Prediction of Adsorption and Diffusion of Shale Gas in Composite Pores Consisting of Kaolinite and Kerogen using Molecular Simulation

Natural gas production from shale formations is one ofthe mostrecent and fast growing developments in the oil and gas industry.The accurate prediction of the adsorption and transport of shale gasis essential for estimating shale gas production capacity and improvingexisting extractions. To realistically represent heterogeneous shaleformations, a composite pore model was built from a kaolinite slitmesopore hosting a kerogen matrix. Moreover, empty slabs (2, 3, and4 nm) were added between the kerogen matrix and siloxane surface ofkaolinite. Using Grand-Canonical Monte Carlo (GCMC) and moleculardynamics (MD) simulations, the adsorption and diffusion of pure methane,pure ethane, and a shale gas mixture were computed at various highpressures (100, 150, and 250 atm) and temperature of 298.15 K. Theaddition of an inner slit pore was found to significantly increasethe excess adsorption of methane, as a pure component and in the shalegas mixture. The saturation of the composite pore with methane wasobserved to be at a higher pressure compared to ethane. The excessadsorption of carbon dioxide was not largely affected by pressure,and the local number density profile showed its strong affinity tokerogen micropores and the hydroxylated gibbsite surface under allconditions and pore widths. Lateral diffusion coefficients were foundto increase with increasing the width of the empty slab inside thecomposite pore. Statistical errors of diffusion coefficients werefound to be large for the case of shale gas components present atlow composition. A larger composite pore configuration was createdto investigate the diffusion of methane in different regions of thecomposite pore. The calculated diffusion coefficients and mean residencetimes were found to be indicative of the different adsorption mechanismsoccurring inside the pore.

Automated summary

Shale gas production is a rapidly growing sector in the oil and gas industry, and accurate prediction of its adsorption and transport is crucial for estimating production capacity. This study used molecular simulations to investigate the adsorption and diffusion of methane, ethane, and shale gas in a composite pore model representing heterogeneous shale formations. The addition of an inner slit pore significantly increased the adsorption of methane, and the saturation of the composite pore with methane occurred at a higher pressure than ethane. Carbon dioxide adsorption was not strongly affected by pressure, and its affinity to kerogen micropores was observed in all conditions. Diffusion coefficients were found to increase with the width of the empty slab inside the composite pore. The results provide insights into the adsorption mechanisms occurring inside the pore.