Characterization of CO2/CH4 Competitive Adsorption in Various Clay Minerals in Relation to Shale Gas Recovery from Molecular Simulation

CO2 sequestration and enhanced gas recovery (CS-EGR) is a viable option with enormous potentials to produce shale gas. However, the microscopic competitive sorption behaviors of CH4 and CO2 in various clay minerals that are an important constituent of shale at actual formation conditions are still less clear. In this work, we study CO2/CH4 binary mixture competitive sorption in various clay minerals (montmorillonite, illite, and kaolinite) by using grand canonical Monte Carlo simulations. The effects of the clay mineral types and possible stratigraphic conditions, including temperature, pressure, CO2/CH4 molar fraction, and selectivity, are discussed in detail. The results demonstrate that the CO2 sorption capacity in the clay mineral follows an order of montmorillonite > illite > kaolinite. CO2 molecules are prone to be adsorbed on the surfaces of montmorillonite and illite nanopores with cation exchange than on the surface of the kaolinite nanopore without cation exchange. Moreover, cation exchange could distinctly increase the CO2/CH4 adsorption ratio so that the first layer of CH4 molecules can be displaced by CO2 molecules. The replacement ratio of CH4 is related to the type of adsorbent, which is independent of the original formation pressure. In addition, a case study is designed to quantify the enhanced gas recovery (EGR) and CO2-CH4 displacement efficiency. With a higher reservoir initial pressure when injecting CO2, the EGR of adsorbed CH4 gas could increase up to 28.97%. Our findings provide insights into gas mixture sorption in shale reservoirs and provide important guidelines for CS-EGR projects.