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Critical Factors Controlling Shale Gas Adsorption Mechanisms on Organic/Inorganic Minerals Investigated Using GCMC Simulations

Abstract

Understanding the adsorption mechanisms of different gas molecules on various minerals is crucial for accurately modelling shale gas adsorption behaviors and for objectively evaluating adsorbed gas contents under geological conditions. We simulated the adsorption behaviors of CH4, CO2 and N2 on both organic matter and inorganic minerals at 60 ºC and 90 ºC over a range of pressures up to 50 MPa by using the Grand Canonical Monte Carlo (GCMC) method. It has been found that both the comprehensive effect of the adsorption sites with differential adsorption capacity and the distribution density of the adsorption sites on the organic matter and inorganic mineral surfaces control the adsorption capacity in terms of per unit surface area of minerals. For individual minerals with a certain adsorption capacity in terms of per unit surface area, the specific surface area of individual minerals is the critical factor that determines the adsorption capacity in terms of per unit mass of the minerals. The interaction among gas molecules also affects the adsorption behavior slightly. We further compared the adsorption capacity among various gas molecules on both organic matter and inorganic minerals by inspecting the strength and distribution density of the adsorption sites on mineral surfaces, the specific surface area of the minerals and the interaction strength among gas molecules. For CH4 (with no polarity), the stronger intensity and the larger density of the adsorption sites on the kerogen surface make its adsorption capacity significantly larger than that on the clay surfaces in terms of per unit surface area. The difference of the adsorption capacity of both CO2 and N2 (with polarity) between on the kerogen surface (with weaker intensity but larger density of adsorption sites) and on the clay surface (with stronger intensity but smaller density of adsorption sites) is not distinct. The specific surface area of the kerogen is much larger than that of clay minerals, leading to a much stronger adsorption capacity in terms of per unit mass of the kerogen than that of the clay minerals. These investigations allowed us to identify the critical factors controlling shale gas adsorption mechanisms on different minerals, which provide some helpful insights for both of the exploration and the development of shale gas.