The Mechanism and Influencing Factors of Methane Adsorption on Shale Surfaces

Understanding the mechanism of methane adsorption on shale surfaces and the key influential factors is very critical for the accurate prediction of shale gas EUR.In this paper,a model for gas adsorption in shale was built based on the Grand Canonical Monte Carlo (GCMC) method in MS software.In this model,the molecular dynamics of shale gas were simulated to reflect gas existence state in shale reservoirs.The process of methane (CH4) adsorption in graphite with varying pore size and also on the surface of silica with different polarity was simulated using graphite to replace organic kerogen,silica crystals to replace clay minerals,and methane to replace shale gas.The purpose was to analyze the adsorption state,adsorption energy,and other parameters.The results of molecular simulation showed that the interaction between CH4 and silica surface is dominated by Coulomb force,in which the adsorption energy and the adsorption volume decrease with an increase in surface polarity.On the other hand,the interaction between CH4 and the graphite surface is dominated by van der Waals force,in which both total system energy and adsorption energy decrease with the increase in pore size.The research results in the paper indicates that organic matter is the main carrier of CH4 adsorption and that the CH4 adsorption capacity of graphite is much larger than that of silica.Further,the adsorption of CH4 on the graphite surface is a multi-layer physical adsorption,and it is the major influencing factor in CH4 reserves in shale reservoirs.