Adsorbed gas confined in nanopores is a significant component of shale gas, and understanding the mechanisms of gas adsorption in shale nanopores is crucial for enhancing shale gas recovery and carbon dioxide geological sequestration. Due to the nanoscale pore sizes, complex pore structures, and diverse mineral types, adsorption experiments have a limited capacity to elucidate the microscopic mechanisms of gas adsorption. Compared to expensive adsorption experiments, molecular simulation methods can not only simulate reservoir in-situ conditions but also reveal the adsorption mechanisms from the molecular scale perspective. This work provides a brief review for the characteristics of methane adsorption in shale inorganic minerals and organic matter. Additionally, the competitive adsorption behavior of methane and carbon dioxide in shale is introduced to clarify the potential of shale reservoirs for carbon dioxide geological storage. Finally, the challenges faced by molecular simulation methods in gas adsorption research are discussed.

Shale oil and gas, as important unconventional resources, have been widely discussed in the last decade. The occurrence characteristics of fluids (oil, gas, and water) in shale reservoirs are closely related to the exploitation of shale oil and gas, therefore the quantitative characterization of fluid occurrence in shale reservoirs has received extensive attention. In this paper, the latest advances and potential challenges on this subject are summarized. With respect to shale oil, the amounts, ratios and micro-distributions of shale oil in different states can be determined using the state equation of liquid and adsorption ratio equation, which contributes to identifying high-quality shale oil reservoirs. However, it is still necessary to strengthen the research on the multi-attribute coupling relationship and oil-rock interaction of shale oil reservoirs, and the determination of occurrence characteristics of adsorbed and free oil under in situ reservoir conditions. In terms of shale gas evaluation, the process analysis method and isotope fractionation method effectively solve the problem of evaluating in situ gas-bearing characteristics of shale, and can accurately estimate the amounts of total, adsorbed and free gas. The quantum physisorption behavior of gas could be a new research direction to reveal the microscopic occurrence mechanism of shale gas. As for shale pore water, a complete evaluation procedure for determining the amounts and micro-distributions of adsorbed and free water in shale matrix pores has been established, which provides insight into the storage and flow of oil and gas. In future work, a study on the quantitative evaluation of water-rock interaction is significant for obtaining the adsorbed and free water under in situ reservoir conditions.

There have been many theories to describe adsorbed or free gas. However, quantitative description of the occurrence characteristics of liquids in porous media has always been a great challenge due to a lack of basic theory. Through years of research, two theoretical equations, i.e., adsorption ratio equation and state equation of liquid, have been proposed to describe the characteristics of liquids in porous media, and revealed the mechanism of liquid occurrence. Further, a quantitative evaluation technology for the microscopic distribution of liquids was established by combining nuclear magnetic resonance theory. This research will be of great significance for studying the microscopic distribution of liquids in tight reservoirs, such as shale, coal, and tight sandstone.