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The in situ state of rock mass stresses is a key design parameter, e.g., for deep engineered geothermal systems. However, knowledge of the stress state at great depths is sparse mostly because of the lack of possible in situ tests in deep boreholes. Among different options, core-based in situ stress estimation may provide valuable stress information though core-based techniques have not yet become a standard. In this study we focus on the Diametrical Core Deformation Analysis (DCDA) technique using monzogranitic to monzonitic rock drill cores from 4.9 km depth of the Basel-1 borehole in Switzerland. With DCDA the maximum and minimum horizontal stress (SHmax and Shmin) directions, and the horizontal differential stress magnitudes (∆S) can be estimated from rock cores extracted from vertical boreholes. Our study has three goals: first, to assess photogrammetric core scanning to conduct DCDA; second, to compare DCDA results with borehole breakout and stress-induced core discing fracture (CDF) data sets; and third, to investigate the impact of rock elastic anisotropy on ∆S. Our study reveals that photogrammetric scanning can be used to extract reliable core diametrical data and CDF traces. Locally aligned core pieces showed similar SHmax orientations, conform to borehole breakout results. However, the variability of core diametrical differences was large for the Basel-1 core pieces, which leads to a large spread of ∆S. Finally, we demonstrate that core elastic anisotropy must be considered, requiring robust estimates of rock elastic moduli, to receive valuable stress information from DCDA analyses. [ABSTRACT FROM AUTHOR]

Petrothermal systems seek to extract energy from hot, low-permeability rocks that in northern Switzerland lie at 4–6 km depth. Permeability is enhanced by performing large 'hydraulic stimulation' injections, such as occurred in the 5 km deep well at Basel, Switzerland. Knowledge of the discontinuity properties and distribution in target reservoirs can help design such stimulations, but characterisation is challenging because of limited rock mass exposure. Rock mass fractures below Basel were investigated in previous studies by analyses of acoustic televiewer data of the Basel-1 well and by fault plane solutions. In addition, seismic events with high similarity of seismic waveforms were grouped into clusters and interpreted to have originated from slip of patches of a common fault or fault zone. In this study we investigate the orientations of fault patches from seismic clusters, considering relative location errors of cluster events with Monte-Carlo simulations, and use common events in clusters ranging in size from few 10s to many 100s of metres to explore the internal architecture of larger fault zones at Basel. We find that the orientations of microseismically-inferred faults and borehole fractures are similar and that fracture zones consist of sub-parallel or 'anastomosing' fractures. • We infer fault plane orientations from microseismic clusters of different sizes. • Orientations of microseismically-inferred faults and borehole fractures are similar. • Also orientations of fault planes from FPS and wellbore fracture zones are similar. • Fracture zones below Basel consist of sub-parallel or 'anastomosing' fractures. • Our results help extrapolating borehole fracture properties to the reservoir. [ABSTRACT FROM AUTHOR]