Construction of a visualization model for coal matrix pore structure based on small-angle X-ray scattering

The complex microscopic nanoporous structure within coal not only serves as a site for gas accumulation but also plays a pivotal role in determining the macroscopic transport properties of coal reservoirs. Existing gas intrusion experiments struggle to detect closed pores, and traditional pore models based on tomographic scanning technology exhibit shortcomings in constructing connectivity and realism. The study selected 12 coal samples with varying degrees of metamorphism and conducted systematic research by integrating theoretical analysis, experimental testing, and numerical simulation. At the experimental level, high-precision data were obtained using third-generation synchrotron radiation small-angle X-ray scattering (SAXS) technology and the Eiger2 detector. Combined with cryogenic liquid nitrogen adsorption experiments, this enabled detailed characterization of the surface morphology and pore distribution of coal samples. At the modeling level, the scaling factor was introduced and combined with random walk algorithms and Eulerian rotation techniques. Based on three simplified geometric models—cylinders, frustums, and ellipsoidal frustums-a three-dimensional pore structure model of coal with a size of 1 μm 3 was constructed, and corresponding pore equivalence maps were generated. The results indicate that nano-scale pores occupy a significant proportion within coal bodies, constituting the primary characteristic of coal pore structure. This surface pore structure of coal exhibits pronounced fractal characteristics, demonstrating the self-similarity of its physical structure. Upon removing the influence of mineral impurities, the pore structure data displays extremely high linearity on a double- logarithmic (ln-ln) curve, with the coefficient of determination R 2 ranged from 0.9992 to 0.9999, which validates the high reliability of the pore characterization. Furthermore, by establishing the relationship between pore diameter and equivalent length based on three fundamental geometric models, the geometric parameters of internal pores in different coal samples were quantified. The parameters of the constructed three-dimensional model were found to closely match the pore characteristics within actual coal. Additionally, adsorption significantly influences the nanopore parameters of the coal matrix. Ultimately, the constructed model profoundly reveals the physical essence of the exceptionally complex pore structure of coal from a morphological perspective, clarifying that pore structure characteristics fundamentally govern gas diffusion and migration mechanisms.