Abstract
The increased interest in subsurface development (e.g., unconventional hydrocarbon, deep geothermal, 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 (‘Bedretto Lab’) was established in 2018 in the existing Bedretto Tunnel (Ticino, Switzerland), with an average overburden of 1000 m. In this paper, we introduce the Bedretto Lab, 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 Bedretto Lab can adequately serve as a test-bed that allows for in-depth study of the hydro-seismo-mechanical response of fractured crystalline rock masses.
Highlights
The coupled hydro-seismo-mechanical characteristics of crystalline basement rock masses have traditionally been of broad scientific and engineering interest
Combining the characterization efforts to date, we evaluate the suitability of the Bedretto Lab rock volume as a test-bed to host upcoming experiments, and offer an outlook on the challenges and opportunities to advance the understanding of hydro-seismo-mechanical processes taking place in fractured crystalline rock masses
Since 2018, the Bedretto Tunnel has been made available by its owner, Matterhorn Gotthard Bahn (MGB), to ETH Zürich for long-term 115 research, which prompted the establishment of the Bedretto Lab
Summary
The coupled hydro-seismo-mechanical characteristics of crystalline basement rock masses have traditionally been of broad scientific and engineering interest. 55 The need to better understand the hydro-seismo-mechanical coupling in fractured rock masses becomes even more crucial in the recent context of unconventional oil and gas and deep engineered geothermal systems (EGS) and the associated (triggered or induced) seismicity (Cornet, 2015; Ellsworth, 2013; Elsworth et al, 2016; Giardini, 2009). A handful of in situ field experiments have been conducted in recent years (Fu et al, 2021; Hertrich et al, 2021; Ingraham, 2021; Krietsch et al, 2020; Ma, 2021; Schoenball et al, 2020), which have significantly advanced our understanding of the hydro-seismo-mechanical processes at decameter scales; to what extent such experiments are representative of the 85 realistic in situ heterogeneous rock mass remains an open question. Combining the characterization efforts to date, we evaluate the suitability of the Bedretto Lab rock volume as a test-bed to host upcoming experiments, and offer an outlook on the challenges and opportunities to advance the understanding of hydro-seismo-mechanical processes taking place in fractured crystalline rock masses. Since 2018, the Bedretto Tunnel has been made available by its owner, Matterhorn Gotthard Bahn (MGB), to ETH Zürich for long-term 115 research, which prompted the establishment of the Bedretto Lab
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