Molecular Simulation of the Competitive Adsorption of Methane and Carbon Dioxide in Kerogen

Abstract

Carbon dioxide enhanced shale gas extraction (CO2-EGR) technology is of great significance for shale gas exploration and carbon dioxide storage in subsurface, which involves the competitive adsorption in shale during the displacing process. Kerogen matrix and slit models are built with the real type II-A kerogen macromolecules and the adsorption of CH4 and CO2 in the kerogen models are modelled. It is seen: 1) the gas absolute adsorption increases with its molar fraction while decreases with temperature. The Langmuir pressure for CO2 decreases as its molar fraction increases due to its high adsorption capacity, while that for CH4 increases with its molar fraction. In addition, the Langmuir amount of CO2 is more sensitive to temperature. The adsorption selectivity of CO2 over CH4 decreases with the increase in pressure and the CO2 fraction. And water significantly reduces the gas adsorption especially that in the matrix. 2) CO2 has high affinity to the Sulfur- and Nitrogen-containing functional groups, while CH4 molecules mainly adsorb on the Sulfur-, Nitrogen- and Carbon- containing functional groups, though these affinities are weak compared with CO2. While water molecules are strongly bound to the Oxygen- containing functional groups with water cluster formed at high water contents. 3) Lower interaction energies are shown in the matrix compared with the slit due to the adsorption superposition. The water interaction energy is lowest due to the hydrogen bond, while the interaction energy of CO2 is much smaller than that of CH4 indicating its adsorption advantage.

Keywords: Competitive adsorption, kerogen, water content, adsorption selectivity