Quantitative analysis of microscopic structure and gas seepage characteristics of low-rank coal based on CT three-dimensional reconstruction of CT images and fractal theory

The pores/fractures are places for gas migration and storage in coal. Their structure complexity will directly influence the pore structure parameters and gas seepage properties of coal. Therefore, we selected 6 low-rank coal samples and quantitatively studied these influences utilizing the high-resolution micro computed tomography (mu CT) and fractal theory. The total pore structure and connected pore structure parameters of 6 samples were acquired from three-dimensional (3D) reconstruction of CT images. The fractal dimensions of total pore structures (D-f), solid structures (D-s) and connected pore structures (D-c) were calculated using 3D box-counting method. The relationship between D-f and porosity was analyzed through the 3D total pore model, and the relationships between D-c and connectivity as well as permeability were analyzed through the 3D connected pore model. The results show that D-f, which is between 2.10 and 2.43, could accurately describe the fractal characteristics of coal and the solid structure of coal is not fractal. The porosities of the 6 samples are between 3.33% and 12.18% and their relationship with D-f can be defined by a monotonically increasing power function. The larger the D-f is, the larger the porosity of the tested coal sample. D-c is between 2.28 and 2.32, the connectivity is between 0.54 and 0.83, and the permeability is in the range of 1.76-11.63 x 10(-8) m(2) (except samples 1 and 6). D-c is negatively correlated with connectivity and permeability and its relationship with connectivity and permeability can be represented by the quadratic function and monotone decreasing power function, respectively.