Sorption of Deep Shale Gas on Minerals and Organic Matter from Molecular Simulation

Deep shale gas is one of the key targets of China's natural gas exploitation in the future. Abnormally high reservoir pressures cause an unclear occurrence state of shale gas. In this paper, molecular modeling techniques are first used to establish slit-pore models of illite, quartz, and kerogen with different widths. Monte Carlo and molecular dynamics methods are second applied to simulate the sorption of methane under the geological conditions of deep reservoirs. Then, the gas distributions in the pores and changes of energies in sorption are analyzed. The applicability of the Langmuir model is also verified. The results show that (1) shale gas is approximately adsorbed as strong monolayers in a pore; the density of the adsorbed gas in mesopores is not sensitive to the pore size; (2) the Langmuir equation is applicable for the sorption of shale gas over a wide range of pressures; the surface excesses and maximum adsorbed amounts for the mesopores follow the order of illite > quartz > kerogen, which results from the different actual surface areas depending on the roughness and holes on the surfaces; (3) in a mesopore, total gas energy reduces more in adsorption under higher pressure; van der Waals force dominates gas adsorption, while electrostatic force affects weakly; under high pressure, the total gas energy for kerogen matrix decreases less when pressure increases, which is related to the extremely dense absorbed gas and nonmonotonic variation of van der Waals force with the distance between molecules; the interactions between shale gas and different types of pores follow the order of kerogen matrix > kerogen mesopore > illite > quartz. This study clarifies the sorbing behaviors of deep shale gas as well as the applicability of the Langmuir model, which can be a reference for developing the deep shale gas.