Simulation of Adsorption–Desorption Behavior in Coal Seam Gas Reservoirs at the Molecular Level: A Comprehensive Review

This work reviews the processes of methane (CH4) and carbon dioxide (CO2) adsorption and desorption, as well as the displacement of CH4 by CO2, at the molecular level. For the adsorbate, laboratory experiments and numerical simulations have been carried out to examine the adsorption and desorption behavior of CH4 and CO2. For the adsorbent, graphite, graphene, carbon nanotube, and heteroatom-containing carbon-based models have been developed for adsorption and desorption processes. In recent decades, simulations of carbon dioxide enhanced coalbed methane (CO2-ECBM) have achieved significant progress. This success has generated significant interest among the scientific communities, as coalbed methane is one of the cleanest forms of energy. Furthermore, an improved understanding of this process can help to sequester CO2 in deep unminable coal seam gas reservoirs to mitigate CO2 emission. This work reviews the advantages and weaknesses of three simulation methods, namely, molecular dynamics (MD), Monte Carlo (MC), and the density functional theory (DFT), with respect to their applications in CO2-ECBM and CO2 sequestration in CBM. Based on such simulations, dynamic and thermodynamic properties have been analyzed to evaluate the effectiveness of CO2-ECBM and CO2 sequestration. In recent studies, MD simulation has been commonly used to optimize the configuration of the adsorbate and adsorbent, as well as to define the self-diffusion coefficient and mean squared distance. However, this approach only works for a small atomic system or a short time simulation process. MC, on the other hand, is used to predict the adsorption behavior of pure component and binary and ternary mixtures of adsorbates. Nevertheless, the simulation cannot obtain information on the dynamic processes. Meanwhile, DFT is commonly used to predict the adsorption site, orientation, and adsorption energy. Dynamic and thermodynamic properties are discussed based on the three simulation methods. Certain aspects facing the future are also addressed.