Characterisation of the Full Pore Size Distribution of and Factors Influencing Deep Coal Reservoirs: A Case Study of the Benxi Formation in the Daning–Jixian Block at the Southeastern Margin of the Ordos Basin

The complex geological environment in deep layers results in differences in the pore and fracture structures and states of coalbed methane (CBM) occurrences between deep and shallow coal reservoirs. The coexistence of multiphase gases endows deep CBM with both “conventional” and “unconventional” geological attributes. Based on systematically collected coal samples from the Benxi Formation in the Daning–Jixian area of the Ordos Basin, high-pressure mercury intrusion (HPMI), low-temperature N2 adsorption (LTN2A), and low-pressure CO2 adsorption (LPCO2A) experiments were conducted to characterise the pore structures across the full pore size distribution of the Benxi Formation coals. The aim of this research is to gain an in-depth understanding of the pore size distribution of full-size pores and to explore the factors influencing their pore structure and control over the gas content in coal reservoirs. The results indicate that the pore size distribution of the coal samples from the Benxi Formation in the study area is unimodal and that nanopores are present. The pore sizes are relatively small, with an average total pore volume (PV) of 0.073 cm3/g and an average total specific surface area (SSA) of 227.87 m2/g. Among these, micropores account for 92.26% of the total PV and 99.57% of the total SSA, making micropores the primary contributors to the gas storage space in the Benxi Formation coals. Mesopores and macropores contribute relatively little to the PV and SSA, which is unfavourable for CBM permeability. The development of pores in the Benxi Formation coals in the study area is influenced by the coal maturity, vitrinite content, and ash yield. Generally, the PV increases when the coal’s rank increases; an increase in the vitrinite content promotes the development of micropores, whereas a relatively high ash yield leads to decreases in the PV and SSA. The influence of the SSAs of coal pores on the gas content is reflected mainly by its effect on the adsorbed gas content. Since adsorbed gas molecules exist mainly in coal pores in the adsorbed state, the SSAs of coal pores strongly affect the storage capacity of coal for adsorbed gas.