Nanopore Surfaces Control the Shale Gas Adsorption via Roughness and Layer-Accumulated Adsorption Potential: A Molecular Dynamics Study

By performing molecular dynamic simulations, we study the methane adsorption on rough graphite surfaces prepared by rotating the graphite (0001) face with different tilt angles from 0 degrees to 90 degrees. It is found that the methane adsorption on the surface with the tilt angle of 90 degrees is much weaker than that of the (0001) surface, and density profiles of the methane adsorption region show irregular shapes of adsorption peaks compared to the common regular peak on the graphite (0001) surface. With increasing the tilt angle from 0 degrees to 90 degrees, the average density of adsorption methane increases first, then decreases, and shows a maximum value at 5 degrees. These observations could be explained qualitatively with two controlling factors. One is the surface roughness affecting the contact area for methane adsorption, and the other is the layer-accumulated adsorption potential (LAAP), newly proposed, reflecting the actual surface-methane interactions with considering different inner structures of surfaces. We believe that the average adsorption density is determined by the LAAP intrinsically but that it can be enhanced by the surface roughness when the adsorption area increases due to the zigzag shape of surface. Anyway, the widely used graphite (0001) surface is too smooth and its structure is too ordered to represent the surfaces of organic nanopores in shale samples, and one should be cautious to use its adsorption density reported previously. This study is very helpful for understanding the gas adsorption mechanisms and has potential applications in many fields, such as the prediction for shale gas reserves, the research and development of nano materials, precise surface treatments, atomic layer deposition, and so on.

Automated summary

The study found that the methane adsorption on rough graphite surfaces with different tilt angles has irregular density profiles and varying adsorption strength. The average adsorption density of methane initially increases and then decreases with increasing tilt angle, reaching a maximum at 5 degrees. The findings suggest that surface roughness and layer-accumulated adsorption potential play key roles in determining methane adsorption behavior.