How Does In Situ Stress Rotate Within a Fault Zone? Insights From Explicit Modeling of the Frictional, Fractured Rock Mass

Zhang, Shihuai; Ma, Xiaodong

doi:10.1029/2021jb022348

Cited by 19 publications

(10 citation statements)

References 93 publications

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“…In addition, the systematic breakout rotation potentially suggests overall increasing deformation (and thus increasing stress drop) toward the fault zone center. This inference is generally consistent with other field observations (Faulkner et al., 2010) and model predictions based on fracture deformation (Zhang & Ma, 2021b). If this is the case for the major fault zone in CB1, it can be assumed that the stress ratio decreases rapidly from 0.9 to a lower value from 150 to 160 m MD.…”

Section: Discussionsupporting

confidence: 91%

“…The mechanisms responsible for the stress variations around fault zones are intricate. According to the fact that fault zones are usually composed of one or multiple fault core(s) and damage zones (e.g., Caine et al., 1996; Chester et al., 1993; Faulkner et al., 2010; Mitchell & Faulkner, 2009; Ross et al., 2020), stress variations near fault zones are likely to be the consequence of cumulative deformation and the accompanying changes of mechanical properties across the fault zones (Faulkner et al., 2006; Healy, 2008; Heap et al., 2010; Zhang & Ma, 2021b). Such explanations can be plausibly validated by the focal mechanism stress inversion results associated with tectonic fault zones at seismogenic depths (Hardebeck & Hauksson, 1999; Provost & Houston, 2001).…”

Section: Introductionmentioning

confidence: 99%

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Fault Zone Spatial Stress Variations in a Granitic Rock Mass: Revealed by Breakouts Within an Array of Boreholes

Zhang

Bröker

et al. 2023

JGR Solid Earth

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The in situ stress state within fault zones is technically challenging to characterize. At the Bedretto Underground Laboratory in the Swiss Alps, the breakouts observed in an array of eight inclined boreholes penetrating a fault zone offer a unique opportunity to characterize the fault‐associated spatial stress variations. Synthesizing multiple geophysical logs, natural geologic structures intersecting these boreholes are identified, revealing a hierarchy of a major fault zone along with secondary structures. Within the boreholes, breakout rotations occur over multiple scales, spanning individual fractures and the entire major fault zone. We first estimate and rule out the effect of the fracture‐induced anisotropy on the breakout rotations, which are attributed mainly to the stress variations. Based on the stress field around a circular borehole and Mohr‐Coulomb failure criterion, the observed breakout azimuths are used to invert the stress information. Results show that the stress field outside the fault zone features a stress ratio (quantifying the relative stress magnitude) of about 0.9, an inclined overburden stress (inclination: 12°∼18°), and a maximum horizontal principal stress (SHmax) oriented N100∼120°E. Within the fault zone, a substantial reduction of the stress ratio and complicated stress rotations are constrained, likely induced by the stress drop on local fractures. As a result, less critical stress state inside the major fault zone is expected. Our work provides a semi‐quantitative estimation of the in‐situ stress variations around fault zones in the absence of direct stress measurements, which is beneficial to a number of scientific and engineering applications.

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Section: Discussionsupporting

confidence: 91%

Section: Introductionmentioning

confidence: 99%

Fault Zone Spatial Stress Variations in a Granitic Rock Mass: Revealed by Breakouts Within an Array of Boreholes

Zhang

Bröker

et al. 2023

JGR Solid Earth

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“…Conversely, the latter needs to be minimized concerning certain underground facilities (e.g., CO 2 storage, nuclear waste disposal, X. Ma et al: Multi-disciplinary characterizations of the BedrettoLab tunnels), in which fluid flow should be regulated or even prevented (Zoback and Gorelick, 2012).…”

Section: Introductionmentioning

confidence: 99%

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

Hertrich

Amann

et al. 2022

Solid Earth

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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.

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“…Higher stresses acting on long boreholes often come with borehole deformation, especially borehole breakouts [ 17 ] because the boreholes are located outside of the stress shadow of the underground cavities. In addition, faults or other zones of weakness can cause steps in the borehole direction and reduce the borehole smoothness further.…”

Section: Challenges In the Instrumentation Of Meso-scale Experimentsmentioning

confidence: 99%

“…Pictures taken of monitoring boreholes MB1, MB3, and MB4 of the BRP project. Figure 2 b modified from [ 17 ].…”

Section: Figurementioning

confidence: 99%

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

Plenkers

Reinicke

Obermann

et al. 2023

Sensors

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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.

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How Does In Situ Stress Rotate Within a Fault Zone? Insights From Explicit Modeling of the Frictional, Fractured Rock Mass

Cited by 19 publications

References 93 publications

Fault Zone Spatial Stress Variations in a Granitic Rock Mass: Revealed by Breakouts Within an Array of Boreholes

Fault Zone Spatial Stress Variations in a Granitic Rock Mass: Revealed by Breakouts Within an Array of Boreholes

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

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

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