Multi-disciplinary characterizations of the BedrettoLab – a new underground geoscience research facility

Abstract

Abstract. The increased interest in subsurface development (e.g., unconventional hydrocarbon, engineered geothermal systems (EGSs), waste disposal) and the associated (triggered or induced) seismicity calls for a better understanding of the hydro-seismo-mechanical coupling in fractured rock masses. Being able to bridge the knowledge gap between laboratory and reservoir scales, controllable meso-scale in situ experiments are deemed indispensable. In an effort to access and instrument rock masses of hectometer size, the Bedretto Underground Laboratory for Geosciences and Geoenergies (“BedrettoLab”) was established in 2018 in the existing Bedretto Tunnel (Ticino, Switzerland), with an average overburden of 1000 m. In this paper, we introduce the BedrettoLab, its general setting and current status. Combined geological, geomechanical and geophysical methods were employed in a hectometer-scale rock mass explored by several boreholes to characterize the in situ conditions and internal structures of the rock volume. The rock volume features three distinct units, with the middle fault zone sandwiched by two relatively intact units. The middle fault zone unit appears to be a representative feature of the site, as similar structures repeat every several hundreds of meters along the tunnel. The lithological variations across the characterization boreholes manifest the complexity and heterogeneity of the rock volume and are accompanied by compartmentalized hydrostructures and significant stress rotations. With this complexity, the characterized rock volume is considered characteristic of the heterogeneity that is typically encountered in subsurface exploration and development. The BedrettoLab can adequately serve as a test-bed that allows for in-depth study of the hydro-seismo-mechanical response of fractured crystalline rock masses.

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

“…The chosen rock volume comes in three distinct lithological units located at approx. 60 m to 120 m, 120 m to 200 m, and beyond 200 m distance (borehole depth) to the tunnel where the Rotondo granite comes with varying degrees of pre-existing deformation, as described in detail in [ 31 ]. In the BRP, stimulation experiments are performed in all three units.…”

“…Designing the sensor geometry requires knowledge on the location of fractures and shear zones. In the BRP, very reliable data were available from the rock characterization, as described in detail in [ 31 ] and extensive borehole logging in all monitoring boreholes. Methods used include core analysis, structural mapping, hydraulic characterization, georadar, televiewer, and camera inspection.…”

“…In the first phase (September 2019), monitoring boreholes MB1, MB2, and MB3 were drilled. The first three boreholes drilled were used for extensive characterization of the rock volume [ 31 ]. They span a tripod in such a way that they penetrate potential fractures across the stimulation volume.…”

“…Moreover, for redundancy, the single-mode fiber-optic cable connecting the FBG was installed inside a loop, which allows access to the gratings from both sides. We placed the sensors at fractures with a high likelihood of being open and conductive based on observations in the geophysical logging [ 31 ]. FBG sensors were cemented into the borehole in order to ensure proper coupling to the surrounding rock volume.…”

A Spectral Boundary‐Integral Method for Faults and Fractures in a Poroelastic Solid: Simulations of a Rate‐and‐State Fault With Dilatancy, Compaction, and Fluid Injection

“…That is a fault without a significant damage zone or fault core. Such simple faults are particularly relevant for field‐scale fault injection experiments and have been targeted in previous experiments (Guglielmi et al., 2015) and will be targeted in future (Ma et al., 2022) experiments. We suggest, however, that our problem setup could be reinterpreted and applied to a larger class of problems with minor modifications.…”