The dependence of fracture mechanical and fluid flow properties on fracture roughness and sample size

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Abstract
A parameter study has been carried out to investigate the interdependence of mechanical and fluid flow properties of fractures with fracture roughness and sample size. A rough fracture can be defined mathematically in terms of its aperture density distribution. Correlations were found between the shapes of the aperture density distribution function and the specific fractures of the stress‐strain behavior and fluid flow characteristics. Well‐matched fractures had peaked aperture distributions that resulted in very nonlinear stress‐strain behavior. With an increasing degree of mismatching between the top and bottom of a fracture, the aperture density distribution broadened and the nonlinearity of the stress‐strain behavior became less accentuated. The different aperture density distributions also gave rise to qualitatively different fluid flow behavior. Findings from this investigation make it possible to estimate the stress‐strain and fluid flow behavior when the roughness characteristics of the fracture are known and, conversely, to estimate the fracture roughness from an examination of the hydraulic and mechanical data. Results from this study showed that both the mechanical and hydraulic properties of the fracture are controlled by the large‐scale roughness of the joint surface. This suggests that when the stress‐flow behavior of a fracture is being investigated, the size of the rock sample should be larger than the typical wave length of the roughness undulations.

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