Abstract
As kerogen is the main organic component in shale, the adsorption capacity, diffusion and permeability of the gas in kerogen plays an important role in shale gas production. Based on the molecular model of type II kerogen, an organic nanoporous structure was established. The Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) methods were used to study the adsorption and diffusion capacity of mixed gas systems with different mole ratios of CO2 and CH4 in the foregoing nanoporous structure, and gas adsorption, isosteric heats of adsorption and self-diffusion coefficient were obtained. The selective permeation of gas components in the organic pores was further studied. The results show that CO2 and CH4 present physical adsorption in the organic nanopores. The adsorption capacity of CO2 is larger than that of CH4 in organic pores, but the self-diffusion coefficient of CH4 in mixed gas is larger than that of CO2. Moreover, the self-diffusion coefficient in the horizontal direction is larger than that in the vertical direction. The mixed gas pressure and mole ratio have limited effects on the isosteric heat and the self-diffusion of CH4 and CO2 adsorption. Regarding the analysis of mixed gas selective permeation, it is concluded that the adsorption selectivity of CO2 is larger than that of CH4 in the organic nanopores. The larger the CO2/CH4 mole ratio, the greater the adsorption and permeation selectivity of mixed gas in shale. The permeation process is mainly controlled by adsorption rather than diffusion. These results are expected to reveal the adsorption and diffusion mechanism of gas in shale organics, which has a great significance for further research.
Highlights
With the development and utilization of unconventional natural gas resources around the world, shale gas, as a kind of the unconventional gas, plays an important role in the exploration and development of natural gas reservoirs [1,2,3]
The results showed that, if the kerogen pore is water wetted, the water molecules are adsorbed near the functional groups and the methane molecules were adsorbed on the surface of the organics
According to the principles of Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD), we studied the interactions between kerogen and theAccording different proportion of CH4of
Summary
With the development and utilization of unconventional natural gas resources around the world, shale gas, as a kind of the unconventional gas, plays an important role in the exploration and development of natural gas reservoirs [1,2,3]. Researchers have studied the mechanical properties of shale rock and the migration of shale gas through experimental and theoretical methods [23]. Botan et al used microporous carbon materials to replace the organic structure of shale and coal and simulated the separation process and the permeability of natural gas. Deep examination of this problem, the adsorption and diffusion process of shale gas and coalbed methane were studied using the average unit structure of organics or organic structure fragments. Scholars have researched the adsorption and diffusion of shale gas using molecular simulation methods. By simulating the adsorption and diffusion and analyzing the permeation selectivity of CO2 /CH4 mixtures in the organics, this research has important significance for the further determination of mixed gas permeability in organic pores
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