The exploration and exploitation of hydrocarbon resources within coal and shale reservoirs is an engineering challenge. Well-developed internal micro-pore structures, complex sorption mechanism as well as numerous influencing factors affecting the gas flow are generally not well-accounted in the commercial life-cycle of shale gas and coalbed methane wells. Although large number of studies have been conducted to propose improved sorption models and study the influencing factors on adsorption and desorption characteristics of methane and CO₂ in coal and shale reservoirs, a systematic review of such studies for efficient understanding of the accumulated literature is missing, especially with a focus towards coal and shale reservoirs. In that context, this study presents a review of sorption characteristics of methane in coal and shale. Firstly, theoretical mechanisms for methane sorption are introduced, followed by description of sorption models. Further, three factors influencing the sorption of gas in coal and shale are described: total organic carbon and clays, pore structures, and reservoir conditions. Finally, the preferential sorption characteristics of hydrocarbons and carbon dioxide are described, and the methods to promote methane desorption for enhanced recovery are discussed, which include technologies such as gas injection, microwave heating, and hydraulic fracturing.

Spontaneous imbibition in shale is commonly observed during and after hydraulic fracturing. This mechanism greatly influences hydrocarbon recovery in shale plays, therefore attracting increasing attention from researchers. The number of related publications is increasing. In this review, we summarize the recent advances in shale spontaneous imbibition from three aspects, namely, conventional spontaneous imbibition models, common experimental methods, and key mechanisms that need to be focused on. The major influencing factors on the shale imbibition process are discussed, and promising future works are presented on the basis of the merits and demerits of the recent studies. This study discusses the mechanisms of shale-liquid interaction, the complexity of multiphase flow in shale plays, and highlights the achievements and challenges in shale imbibition research.