Geometrical description and permeability calculation about shale tensile micro-fractures

Abstract

Abstract: To study fluid flow in shale tensile micro-fractures, five shale core samples, taken from Barnnet Shale, were splitted into artificial tensile fractures based on Brazilian test. The morphology of the artificial fractures was obtained by a 3-D laser sensor profilometer. Then, 3-D information was transformed into 2-D information and the quantitative parameters, such as tortuosity, surface angularity and roughness, were calculated based on the scanning principle. Cluster analysis was introduced to make sure the distance among the parameters, the fluid flow in shale micro-fractures with the opening of 0.05−0.40 mm was simulated by Lattice Boltzmann Method (LBM), and an equation was derived for calculating the shale tensile micro-fracture permeability. The results show that, the tortuosity of the samples is close to 1.10, the angularity is among 0.99°–8.86°, and the roughness is among 0.062−0.162 mm; the parameters cannot be substituted by one another and their effects should be considered at the same time; the micro-fracture permeability is less 19%−29% than the parallel plate model permeability, so the roughness should be included. It is verified that the deviation of the equation is less than 4%, and it can be used to calculate shale tensile micro-fracture permeability. Key words: shale, tensile micro-fracture, fracture morphology, fracture tortuosity, fracture angularity, fracture roughness, fluid flow simulation, fracture permeability model