Influence of three-dimensional roughness of rock fracture on seepage characteristics based on the digital image technology

Digital image processing technology can objectively reflect the surface roughness of coal and rock mass fractures and turn physical properties such as fluctuation height to physical data of the rock fracture surface to be used for numerical analyses; thus, it can effectively be applied to the seepage flow analysis of rock fractures. First, clear digital images of the fracture surface height are obtained under the same camera conditions, and then characteristic function values specific to various structure are normalized. Subsequently, the fluctuation height distribution of the rock surface is restored according to the measured fluctuation heights of local points. Finally, according to the surface fluctuation of fractures, invasive restructuring is carried out for three-dimensional fractures in the same coordinate system. Obtained physical parameters on the three-dimensional fractures of the rock are inputted into the COMSOL Multiphysics software to obtain the characteristic three-dimensional model of the rock fracture surface. The numerical analysis results are compared with experimental data on fracture seepage obtained from a seepage coupling true triaxial test system, and the relevance between the simulated result and the physical experiment is higher than 90%, which confirms that the integration of the digital image technology and the numerical analysis method can effectively simulate seepage in rough fractures. The hydraulic gradient of rough fractures and seepage velocity are also consistent with Forchheimer flow characteristics.