subsequent finite-discrete numerical simulations, in which the experimental results are implemented, show varying fracture patterns in the rock mass caused by drilling. The fracturing depends on the rock type, the pre-existing discontinuities, and the stresses in up to 5 km depth. Further expansion of this investigation to other scenarios and rock types, paired with a reliable geological prediction, reduces the associated risks for deep geothermal projects.
In the metropolitan area of Munich, the well-known hydrothermal reservoir in the North Alpine Foreland Basin (NAFB) provides a suitable renewable source for domestically and industrially demanded heat (Agemar et al. 2014). However, the heterogeneity of the reservoir is widely known and causes different hydraulic and mechanical properties of the rock mass. Increasing the understanding of subsurface processes is therefore a crucial task to significantly expand the share of geothermal heat supply in the heating sector. Besides the acting stress conditions, the geomechanical behavior of these deep wells is highly dependent on mechanical rock mechanical properties like strength, elastic behavior, and internal friction parameters. Furthermore, the pre-existing joints and faults in the rock mass, henceforth referred to as "discrete fracture network" (DFN), influence the rock mass behavior significantly with their roughness and their frictional and cohesive properties (Stockinger 2022; Zoback 2010).
The Upper Jurassic sedimentary rocks are considered an important aquifer due to their permeable, fluid-bearing, and deep rocks, within the NAFB in the south of Germany. They consist of predominantly marine limestones, marls, and dolomites. Based on different characteristics, the Upper Jurassic carbonates are usually divided into a massive reef facies and a stratified basin facies (Meyer & Schmidt-Kaler 1989). After deposition of the Mesozoic sediments, the complicated tectonic processes of the Alpine orogenesis formed a trough filled with debris from the Alps and the Bohemian Massif (Lemcke 1973). Due to the southwards increasing overburden of Tertiary sediments, the Upper Jurassic Carbonates now dip from north to south towards the Alps and can be found at depths of up to 4,500 m (Agemar et al. 2014). At this depth, the geothermal gradient causes rock and fluid temperatures of up to 140 °C (Agemar et al. 2014).
