Simulation of anisotropic seepage in 3D heterogeneous coal fractured by liquid nitrogen

Liquid nitrogen (LN₂) fracturing represents an innovative waterless permeability enhancement technology, capable of effectively improving coal seam permeability. To quantify pore space distribution heterogeneity and permeability anisotropy, the pore microstructure of low-order bituminous coal after LN₂ fracturing was characterized via Micro-CT scanning. Based on the three-dimensional reconstructed gas pore model, COMSOL software was used to simulate the single-phase seepage process. The results show that the frequency distribution histograms of pore throat parameters follow a lognormal distribution. LN₂ fracturing results in an increase in the number of pore throats and an expansion of their radius range. Specifically, the average coordination number increases by 0.86, while the average tortuosity decreases by 0.75. For fractured coal samples, the Z-direction manifests the highest porosity and connectivity, with porosity increasing by 182%. The pore surface area and shape factor follow a logarithmic distribution, while the pore volume, equivalent diameter, and sphericity display a power function distribution. A positive correlation exists between pore radius and coordination number, whereas a negative correlation is observed between throat radius and pore-throat ratio. The gas flowed along the X, Y, and Z directions, respectively, with the pore pressure gradually decreasing along the flow direction. Notably, the pressure exhibited the most rapid changes in narrow and rough regions. Additionally, permeability and seepage velocity exhibited pronounced directional anisotropy, while LN₂ fracturing enhanced seepage velocities across all directions. With increasing pressure gradient, seepage velocity displayed a nonlinear upward trend in each direction.