Modeling Adsorption of Gases in Nanoscale Pores Using Grand Canonical Monte Carlo Simulation Techniques

摘要

Shale gas and liquid-rich shales have become important energy sources in the US and other parts of the world.Shale rocks contain a very heterogeneous pore system.The pore size varies between micro-,meso-and macroscales.One critical phenomenon that governs the flow of hydrocarbons and is strongly affected by pore size is the adsorptive behavior.The adsorbate packing and density is influenced strongly by the force fields of the closely opposing pore walls. In this study,molecular simulation was applied to generate adsorption isotherms of pure methane,pure n-butane and methane/n-butane mixtures using grand canonical Monte Carlo(GCMC)techniques.The simulations were conducted on a slit pore model composed of parallel planar graphitic surfaces.Various pore sizes were studied ranging from 1 nm to 20 nm.The nonbonded interactions between the molecules, as well as between the molecules and the pore walls were described with Lennard-Jones potentials.To verify the model,the simulated adsorption isotherms were compared with adsorption isotherms obtained from published experimental data where the adsorption measurements of methane and n-butane were conducted on a Marcellus shale sample and carbon materials;respectively. In the pure molecule cases,results indicate that as the pore width increases,the density of the adsorbed molecules decreases due to the reduction of the fluid-wall interaction.For example,above 10 nm pore-width,the density of methane inside the slit pore becomes comparable to the methane density in the bulk.In the methane/n-butane mixture case,results indicate that the selectivity of n-butane over methane was greater than 1 at low pressures(~200 psi).Therefore,n-Butane is the more favorable component to adsorb and form a layer near the pore walls.However,at high pressures(~1,000 psi),“selectivity reversal” was observed due to size entropy effects.Preferential adsorption leads to compositional variations in the gas produced from the shale. This study provides insights on the adsorptive behavior of light and heavy hydrocarbon molecules. Determining the effect of fluid confinement on adsorption will lead to better understanding of the fluid behavior in rocks with nanoscale pores such as shales.This is especially important when the composi-tional behavior of confined multicomponent gas-condensate mixtures is investigated during depletion to develop strategies for more effective hydrocarbon recovery.