Micro-CT Three-Dimensional Digital Modeling and Seepage Simulation of Coal under Liquid Nitrogen Fracturing

The connectivity characteristics of the pore-fracture structure and its seepage characteristics are not only important for the microstructural evolution of coal but also affect the adsorption, desorption, and diffusion. This study describes the quantitative analysis of computed tomography (CT) images with three-dimensional pore and fracture structures of low-rank bituminous coals before and after fracturing by cryogenic liquid nitrogen using MIP and CT scanning techniques. The porosity and tortuosity were calculated, and the connectivity and pore-fracture spatial distribution of the coal were displayed. The results show that the porosities of the original and fractured coal samples are 6.32 and 10.39%; the pore connectivities are 0.42 and 0.75, respectively; the connectivity pore area increases by 1.326 × 10⁸ μm²; and the volume increases by 7.59 × 10¹¹ μm³. The fractured coal samples have a more complex pore structure with lower tortuosity, which is helpful for the transport and permeation of fluids. The pore network model (PNM) was extracted to reflect the connected pore-throat structure. The results showed that the frequency histograms of the pore-throat parameters and the coordination number of the fractured coal showed a log-normal distribution with a high degree of fit, and the coordination number increased by 1.81. Pore permeability network modeling was done to calculate absolute permeability. The results show that the fractured coal has obvious anisotropy and is consistent with the distribution of face porosity, and the fitted curves of permeability versus the pore-throat ratio, coordination number, tortuosity, and fractal dimension are consistent with the power function trend. There was a significant correlation between connectivity and related parameters, with the coordination number having the greatest effect. This research is of great significance for improving the efficiency of coalbed methane extraction, optimizing fracturing technology, and promoting unconventional natural gas development.