In situ stress profiling along Bedretto boreholes via integrated geomechanical approaches
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Author
Date
2022Type
- Master Thesis
ETH Bibliography
yes
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Abstract
With the Bedretto Underground Laboratory for Geoscience, ETH Zurich has an excellent opportunity to gain a deeper insight into underground processes relevant for hydro-seismo-mechanical fractured granitic rock mass behaviour and subsequent applications. Improvement of state-of-the art equipment and research progress in geothermal energy, earthquake physics and rock stability can be achieved by conducting essential experiments directly into the fractured crystalline underground.
In an unique research infrastructure, it is possible to overcome the challenging transfer from small laboratory experiments to large-scale reservoir projects.
Within this work, the in-situ stress was mapped and characterised along three inclined boreholes of varying lengths (2 x 40 m and 216.5 m) in the Bedretto tunnel using two different methods: Mini-frac tests and Ellipticity analysis. The two methods focus on the local in-situ stress estimation and complement each other to improve understanding of the in-situ stress state.
The Mini-frac experiments were conducted to infer the minimum principal stress magnitude from fracture closure, shut-in and jacking pressure. In summary, the minimum principal stress has been most accurately estimated from the shut-in pressure. Based on the pressure decay analysis after the shut-in, a minimum principal stress of 15.6 ± 2.8 MPa was measured using the inflection point
method inside the short boreholes. An increase of the pore pressure with the depth from 2.5 to 3.7 MPa was detected. In the Ellipticity analysis, the borehole deformation is investigated using Acoustic televiewer data (ATV). An elliptical fitting on the borehole shape allowed inferring a continuous stress orientation profile along the boreholes. Hence, a maximum stress orientation of 77° ± 29°N was measured along the short boreholes and 151° ± 23°N along the long borehole.
Significant spatial intra- and inter-borehole variations of the prevailing local in-situ stress were observed. Natural and induced fractures and fault zones reveal to decrease and locally rotate the in-situ stress. Further, a significant influence of tunnel-induced effects on the stress along the boreholes was observed up to a distance of 4 times the tunnel diameter.
Compared to previous stress measurements, the combination of the two chosen methods allows a profound description of the stress distribution and anomalies in the underground of the Bedretto tunnel. This leads to extended understanding of fault reactivation, tunnel-induced effects, in-situ stress variability from local heterogeneities and fracture behaviour inside the crystalline underground. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000592213Publication status
publishedContributors
Examiner: Ma, Xiaodong
Examiner: Hertrich, Marian
Examiner: Zhang, Shihuai
Examiner: Bröker, Kai
Publisher
ETH ZurichSubject
Geomechanics; In-situ stress; Bedretto Underground Laboratory for Geosciences and Geoenergies; Hydraulic fracturing; Ellipticity analysisOrganisational unit
03465 - Löw, Simon (emeritus) / Löw, Simon (emeritus)
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ETH Bibliography
yes
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