Multi-Disciplinary Monitoring Networks for Mesoscale Underground Experiments: Advances in the Bedretto Reservoir Project

Abstract

The Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG) allows the implementation of hectometer (>100 m) scale in situ experiments to study ambitious research questions. The first experiment on hectometer scale is the Bedretto Reservoir Project (BRP), which studies geothermal exploration. Compared with decameter scale experiments, the financial and organizational costs are significantly increased in hectometer scale experiments and the implementation of high-resolution monitoring comes with considerable risks. We discuss in detail risks for monitoring equipment in hectometer scale experiments and introduce the BRP monitoring network, a multi-component monitoring system combining sensors from seismology, applied geophysics, hydrology, and geomechanics. The multi-sensor network is installed inside long boreholes (up to 300 m length), drilled from the Bedretto tunnel. Boreholes are sealed with a purpose-made cementing system to reach (as far as possible) rock integrity within the experiment volume. The approach incorporates different sensor types, namely, piezoelectric accelerometers, in situ acoustic emission (AE) sensors, fiber-optic cables for distributed acoustic sensing (DAS), distributed strain sensing (DSS) and distributed temperature sensing (DTS), fiber Bragg grating (FBG) sensors, geophones, ultrasonic transmitters, and pore pressure sensors. The network was realized after intense technical development, including the development of the following key elements: rotatable centralizer with integrated cable clamp, multi-sensor in situ AE sensor chain, and cementable tube pore pressure sensor.

Insights from Subsurface Monitoring for Engineering of the Stimulation Pattern in Fractured Reservoirs

“…Nine boreholes (see Figure 1-(c)), ranging in depth from 101 meters to 404 meters, have been drilled in the BULGG for monitoring, characterization, and stimulation purposes, with a comprehensive characterization available in the work of Ma et al (2022). The boreholes are equipped with various sensors, including piezoelectric accelerometers (ACC), Acoustic Emission sensors (AE), fiber-optic cables for Distributed Acoustic Sensing (DAS), Distributed Strain Sensing (DSS), Distributed Temperature Sensing (DTS) and Fiber Bragg Grating (FBG), geophones, ultrasonic transmitters, and pore pressure sensors (Plenkers et al 2023)…”

“…To monitor the seismic activity generated by the stimulations an ultra-high frequency network composed of 34 Acoustic Emission sensors and Accelerometers was installed, covering a frequency band from 50Hz to 100kHz (Plenkers et al 2023. The open-source python-based DUGSeis package (Rosskopf et al 2024b) was used to detect event candidates with a standard STA/LTA method applied to the 5 stations closest to the stimulated interval.…”

“…The spatial distribution of sensors and boreholes within the reservoir is depicted in Figure A-1 in Appendix-A. The sensor network design addresses installation challenges such as long and fractured boreholes, high pressure, and cementing requirements (Gholizadeh, Plenkers et al 2023. ST1 is the main stimulation borehole and has an approximate dip angle of 49.2°.…”