Coal Wettability: A Holistic Overview of the Data Sets, Influencing Factors, and Knowledge Gaps

Coal seams are naturally occurring geological media offering tremendous potential for gas storage. The wetting characteristics of coals at typical geological formations underpin a diverse array of processes spanning coal resource recovery, combustion, enhanced coal beneficiation, enhanced methane recovery, and CO₂ storage for sustainable energy transition. An accurate characterization of coal wettability is thus crucial and remains an active area of research. The intrinsic heterogeneity of coal surfaces and the presence of multicomponent systems add layers of intricacy to wetting behavior. In particular, coal wettability characterization is challenging because it is a complex and multifaceted function of a range of influencing parameters. These include coal geological parameters (such as coal rank, ash content, and microstructure), operating conditions (e.g., CO₂ injection pressure, coal seam temperature, and salinity), and sample conditioning factors (e.g., surface roughness, polishing, cleaning, etc.). This study develops a repository of the coal wettability data sets (using contact angle measurements, the nuclear magnetic resonance method, and spontaneous imbibition) for a range of conditions. The factors influencing coal wettability are critically analyzed and explained. We also identify potential limitations related to wettability measurement techniques and present the outlook for future research in this area. The findings suggest that the coal/CO₂/brine systems exhibit wetting characteristics from weakly water-wet to strongly CO₂-wet. The main factors contributing to the increased CO₂ wettability of coals include but are not limited to high injection pressure, low to moderate temperatures, high salinity, low ash content, and high vitrinite reflectance. Thus, this study offers a succinct analysis of the coal wettability data sets, presents an overview of cutting-edge technologies, and discusses potential future advances in this field to improve the understanding of coal wettability and the associated impact on fluid flow in the coal microstructure.