GCMC-MD prediction of adsorption and diffusion behavior of shale gas in nanopores

In natural gas exploitation, CO2 has become an effective development method with its advantages of adsorption replacement from CH4, through continuous convection displacement and gas reservoir energy supplement for CH4. However, one limitation is how to explain the behavior of adsorption replacement based on microscopic mechanisms. In this work, grand canonical Monte Carlo incorporating with molecular dynamics (GCMC-MD) method are used to numerically simulate the adsorption and diffusion characteristics of CH4 in quartz pores with various sizes (1, 1.5 and 2 nm), temperature, pressure and CO2 content. Simulation results indicate that the adsorption amount of CH4 decreases as CO2 content and temperature rise, while it increases with increments in pressure and pore size. Meanwhile the CH4 density peak occurs only on the pore wall for 1 nm pore with a thickness in 3.8 & Aring;, while its secondary peak appears in the pore center as the pore size and pressure increase. The diffusion coefficient is proportional to the pore size and temperature, but goes down sharply with increasing pressure. Compared to CO2 diffusion properties, CH4 diffusion is more likely. According to the selectivity parameter SCH4/CO2, CH4 shows the characteristics of being easily displaced under low pressure and small pores. The energy of CH4 adsorption sites is always higher than that of CO2, and the energy of adsorption sites is higher with large pore sizes. The adsorption behaviors of CH4 and CO2 are regards as physical adsorption according to their maximum adsorption heat less than 42 kJ/mol. In addition, the increments in pressure and CO2 content can promote the average isosteric heat of adsorption. This study expands the understanding of the adsorption and diffusion behavior of shale gas in nanopores and also provides theoretical and technical support for the development of shale gas.