Chemical Engineering Journal | odeling and evaluation on gas adsorption ratio: Insight into the coexistence mechanism of adsorbed and free gas in shales

The coexistence of adsorbed and free gas in nanopores is an important characteristic in shale gas reservoir, while the coexistence mechanism between adsorbed and free gas is still unclear. Gas adsorption ratio (GAR) can be utilized as a characteristic parameter of reflecting the coexistence of adsorbed and free gas in nanoporous shales. In this study, we developed a theoretical model to describe the GAR according to the quantum physisorption theory, and established an experimental method for evaluating the GAR using two shales from the southern Sichuan basin of China. Further, the coexistence mechanism of adsorbed and free gas in nanoporous shales was investigated by experimental data and the theoretical model. Key findings are as follows: (1) the GAR is equal to P1 × P2, where P1 is the probability of achieving adsorption of each molecule; P2 is the probability of gas molecules participating in adsorption. The P2 is primarily impacted by the minimum potential energy unit (E0), while the P1 is mainly impacted by gas temperature. (2) Under experimental temperature (30.6°C) condition, GARs were obtained as 80.06% (core S1) and 77.85% (core S2). During the whole isothermal adsorption process, about 81–83% of gas molecules participate in adsorption, and the probability of achieving adsorption for each molecule participating in adsorption is about 0.95–0.96, which microscopically explains the high GAR values. (3) The GAR is independent of gas pressure during isothermal adsorption process. Gas temperature is the most important factor leading to the conversion between adsorbed gas and free gas and plays a dominant role in the GAR variation. This study is of great help to re-understand the microscopic occurrence of methane in shale nanopores.