Study on the Pore Structure and Multifractal Characteristics of Medium- and High-Rank Coals Based on the Gas Adsorption Method: A Case Study of the Benxi Formation in the Eastern Margin of the Ordos Basin

Abstract

Nanopore heterogeneity significantly affects the adsorption, desorption, and diffusion processes of coalbed methane.Here, the medium-and high-rank coals of the Benxi Formation in the eastern margin of the Ordos Basin were taken as the research object, and low-pressure CO 2 adsorption and low-temperature N 2 adsorption experiments were used to study the pore characteristics; combined with multifractal theory, the multifractal characteristics of micropores (0.3−2.0 nm) and meso-/macropores (2−300 nm) in MRC and HRC are characterized and explored, and the evolutionary trends and influential factors during metamorphism are explored. The results show that the pore distribution of micropores and meso-/macropores in MRC to HRC coal has multifractal characteristics. Compared with meso-/macropores, micropores exhibit smaller Δα values and larger H values; that is, micropores have weaker heterogeneity and better pore connectivity. As the coal rank increases, the percentage of the micropore volume in the coal increases, and the pore size distribution of micropores and meso-/macropores tends to be more homogeneous, which leads to a decrease in the pore structure uniformity and improvement in the pore connectivity in coal reservoirs. An increase in the vitrinite content leads to better connectivity and a more uniform pore structure of micropores and meso-/macropores. An increase in the mineral content and A d leads to a decrease in the connectivity of micropores and meso-/macropores and tends to increase the complexity of pore structures. With increasing micropore volume and specific surface area, the degree of pore aggregation and heterogeneity decreases, and the pore connectivity increases. However, there is no significant correlation between the pore structure and the multifractal parameters of the meso-/macropore structure. The HRC samples have more micropores than the MRC samples and thus exhibit greater pore structure homogeneity and better pore connectivity. A uniform distribution and good connectivity of the pores help to improve the methane adsorption capacity.