Molecular simulation of methane adsorption in nanoscale rough slits

Abstract

There are large amounts of nanoscale rough pores in shale, which provides the sites for methane adsorption. Proper understanding of the mechanisms of methane adsorption in nanoscale rough slits is important for estimating the shale gas reserves. In this study, graphene was used instead of shale to construct nanoscale slits with different widths and roughness, and the effects of roughness, slit width and pressure on the methane adsorption characteristics are investigated by using the GCMC and molecular dynamics methods. The results showed that the methane adsorption configurations are influenced by the slit width and surface roughness. The adsorption layers in rough slits are discontinuous and weak second adsorption layers are formed in large slits at high pressures. The inaccessible volume increases with the roughness resulting in the reduction of accessible volume. Therefore, both the excess adsorption and absolute adsorption decrease with the roughness, and they increase with the slit width in the microscopic scale due to the increased adsorption. While decrease slightly with the slit width in the mesoscopic scale due to the reduction of adsorbed phase density. The excess adsorption isotherms are well fitted by the Langmuir-Freundlich model. The adsorption is found more stable in smaller and rougher slits due to the superposition of adsorption potentials through the analysis of adsorbate potential energy and adsorption heat.