Microscopic Structure of Coal through Liquid Nitrogen Treatment Based on Micro-CT and 3D Visualization of Permeability Simulation

Liquid nitrogen (LN2) fracturing effectively enhances the coal seam permeability. Utilizing micro-CT scanning and Avizo software, pore networks and topological models were reconstructed to analyze structural changes before and after LN2 treatment. By examining pore and throat characteristics, correlations with seepage indicators were established. Numerical simulations visualized gas flow distribution, focusing on pressure and velocity fields, and studied permeability evolution under varying pressure gradients. The results show that after undergoing LN2 treatment, the pore radius of the coal sample exhibited a significant increase within the range of 100–500 μm, with an increase of 155% compared to the dry coal sample. The pore area and volume exhibit a bimodal distribution, with the primary growth regions for pore area being between 1 × 104–5 × 105 μm2 and 7.5 × 105–7.5 × 106 μm2, and for pore volume, the main growth region is within the range of 106–2.5 × 108 μm3. Through the fitting of structural characteristic parameters and seepage characteristic parameters, it was found that the pore radius and coordination number, as well as the throat length and tortuosity, are positively correlated before and after LN2 treatment. Conversely, the throat radius is negatively correlated with the coordination number. Based on the three-dimensional (3D) reconstructed model, the visualized gas seepage in the fracture space exhibits significant heterogeneity. The permeability of the coal body shows significant differences before and after LN2 treatment. The seepage velocity along the Y- and Z-axes is significantly higher than that along the X-axis, and the permeability exhibits a nonlinear increase with a rising pressure gradient.