Multi-Scale Characterization and Formation Mechanism of Pore Structure Heterogeneity in Deep-Buried Coal Reservoirs: A Case Study of the Benxi Formation, Ordos Basin

The prominent multi-scale pore heterogeneity widely developed in deep-buried coal reservoirs, which seriously restricts accurate reservoir characterization and precise resource evaluation for deep coalbed methane exploitation. Taking deep-buried coal reservoirs of the Benxi Formation in the Ordos Basin as the research target, this study integrates data from low-temperature CO2/N2 adsorption and high-pressure mercury intrusion experiments. Segmented monofractal quantification and multi-fractal singularity analysis are further adopted. Fractal differentiation characteristics, scale effects of pores at different scales, and the synergistic control mechanism of multiple geological factors were indicated. Eight segmented single-fractal dimensions (D1~D8) are defined. The results indicate an obvious scale-dependent zonal distribution of pore heterogeneity in deep-buried coal. Micropores smaller than 1.2 nm possess the largest fractal dimension and the strongest heterogeneity. They act as the primary adsorption and storage space for coalbed methane. Mesopores ranging from 1.1 nm to 8 nm have the lowest fractal dimension with the most homogeneous structure, serving as major gas migration pathways. The heterogeneity of macropores gradually increases with pore size. A continuous full-scale pore size distribution curve is reconstructed. Coal reservoirs exhibit a typical bimodal pore structure dominated by micropores and macropores, with the micropore-dominated storage and macropore-dominated seepage. Key multi-fractal indicators including Δα, Δf and the Hurst index are used for quantitative comparison. Micropores display strong aggregation and weak interpore connectivity. Mesopores own superior connectivity, while their heterogeneity differs greatly between individual samples. Macropores feature moderate aggregation and connectivity. Coalification degree, organic macerals, clay minerals and industrial parameters are associated with the formation and differentiation of pore heterogeneity. Each factor differentially regulates the fractal evolution of pores across various scales.