EGU24-10618, updated on 10 Dec 2024
https://doi.org/10.5194/egusphere-egu24-10618
EGU General Assembly 2024
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

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

Kai Bröker1,3, Xiaodong Ma2, Nima Gholizadeh Doonechaly3, Antonio Pio Rinaldi4, Anne Obermann3,4, Martina Rosskopf3, Marian Hertrich3, Domenico Giardini3, and the BedrettoLab Team*
Kai Bröker et al.
  • 1Center for Hydrogeology and Geothermics, University of Neuchâtel, Rue Emile-Argand 11, 2000 Neuchâtel, Switzerland (kai.broeker94@gmail.com)
  • 2School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China
  • 3Institute of Geopyhsics, ETH Zürich, Zürich, Switzerland
  • 4Swiss Seismological Service (SED), ETH Zürich, Zürich, Switzerland
  • *A full list of authors appears at the end of the abstract

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

BedrettoLab Team:

The team involves more than 30 people from ETHZ and 10 research institutes and companies involved in the Bedretto Laboratory (see http://www.bedrettolab.ethz.ch/en/home/ for more details)

How to cite: Bröker, K., Ma, X., Gholizadeh Doonechaly, N., Rinaldi, A. P., Obermann, A., Rosskopf, M., Hertrich, M., and Giardini, D. and the BedrettoLab Team: Systematic multi-stage hydraulic stimulation experiments in a hectometer-scale fractured rock volume at the Bedretto Underground Laboratory, Switzerland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10618, https://doi.org/10.5194/egusphere-egu24-10618, 2024.