Quantitative 3D spatial characterization and flow simulation of coal macropores based on μCT technology

- A numerical model for coal is built and the equivalent pore network model extracted.
- The pore space is characterized and quantitatively analyzed.
- CBM flow in macropores is simulated and the absolute permeability calculated.

The macropores in coal are an important link connecting nanometre-scale pores and millimetre-scale fractures. The 3D spatial characterization and flow simulation of macropores laid the foundation for the study of Coal Bed Methane (CBM) migration in pore-fractures. A 3D digital coal model with macropores (>13.85 μm) was constructed by using μCT scanning combined with Avizo image processing, and the equivalent pore network model (PNM) was extracted to calculate the absolute permeability of the coal samples. The CBM flow process in macropores is simulated by COMSOL, and the streamline distribution for the pressure field and velocity field are revealed. The absolute permeability in the X-, Y-, and Z-directions is calculated. The results showed that the macropores have good connectivity in 3D space, and the shapes of the macropores are either spherical or sheet-like. The mean coordination number of the pore throat is 2.55 and the mean throat radius is 43.76 μm. The differences between the permeability simulation calculation method and representative elementary volume (REV) selection scale lead to different errors in the measured results, but the permeability values are still of the same order of magnitude. Among them: when REV is 200 × 200 × 200 voxels, the equivalent PNM calculation result and the test result of actual coal sample have small error, when REV is the 50 × 50 × 50 voxels, the finite element simulation result and the actual coal sample have a relatively large error. Along the gas flow direction, the pore pressure decreases, the flow rate becomes larger; because of the heterogeneity of pore structure, the micro-fracture velocity increased rapidly, especially where the fractured channel was narrow, and the CBM flow rate suddenly increased. In some complicated pore structures this inevitably leads to a gas stagnation and no flow occurred.