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.

Keywords: Bedretto Underground Laboratory for Geosciences and Geoenergies; fiber-optic monitoring; geothermal exploration research; in situ rock experiments; microseismic monitoring; multi-discipline monitoring; ultrasonic monitoring.

Figure 1

Boreholes of the Bedretto Reservoir Project (BRP) shown in (a) map view and (b) side view. Monitoring boreholes are shown as black lines; production boreholes are shown as red lines. In (c) the sum deviation between the straight boreholes trajectories planned and boreholes realized in the z-direction is shown. Most boreholes deviated upwards at distances >100 m from the borehole mouth. The maximum deviation is 20 m. Horizontal and vertical deviation is summarized in (d).

Figure 2

Quality of hectometer-scale boreholes with rough borehole contours: (a) step over at end of conductor pipe, (b) borehole breakouts, and (c) shear zones are common observations and major challenges for lowering monitoring equipment. In addition, (d) borehole blockage by rock fractures is not unusual. White arrows highlight features; red arrows point towards the borehole bottom, as the borehole camera is rotated. Pictures taken of monitoring boreholes MB1, MB3, and MB4 of the BRP project. Figure 2b modified from [17].

Figure 3

Illustration showing the limited space inside the borehole (crosscut). The sketch shows the geometries of the most dominant devices installed in the BRP project with respect to the borehole diameter (most outside thin black line): the solid lines show the outer diameter of the centralizer springs; the dashed line shows the diameter of the cable clamps; the dark grey ring shows the cementable tube pore pressure; the bright grey circle shows the largest sensor installed next to the central rod (accelerometer); the solid black line in the center shows the central tubing; and the solid black circles show the different cables. All devices are separated in space to improve consistent cementation. Details in text.

Figure 4

Location and setting of the Bedretto Underground Laboratory for Geosciences and Geoenergies: (a) geographical overview of Switzerland; (b) map view of local setting with regard to the Furka Tunnel; and (c) cross section after Keller and Schneider, 1982. The Bedretto tunnel is shown as a red line.

Figure 5

Final sensor positions inside the long monitoring boreholes in the BRP. Monitoring boreholes are sorted regarding the location of the borehole mouth in tunnel meters (TMs). For the borehole geometry see Figure 1.

Figure 6

Main components of the BRP multi-sensor monitoring network installed together in the same monitoring boreholes: (a) centralizer with integrated cable clamp; (b) frontshoe with integrated geophone; (c) top plug; (d) fiber Bragg grating (FBG) sensor; (e) cementable tube pore pressure (CTPP) sensor; (f) high-frequency accelerometer with pre-amplifier; (g) in situ acoustic emission (AE) sensor; and (h) ultrasonic transmitter. For details, see Section 3.4 Installation and Guidance System for (a–c); Section 3.5 Geomechanics for (d,e); Section 3.6 Seismology for (b,f,g); and Section 3.7 Active Seismics for (h). An overview of size, numbers, and locations is given in Table 1.

Figure 7

Centralizer with integrated cable clamp: (a) crosscut and (b) side view. Silicon inlays allow the installation of thin cables.

Figure 8

Composition of the cementable tube pore pressure sensor (CTPP). Shown is (a) crosscut along tubing; (b) crosscut orthogonal to tubeing, and (c) disjointed components. The sensor was developed to create an open space within the cemented borehole to allow the measurement of dynamic pressure signals. The construction ensures that all cables of other devices can pass through the interior while also installing an FBG sensor at the same location.

Figure 9

Data examples for BRP sensors (geomechanics). The thermo-hydromechanical monitoring of the reservoir volume during an injection operation in Interval-10 in the ST1 wellbore is shown: (a) injection pressure (measured downhole) and flow rate; (b) pressure change measured at four CTPP sensors in MB5 and MB8 wellbores, all zeroed at the first data point in the plot for comparison purpose; (c) temperature profile measured using DTS in injection wellbore ST1 (the packed injection interval is bounded with dotted lines); (d) temperature profile measured using DTS in monitoring borehole MB5; and (e,f) strain measurements in two wellbores, MB5 and MB8, using FBG sensors with compression shown as negative values, smoothed over 20 s time window. The start and end of the injection are marked in all subplots. The grouted CTPP pressure sensor MB5-162.5 m and the corresponding collocated FBG sensor at the depth of 162.5 m in MB5 clearly show the hydromechanical connection to the injection interval (Interval 10) in the ST1 wellbore.

Figure 10

Data of example seismic events. Shown in (a) is the seismic activity from 10 to 28 September 2021. The activity is correlated to the newly cemented boreholes MB5, MB7, and MB8, i.e., to picoseismicity triggered by the hydration and temperature input from cementation. Monitoring boreholes are shown as black lines; production boreholes are shown as red lines; the tunnel is shown as a grey line. In (b) we show the waveforms of a typical seismic event of this time period and recorded on AE sensors of two boreholes. The waveforms are normalized; the maximum amplitude of each trace is given on the right. The network is capable of recording very small seismic events with good signal-to-noise ratios. As discussed in the text, AE sensors are not-calibrated; therefore, the absolute energy (magnitude) of the event is uncertain at this time and subject to further analysis.

Figure 11

Data of example ultrasonic transmitters in BRP. In (a) the raw waveform recording of transmitter signal GMuG-Tr70 over 43.6 m distance is shown (1738 stacks). The corresponding spectrum is shown in (c). In (b), the waveform of transmitter GMuG-Tr50 over 7.9 m distance is shown (1524 stacks). The corresponding spectrum is shown in (d). Due to intrinsic damping, higher frequencies are transmitted for short distances only. In (d), frequencies above 70 kHz are visible, whereas in (c) only frequencies up to 15 kHz are recorded above the noise floor.