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Influence of gouge thickness and grain size on permeability of macrofractured basalt

Wang, G; Mitchell, TM; Meredith, PG; Nara, Y; Wu, Z; (2016) Influence of gouge thickness and grain size on permeability of macrofractured basalt. Journal of Geophysical Research: Solid Earth , 121 (12) pp. 8472-8487. 10.1002/2016JB013363. Green open access

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Abstract

Fractures allow crystalline rocks to store and transport fluids, but fracture permeability can also be influenced significantly by the existence or absence of gouge and by stress history. To investigate these issues, we measured the water permeability of macrofractured basalt samples unfilled or infilled with gouge of different grain sizes and thicknesses as a function of hydrostatic stress and also under cyclic stress conditions. In all experiments, permeability decreased with increasing effective pressure, but unfilled fractures exhibited a much greater decrease than gouge-filled fractures. Macrofractures filled with fine-grained gouge had the lowest permeabilities and exhibited the smallest change with pressure. By contrast, the permeability changed significantly more in fractures filled with coarser-grained gouge. During cyclic pressurization, permeability decreased with increasing cycle number until reaching a minimum value after a certain number of cycles. Permeability reduction in unfilled fractures is accommodated by both elastic and inelastic deformation of surface asperities, while measurements of the particle size distribution and compaction in gouge-filled fractures indicate only inelastic compaction. In fine-grained gouge this is accommodated by grain rearrangement, while in coarser-grained gouge it is the result of both grain rearrangement and comminution. Overall, sample permeability is dominated by the gouge permeability, which decreases with increasing thickness and is also sensitive to the grain size and its distribution. Our results imply that there is a crossover depth in the crust below which the permeability of well-mated fractures (e.g., joints) becomes lower than that of gouge-filled fractures (e.g., shear faults).

Type: Article
Title: Influence of gouge thickness and grain size on permeability of macrofractured basalt
Open access status: An open access version is available from UCL Discovery
DOI: 10.1002/2016JB013363
Publisher version: http://dx.doi.org/10.1002/2016JB013363
Language: English
Additional information: ©2016. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Keywords: Science & Technology, Physical Sciences, Geochemistry & Geophysics, Effective Pressure Law, Fault Gouge, Confining Pressure, Mechanical-Behavior, Physical-Properties, Deformation Bands, Westerly Granite, Pore Pressure, Rock Joints, Stress
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1545078
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