Molecular simulation of methane adsorption capacity in transitional shale – a case study of Longtan Formation shale in Southern Sichuan Basin, SW China

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It is of great significance to clarify the shale gas occurrence mechanism under formation condition for the exploration and development of transitional shale gas reservoir. Since the formation temperature and pressure are higher than that in facilities, to reveal the methane adsorption characteristics in Longtan Formation, the organic and inorganic slit pore models in shale were constructed by molecular simulation technology. The grand canonical Monte Carlo (GCMC) method and the molecular dynamics method were used to simulate methane’s adsorption behavior on inorganic minerals and graphite. The research results found that the methane isosteric heat on graphite, montmorillonite, illite, and quartz with different pore sizes is less than 42 kJ/mol, indicating that the methane adsorption on transitional shale belongs to physical adsorption. The adsorption capacity of the Longtan Fm. transitional shale main components is in the sequence of organic matter > clay mineral > quartz. The adsorption of methane on transitional shale is not rigorous monolayer adsorption, but mainly a strong adsorption layer. The effect of temperature on the total methane adsorption capacity is stable, and the excess adsorption capacity is greatly affected by pressure. The smaller the pore size, the higher the methane adsorption heat, the stronger the adsorption capacity of methane, and the excess and total adsorption capacity decrease with pore size decreasing. The transitional shale with a high content of organic matter and clay has high adsorbed gas content. Lower temperature and appropriate pressure are conductive to the adsorbed gas preservation.