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Hydro-mechanical measurements and x-ray imaging of sheared crystalline rock fractures from EGS Collab experiments 1 & 2
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  • Meng Meng,
  • Luke P Frash,
  • Wenfeng Li,
  • Nathan J Welch,
  • James William Carey,
  • Craig Ulrich,
  • Timothy J Kneafsey
Meng Meng
Los Almaos National Laboratory

Corresponding Author:[email protected]

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Luke P Frash
Los Alamos National Laboratory (DOE)
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Wenfeng Li
Los Alamos National Lab
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Nathan J Welch
Los Alamos National Laboratory (DOE)
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James William Carey
Los Alamos National Laboratory
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Craig Ulrich
Lawerence Berkeley National Laboratory
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Timothy J Kneafsey
Lawrence Berkeley Laboratory
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Measuring hydro-mechanical properties of natural fractures is a prerequisite for optimizing hydraulic stimulation design and well placement. We completed experiments to characterize shear on natural fractures in schist, amphibolite, and rhyolite specimens drilled from EGS Collab Project’s field sites at the Sanford Underground Research Facility (SURF) in South Dakota. A triaxial direct shear method and coupled x-ray imaging were used to perform hydroshearing and mechanical shearing at the site’s in-situ stress conditions. This produced simultaneous measurements of fracture and matrix strength, permeability, stress-dependent aperture, dilation, and friction strength. Our results identified that only a subset of the natural fractures was weak enough for hydroshearing. Generally, hydroshearing increases fracture permeability by a factor of 10 or more and the enhancement is retainable over time. However, the shear slip does not always result in permeability enhancement. High content of phyllosilicates was found to associate with exceptionally weak fractures that also exhibited poor or even negative enhancement after stimulation. Combining our measurements with site data, we can predict that most observable fractures at the two EGS Collab sites do not meet the criteria for hydroshearing before tensile opening. In some cases, the visible fractures are low permeability and as strong as the adjacent rock. To induce hydroshearing before tensile opening, injection must target known weak and favorably oriented fractures with confirmed pre-existing permeability.