Characterization of pore-fracture networks and their evolution at various measurement scales in coal samples using X-ray μCT and a fractal method

The characteristics of pore-fracture networks, as well as their evolution in coal during loading, are important for exploration of coalbed methane. In this study, the evolution of fracture networks in coal during loading was investigated using X-ray micro-Computed Tomography (X-ray μCT) with a uniaxial compression facility. The cross-sectional slices of the coal sample were obtained by μCT scanning at ten loading levels. The physical and geometrical parameters, such as porosity, fracture aperture, fracture density, and connectivity, were obtained by processing the 3D μCT data. Along with the self-similar characteristics of the 3D fracture networks, a simplified Sierpinski-like fractal model was proposed to further examine the evolution of the pore-fracture networks with higher measurement accuracy. The results showed that the developed fractal model had the ability to characterize the geometrical characteristics of the pore-fracture networks at various measurement scales. With the decreasing measurement scale r, the porosity and volumetric fractal dimensions of the pore-fracture networks increased, which were approximately linear and logarithmic curves with respect to log (1/r), respectively. At the measurement scale of 50 μm, the longitudinal cleavage fractures, which were parallel to or perpendicular to the joints, were generated during uniaxial compression. The fracture porosity, as well as the fracture density, increased with increasing axial stress. The fracture growth process could be divided into three stages: the initial stage, the stable growth stage, and the accelerated growth stage. At the measurement scale of 0.008 μm, the pore-fracture evolution in the coal sample under uniaxial compression occurred in four phases, namely, the pore compaction phase, the production phase of new fractures, the stable development phase of main fractures, and the accelerated production and development phase of the fractures. At the measurement scale of 0.5 μm, the coal permeability varied in a V-shaped curve with axial stress.