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

CO 2 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 CH 4 and CO 2 in various clay minerals that are an important constituent of shale at actual formation conditions are still less clear. In this work, we study CO 2 /CH 4 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, CO 2 /CH 4 molar fraction, and selectivity, are discussed in detail. The results demonstrate that the CO 2 sorption capacity in the clay mineral follows an order of montmorillonite > illite > kaolinite. CO 2 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 CO 2 /CH 4 adsorption ratio so that the first layer of CH 4 molecules can be displaced by CO 2 molecules. The replacement ratio of CH 4 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 CO 2 –CH 4 displacement efficiency. With a higher reservoir initial pressure when injecting CO 2, the EGR of adsorbed CH 4 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.