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.
The hydraulic fracturing technique (also termed mini-frac test) is commonly used to estimate the in situ stress field. We recently conducted a mini-frac stress measurement campaign in the newly-established Bedretto Underground Laboratory (BedrettoLab) in the Swiss Alps. Four vertical boreholes, dedicated for stress characterization of the granitic rock mass, hosted a total of 19 mini-frac test intervals. Systematic pressure transient analysis was performed to carefully estimate the magnitude of the least principal stress ($$S_\mathrm {3}$$ S 3 ). We compared five different methods (inflection point, bilinear pressure decay rate, tangent, fracture compliance, and jacking pressure) to identify an adequate approach best suited for our test scale and the host rock mass. We found that the methods used to determine the fracture closure pressure underestimate the magnitude of $$S_\mathrm {3}$$ S 3 , presumably due to the rapid closure of the hydraulic fracture after shut-in. The most consistent results were found using the inflection point and bilinear pressure decay rate method, which both determine the (instantaneous) shut-in pressure as the proxy for the $$S_\mathrm {3}$$ S 3 magnitude. The determined shut-in pressure, or $$S_\mathrm {3}$$ S 3 magnitude, is $$14.6\pm 1.4$$ 14.6 ± 1.4 MPa from the inflection point method. This allowed us to further estimate the stress environment around the BedrettoLab, which is transitional between normal and strike-slip faulting. The measured local pore pressures from extended shut-in periods are between 2.0 and 5.6 MPa, significantly below hydrostatic. A combination of drainage, cooling, and the excavation damage zone of the tunnel may have significantly perturbed the in situ stress field in the vicinity of the BedrettoLab.
<p>Hydraulic shearing of natural fractures or fault zones is a key mechanism for enhancing permeability in engineered geothermal systems (EGS) in order to extract geothermal energy from crystalline basement rocks. Shear reactivation is achieved by hydraulic stimulation in an injection borehole, involving a complex hydro-seismo-mechanical response of fractured crystalline rock. A major challenge is to predict which fractures are reactivated at which reactivation pressures, in order to efficiently design the injection protocols and create a large fracture network for sufficient fluid circulation and heat exchange.</p> <p>The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) in Switzerland serves as an in situ test-bed where meso-scale hydraulic stimulation experiments are conducted to better bridge the knowledge gap between laboratory scale experiments and complex reservoir scale processes (Ma et al. 2022). The BedrettoLab is located in a 100 m long enlarged section of the Bedretto tunnel (Ticino, Switzerland), 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&#176;-dipping, and was equipped with a multi-packer system that partitions the borehole into 15 intervals. Before conducting two multi-stage hydraulic stimulation phases in borehole ST1, the rock volume was characterized with various geophysical logging tools, hydraulic tests, and mini-frac tests for stress measurements (Br&#246;ker and Ma 2022, Ma et al. 2022).</p> <p>Along the stimulation borehole, we mapped multiple clusters of sub-parallel pre-existing open fractures and fault zones that are preferentially oriented for reactivation in the estimated stress field. In this work, we compare our preceding probabilistic slip tendency and reactivation pressure estimates with the results from hydraulic stimulation experiments. The interval pressure and flowrate data from the stimulations reveal a reactivation of the natural fractures associated with an increase in injectivity. A comparison of the expected stress field around the stimulation interval with the observed reactivation pressure indicates that the fractures were likely reactivated by hydraulic shearing. The observed reactivation pressures are in the range of the preceding estimates, but a precise estimation is challenging due to the large number of input parameters, i.e. stress magnitudes and orientation, fracture orientation, pore pressure, coefficient of friction, and their uncertainties.</p> <p><strong>References:</strong></p> <p>Br&#246;ker, K., & Ma, X. (2022). Estimating the Least Principal Stress in a Granitic Rock Mass: Systematic Mini-Frac Tests and Elaborated Pressure Transient Analysis. <em>Rock Mechanics and Rock Engineering</em>. https://doi.org/10.1007/s00603-021-02743-1</p> <p>Ma, X., Hertrich, M., Amann, F., Br&#246;ker, K., Gholizadeh Doonechaly, N., Gischig, V., Hochreutener, R., K&#228;stli, P., Krietsch, H., Marti, M., N&#228;geli, B., Nejati, M., Obermann, A., Plenkers, K., Rinaldi, A. P., Shakas, A., Villiger, L., Wenning, Q., Zappone, A., &#8230; Giardini, D. (2022). Multi-disciplinary characterizations of the BedrettoLab -- a new underground geoscience research facility. <em>Solid Earth</em>, <em>13</em>(2), 301&#8211;322. https://doi.org/10.5194/se-13-301-2022</p>
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.
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