Anisotropy of fracture and permeability in high-rank coal analyzed using digital rock physics

Abstract

Rock microstructure analysis and physical property simulation by means of digital rock physics (DRP) can overcome the deficiencies such as poor quantification and visibility in traditional petrophysical experiments. The establishment of a complete set of DRP is the basis of analyzing the microstructure of unconventional oil and gas reservoirs. In this study, high-rank coal, a material with anisotropic fracture and permeability properties, was tested using x-ray computer tomography scanning to reconstruct the digital core, and the fracture direction and structural characteristics were quantified with respect to the main inertia axis and the Feret diameter, respectively. A method for calculating the fractal dimension and tortuosity based on DRP is presented, and the optimal interaction between the lattice Boltzmann method seepage simulation and DRP is identified. The results show that the average length, width, aperture, and volume of fractures in the direction of face cleat (DFC) are 1.13, 1.10, 1.11, and 1.09 times that in the direction of butt cleat (DBC), respectively, and their surface area, count, fracture porosity, fractal dimension, and tortuosity are 1.17, 1.16, 1.26, 1.04, and 1.10 times that of DBC, respectively. The permeability of DFC was found to be 3.46 times that of DBC. This study presents an effective method for determining the dominant direction of fracture structure and fluid migration that is not limited to pores and fractures in rock but can also be used to characterize the physical properties of skeletons or solid particles in other materials.