Quantitative characterization of coal microstructure and visualization seepage of macropores using CT-based 3D reconstruction

Investigation of coal microstructure based on 3D reconstruction is of great significance to the development of coalbed methane and the spatial migration of fluids in coal. In this paper, high-resolution X-ray micro-computed tomography (micro-CT) is used to perform CT scanning of the coking coal sample from Shaqu Coal Mine. Then, the quantitative identifying the pores, coal matrix and mineral in coal by using 3D visualization software AVIZO. An equivalent pore network model (PNM) for statistical pore size distribution (PSD) is established based on the "Volume Fraction module" in AVIZO. Finally, the variation of pore pressure, seepage velocity and flow path during the process of methane seepage in pore space are simulated. The results show that the distribution of pores, mineral, and coal matrix are highly heterogeneous, accounting for 8%, 3%, and 89% of the total volume, respectively. PSD statistical analysis showed that most of the pores in the sample space were distributed in the range of 500-1400 nm. With the increase of throat radius and throat length, the number of throat increased first and then decreased. Most of throat equivalent radius is less than 700 nm. The equivalent length of the throat is mainly distributed in the range of 1500-4500 nm. Under the same pressure gradient, the distribution of the pore pressure, seepage velocity and flow path in the three directions are different, which shows the high heterogeneity of the pore structure. During the methane seepage process, the pore pressure gradually decreases, and the smaller the pore radius, the more obvious the pressure change. As the pressure gradient increases, the seepage velocity of methane gradually increases, and the curve presents the obvious nonlinear relationship. In the pore structure, the sudden decrease of the pore radius in some areas causes the seepage velocity increases sharply.