Three-dimensional digitalization modeling characterization of pores in high-rank coal in the southern Qinshui basin

Pore connectivity is an important property of coal. To explore the connectivity of pore-fractures in terms of macropores and mesopores in high-rank coal, two coal samples collected from the coal seam #3 in the southern Qinshui basin were selected. A pore-fracture network model of high-rank coal on the nanometer (10-100 nm) to micrometer (0.1-10 m) scale is constructed, and key parameters are extracted using the 3D (three-dimensional) digital spatial characterization based on 3D scanning with FIB-SEM (Focused Ion Beam Scanning Electron Microscopy). Then, the connectivity of the pore-fractures and the contribution of pores with different genetic types to the connectivity of the high-rank coal are confirmed. The results show that the pores and throats of high-rank coal in coal seam #3 in the southern Qinshui basin are very narrow, with predominant mesopores < 50 nm in width. The tortuosity of the coal samples is low, and the cross-section is predominantly square and triangular in shape, which means that the capillary resistance is small. The connectivity of the pores is poor, and mesopores play an important role in the pore connectivity. Linear differential shrinkage pores are the main connected pores on the nanometer scale and communicate with irregularly rounded and elliptic differential shrinkage pores, secondary pores, and mineral pores. The types and contents of the minerals in coals determine the morphological characteristics and degree of development of the differential shrinkage pores, and have an important influence on the pore connectivity in high-rank coal. The content of quartz determines the degree of development of the linear differential shrinkage pores, and is the primary reasons for the differences in the connectivity of the two samples.