Molecular insights into competitive adsorption of CO2/CH4 mixture in shale nanopores

Abstract

In the present study, competitive adsorption behaviour of supercritical carbon dioxide and methane binary mixture in shale organic nanopores was investigated by using grand canonical Monte Carlo (GCMC) simulations. The model was firstly validated by comparing with experimental data and a satisfactory agreement was obtained. Then the effects of temperature (298-388 K), pressure (up to 60 MPa), pore size (1-4 nm) and moisture content (0-2.4 wt%) on competitive adsorption behaviour of the binary mixture were examined and discussed in depth. It is found that the adsorption capacity of carbon dioxide in shale organic nanopores is much higher than that of methane under various conditions. The mechanism of competitive adsorption was discussed in detail. In addition, the results show that a lower temperature is favorable to both the adsorption amount and selectivity of CO₂/CH₄ binary mixture in shale organic nanopores. However, an appropriate CO₂ injection pressure should be considered to take into account the CO₂ sequestration amount and the exploitation efficiency of shale gas. As for moisture content, different influences on CO₂/CH₄ adsorption selectivity have been observed at low and high moisture conditions. Therefore, different simulation technologies for shale gas production and CO₂ sequestration should be applied depending on the actual moisture conditions of the shale reservoirs. It is expected that the findings in this work could be helpful to estimate and enhance shale gas resource recovery and also evaluate CO₂ sequestration efficiency in shale reservoirs.

Fig. 1. Graphene slit model for the organic nanopore in shale: (a) 2D; (b) 3D vision with adsorbed CO₂/CH₄ binary mixture.

Fig. 2. Comparison of adsorption isotherms from simulation and experiment at 333.15 K.

Fig. 3. Adsorption isotherms of CH₄ at different temperatures.

Fig. 4. Adsorption isotherms of CO₂ at different temperatures.

Fig. 5. Competitive adsorption isotherms of CO₂/CH₄ in shale organic nanopore at different temperatures.

Fig. 6. Adsorption selectivity of CO₂/CH₄ in shale organic nanopore as a function of temperature at different pressures.

Fig. 7. Concentration profile (a) and snapshot (b) of CO₂/CH₄ binary mixture adsorption in the slit-like nanopore of shale organic matter.

Fig. 8. Adsorption amount of CO₂/CH₄ binary mixture as a function of CO₂ injection pressure at different CH₄ partial pressures.

Fig. 9. Snapshots of CO₂/CH₄ binary mixture adsorption for nanopores with different pore sizes: (a) 1 nm; (b) 1.5 nm; (c) 2 nm; (d) 2.5 nm.

Fig. 10. Competitive adsorption isotherms of CO₂/CH₄ in shale organic nanopores with different pore sizes.

Fig. 11. Adsorption selectivity of CO₂/CH₄ in shale organic nanopore as a function of pore size at different pressures.

Fig. 12. Concentration profile of CO₂/CH₄ binary mixture adsorption in the organic shale nanopore.

Fig. 13. Adsorption isotherms of CO₂/CH₄ binary mixture at different moisture contents.

Fig. 14. Adsorption selectivity of CO₂/CH₄ in shale organic nanopore as a function of moisture contents at different pressures.