Competitive adsorption in CO2 enhancing shale gas: Low-field NMR measurement combined with molecular simulation for selectivity and displacement efficiency model

CO2 enhanced shale gas recovery is considered to be a potential method of shale gas development and CO2 reduction. However, core experiments cannot independently analysis competitive adsorption when the partial pressure of CH4 in the core sample keeps changing during displacement. In this study, an experimental method based on low-field nuclear magnetic resonance (NMR) and in situ gas chromatography is developed to quantitatively analyze the competitive adsorption of shale particles. As was demonstrated in our previous study, the adsorption capacity of the nanopores at various pressures can be obtained from the transverse relaxation spectrum of NMR and the change in pressure in a reference cavity. By NMR experiment combined with gas chromatography, the competitive adsorption selectivity of CH4 and CO2 in nanopores is investigated. In addition, molecular simulations are used to simulate the competitive adsorption and selectivity of CO2 and CH4 in shale nanopores with different pore structures, sizes, and shapes. The results reveal that as the pressure, carbon dioxide mole fraction, temperature, and pore size increases, the competitive adsorption selectivity S decreases. However, it remains greater than one, which demonstrates that CO2 has a stronger adsorption ability than CH4. For shale gas development, as the total pressure decreases, the contribution of the competitive adsorption effect becomes significant. A competitive adsorption model for CO2 and CH4 is established, and it is shown to fit the experimental data and simulation data well at different pressures. Finally, based on single-component adsorption results, we can separate the different effects and predict the displacement efficiency, which may provide a criterion in field scale simulations.