Digitized fracture surfaces of andesite retrieved from geothermal area


Publications

  1. Digitized fracture surfaces of andesite retrieved from geothermal area>
    . Scale-independent relationship between permeability and resistivity in mated fractures with natural rough surfaces. Geothermics. .
    Links
    • 10.1016/j.geothermics.2021.102065
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    Abstract — Geothermal systems consisting of fractures in impermeable rocks are difficult to characterize by in situ methods. In an effort to link characteristics of small-scale and large-scale fractures, this study investigated possible relations between their geophysical parameters. We upscaled the relationship between fracture permeability and formation factor in a laboratory specimen to larger fracture dimensions. Microscopic flow characteristics indicate that this relationship is related to the tortuosity of flow paths. We derived an empirical formula that directly predicts changes in fracture permeability from changes in formation factor. This relation may make it possible to monitor subsurface hydraulic activities through resistivity observations.

  2. Digitized fracture surfaces of andesite retrieved from geothermal area>
    . Elastic wave velocity changes due to the fracture aperture and density, and direct correlation with permeability: an energetic approach to mated rock fractures. Journal of Geophysical Research: Solid Earth. .
    Links
    • https://doi.org/10.1029/2021JB022639

    Abstract — In an effort to reveal the subsurface hydraulic changes in fractures by seismic monitoring, aperture-related velocity changes need to be investigated. We developed a numerical approach for calculating changes in elastic wave velocity with fracture aperture opening by determining the internal energy of a digitized fracture model based on natural rough surfaces. The simulated local elastic energy revealed that the interaction energy converged within 1.5 mm of the mean fracture position, and was insignificant unless the fractures intersected. This energetic approach clarified the aperture–velocity relationship and reproduced the experimental results. Further calculations using digital fractures with various sizes and densities demonstrated that the velocity can be accounted for by the superposition of a linear function of fracture density and quadratic function of aperture, and is insensitive to the fracture size. Although the relationship between fracture permeability and elastic wave velocity (i.e., the k–V relationship) depends on the fracture density, the offset-normalized k–V relationship shows clear linearity with the fracture density. The proposed k–V relationship as a function of the aperture and fracture density indicates that laboratory-scale fracture properties of a single fracture can be applied to multiple fractures on a larger scale. Our findings can be used to interpret temporal changes in seismic observations and to monitor fluid flow in fractures.