Research on Coal Reservoir Pore Structures: Progress, Current Status, and Advancing

Coalbed methane (CBM) storage and transport are facilitated by an intricate multi-scale pore structure. It is of great significance to study the characteristics of the pore structure and its role in CBM storage and transport in order to enhance CBM extraction, prevent CBM disasters, and improve the efficiency of CO₂ geological storage. Here, we review the current progress in coal reservoir pore structure research worldwide based on 8199 published papers on "coal pore structure" identified from the Web of Science Core Collection database. Using a bibliometric method with high-frequency core keywords as important database quantitative indices, five clusters with high-frequency keywords were selected as the core content to provide a comprehensive review of the progress of research on the pore structure of the coal body. The findings indicate that, with global attention focused on the storage of greenhouse gases, such as CO₂, and clean energy extraction of CBM, research on pore structure of coal rock reservoirs has increased rapidly since 2010, with studies from China, the USA, Australia, Poland, and Japan the most abundant. With the development of testing technology, research on the basic parameters of coal pore structure, the intrinsic mechanism of pore formation, and the factors influencing the evolution of pore structure has evolved from the macroscopic to the micromolecular level, and from qualitative descriptions to quantitative or semi-quantitative characterization. From keyword analysis, it is evident that the control mechanisms of pore structures with regard to adsorption–desorption–diffusion–seepage of CBM in coal reservoirs have received considerable attention. The development of technologies such as molecular simulation provides important technological support for analyzing the intrinsic mechanisms competitive CO₂, CH₄, and N₂ adsorption in coal–rock reservoirs at the molecular level. The development of molecular dynamics simulations and digital imaging technology will provide crucial support for the quantitative in situ characterization of pore structures and other physical parameters of unconventional reservoirs, such as coal and rock. Moreover, studying the microscopic mechanisms of gas adsorption and fluid flow in porous systems under extreme conditions (e.g., high temperature, high pressure, ultra-microscale) has become a research frontier in this field.