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Permeability evolution during fluid-pressure induced shear slip in saw-cut and natural granite fractures
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  • Zhiqiang Li,
  • Xiaodong Ma,
  • Xiang-Zhao Kong,
  • Martin O Saar,
  • Daniel Vogler
Zhiqiang Li
Shandong University
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Xiaodong Ma
ETH Z├╝rich

Corresponding Author:xiaodongma.rocks@gmail.com

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Xiang-Zhao Kong
ETH Zurich
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Martin O Saar
ETH Zurich
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Daniel Vogler
ETH Zurich
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Abstract

Subsurface engineering applications, such as the development of enhanced geothermal systems and unconventional oil and gas reservoirs, and the geologic disposal of CO 2 and waste water, involve fluid injection into the rock masses deep underground. Elevated fluid pressure can trigger shear slip(s) of pre-existing natural fractures, resulting in permeability changes. The mechanism of slip-induced permeability variation, however, is still not fully understood, and the permeability evolution associated with multiple slips is even more complex. We therefore performed laboratory experiments to investigate the fracture permeability evolution induced by shear slip in both saw-cut and natural fractures with rough surfaces. Our experiments show that the triggering fluid pressures required to induce slip in natural fractures are larger than in saw-cut fractures, likely due to the rougher surface of the natural fractures. We further observe that a critical shear displacement dominates the permeability evolution. When the accumulative shear displacement reaches the critical shear displacement, we find an initial permeability increase for natural fractures, followed by a perme-ability decrease after the most significant slip event. For the saw-cut fractures, the first slip shows the largest shear displacement and related permeability change, while further slips result in diminishing displacements. From the beginning to the end of all experiments, no consistent permeability increase or decrease is observed. Although the change of the fracture surface was not measured quantitatively, we found gouge material after rinsing each natural sample. tribute the slip-induced permeability evolution to the relationship between the damage of the main asperities, which decreases permeability, and the scale of the shear displacement, which increases permeability through shear dilation.