Molecular Dynamics Study On Methane Diffusion In Nanoporous Pores

Shale gas is an important type of unconventional natural gas. Successful development of shale gas is of significance for energy safety assurance. In the field of shale gas development still lie a number of scientific and technical problems. Gas occurrence and diffusion in nanopores of shale is a key characteristic of shale gas which controls the gas production. Most of current studies employ conventional experimental procedures and classic adsorption/diffusion theories to investigate occurrence and diffusion of shale gas. However, those methods have limited capabilities to describe the micro scale effect and to reveal the microscopic mechanism. Introducing new microscopic or mesoscopic methods to studies is becoming an important trend in this field.In this paper, molecular simulation is employed to investigate the occurrence and diffusion of methane in nanopores of shale reservoir. According to the characteristics of shale reservoir, pores in organic carbon which plays an important role in gas occurrence and diffusion, and pores in quartz/illite which are major minerals of shale, are modeled with different pore size and shapes. The united atom methane model and corresponding potential models are also employed. The grand canonical ensemble Monte Carlo simulations are carried out to acquire the configuration of methane in nanopores at typical pressure/temperature of shale reservoir, and canonical ensemble molecular dynamics simulations are further carried out to investigate the gas diffusion.The results show that, at typical pressure/temperature of shale reservoir, the difference of proportions and characteristics of adsorbate phase and free phase fundamental cause of different gas content in different pores. Under the same conditions, the proportion of absorbate phase in smaller pore is higher than that of large pore, and the proportion of absorbate phase in tube-shaped pore is higher than that in slit-shaped pore. The average density of absorbate phase in organic pores is higher than that in quartz/illite pores. The changing trends of absorbate phase and free phase are different with the change of pressure, which results in the more obvious difference of gas content in different pores at lower pressure. With the increase of pressure, the trend of two-layer adsorption shows up in nanopores. In the range of pressure/temperature of shale reservoir, the pressure change has more impact on diffusion coefficient than temperature change. At lower pressure, potential of pore wall has more effect on diffusion coefficient; however, at higher pressure, potential of methane molecules has more effect on diffusion coefficient. The predominant direction of diffusion is paralleled to the pore wall, and diffusion coefficient in this direction is more sensitive to the pressure change. Under the same conditions, diffusion coefficient increases with the increase of pore size; Diffusion coefficient in slit-shaped pore is larger than that in tube-shaped pore; Diffusion coefficient of predominant direction in quartz/illite pore is larger than that in organic pore at lower pressure.