Systematic multi-stage hydraulic stimulation experiments in a hectometer-scale fractured rock volume at the Bedretto Underground Laboratory, Switzerland

Interest in engineered geothermal systems (EGS) has grown in the last decade due to their recognition as a low-emission, renewable energy source. EGS reservoirs with sufficiently high temperatures are located at depths of several kilometers, where the permeability of the crystalline basement rocks is insufficient for geothermal energy extraction. Permeability enhancement is accomplished through hydraulic stimulation, either by hydraulic shearing of natural fractures or shear zones, or through hydraulic fracturing of intact rock. The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) in Switzerland serves as an in situ test-bed where hectometer-scale hydraulic stimulation experiments are conducted to better understand the seismo-hydromechanical response of fractured crystalline rock masses (Ma et al. 2022). The geothermal testbed of the BedrettoLab is located in a 100 m long enlarged section of the Bedretto tunnel in the Swiss Central Alps, with an overburden of more than 1000 m of granite. Several characterization, monitoring, and two stimulation boreholes were drilled. One of the stimulation boreholes (referred to as ST1) is 400 m long, 45°-dipping, and was equipped with a multi-packer system that partitions the borehole into 15 intervals. In this work, we present the structural and seismo-hydromechanical characterization of eight stimulation intervals closely observed using a dense monitoring network (see Plenkers et al. 2023 for the detailed network layout). We injected relatively small fluid volumes (0.35–14 m3) following a standardized injection protocol to compare the response of the targeted geological structures in each interval. Depending on the transmissivity of the interval, the stimulation was conducted pressure- or flow rate-controlled with several steps at constant pressure/flow rate. Despite the similarly oriented structures in each interval, the observed seismo-hydromechanical behavior is complex and heterogeneous. The detected seismicity follows multiple steeply-dipping and NE-SW striking planes (Obermann et al. 2024), which coincides with the direction of known pre-existing fault structures obtained from the geological characterization. In most intervals, a clear bilinear behavior on the pressure vs. flow rate plot marks a strong increase in injectivity above a certain reactivation pressure. Analysis of these reactivation pressures in comparison with the stress field, fracture and seismic cloud orientations implies that the stimulation mechanism is hydraulic shearing of the fractures rather than elastic opening (also known as hydraulic jacking). References: Ma, X., Hertrich, M., Amann, F., Bröker, K., Gholizadeh Doonechaly, N., et al. (2022). Multi-disciplinary characterizations of the BedrettoLab -- a new underground geoscience research facility. Solid Earth, 13(2), 301–322. https://doi.org/10.5194/se-13-301-2022 Obermann, A., et al. (2024). Picoseismic response of hectometer-scale fracture systems to stimulation with cm-scale resolution under the Swiss Alps, in the Bedretto Underground laboratory. In preparation for JGR: Solid Earth. Plenkers, K., Reinicke, A., Obermann, A., Gholizadeh Doonechaly, N., Krietsch, H., et al. (2023). Multi-Disciplinary Monitoring Networks for Mesoscale Underground Experiments: Advances in the Bedretto Reservoir Project. Sensors, 23(6), 3315. https://doi.org/10.3390/s23063315