Study on the microscopic pore structure and seepage mechanism of the Luohe Formation sandstone in the roof of coal seams in the Huanglong Coalfield

The Huanglong Coalfield faces the threat of water hazards from the thick Luhe Formation sandstone roof. This study focuses on the coarse, medium, and fine sandstones of the Luhe Formation in Gaojiabu Coal Mine, and employs a comprehensive suite of techniques, including SEM-EDS, XRD, µ-CT, Rietveld refinement, Avizo, and Comsol to systematically investigate the micro-pore structure characteristics and seepage mechanisms of the three types of sandstones. The results show that in the two-dimensional pore structure, the pore types of the three sandstones are mainly intergranular pores and dissolution pores, with pore-throat types dominated by constricted and curved lamellar throats. The porosity (19.24%, 13.34%, 12.27%), pore size distribution ranges (2.26-401.05 μm, 2.26-206.54 μm, 2.26-181.50 μm), fractal dimensions (1.499, 1.488, 1.453), and average grain diameters (268.494 μm, 242.234 μm, 175.781 μm) of the coarse, medium, and fine sandstones decrease progressively. In terms of mineral composition, the three sandstones are primarily composed of feldspar and quartz. The three-dimensional pore structure shows that the connected porosity (12.175%, 10.790%, 9.949%), pore radius distribution range (3.102-180.119 μm, 3.102-165.043 μm, 3.102-152.544 μm), average pore-throat radius (20.777 μm, 16.804 μm, 15.984 μm), and average coordination number (2.499, 1.632, 1.502) of coarse, medium, and fine sandstones decrease in sequence, while the fractal dimension of connected pores (2.333, 2.344, 2.354) and average tortuosity (1.672, 1.719, 1.822) increase in sequence. In terms of seepage numerical simulation, the connectivity (98.222%, 94.478%, 92.218%) and permeability (1.104 D, 0.310 D, 0.135 D) of coarse, medium, and fine sandstones decrease in sequence, while the frequency of low-speed zones (66.59%, 78.95%, 88.50%) increases in sequence. The research findings can provide a theoretical basis for the roof pre-grouting water-blocking and water-reducing projects in Gaojiabu Mine of the Huanglong Coalfield.