Multiscale storage and transport modeling in unconventional shale gas: A review

Several studies have been performed to model the storage and transport in organic matter present in shale gas. However, the unconventional formation does not only contain organic matter but also inorganic mineral matrix. The studies necessitated by this research gap showed that quartz pores contribute more to the amount of free gas present in shale while clay minerals play a significant role to the adsorbed gas storage capacity, especially in rocks containing low total organic content. Interestingly, methane and a small amount of carbon dioxide, ethane, propane, nitrogen, and water are trapped within the mineral pores and the interphase between two or more minerals. The gases present are stored in a sorbed and free state at different pore scales, which makes the modeling challenging. More so, the adsorbed gas layer influences gas transport through the pore structure. In this work, an extensive review of experimental and simulation outlook to these subjects was considered, with emphasis on key highlights and limitations to previous studies. Besides, there was a recap of commonly utilized field perspectives to compute the producible gas from shale reservoirs, and an insight into an experimental validation point of view. Analytical and numerical transport description methods were broadly debated. Furthermore, there was an overview of the widely availed relationship between heat with chemical and physical adsorption, including the process for evaluating the efficiency of CO₂ sequestration and enhanced gas recovery projects in shale reservoirs alongside the possible drawbacks in their application.