Selective adsorption of CO2/CH4 mixture on clay-rich shale using molecular simulations

Abstract

Shale rocks comprise of a lager proportion of clay minerals; shale gas potentially adsorbs on the clay minerals due to its large specific surface area. In this work, selective adsorption behaviors of CO2/CH4 are investigated in clay-mineral nanopores using the grand canonical Monte Carlo (GCMC) simulation method. In addition, the subsequent implications for CH4 recovery and CO2 sequestration are summarized. Results show that CH4 exhibits the strongest adsorption on kaolinite, followed by that on illite and montmorillonite, while adsorption capacity of CO2 is calculated in the order of montmorillonite > illite ≈ kaolinite, suggesting that CO2 injection has the best performance for CH4 recovery when implemented in montmorillonite-rich shale reservoirs. Comparatively, smaller pores are dominated by the adsorbed gas and are also favorable for CO2 sequestration. The adsorption selectivity of CO2 over CH4 on clay minerals is predicted in the order of kaolinite < illite < montmorillonite; moreover, the adsorption selectivity is reduced as pressure decreases for illite and montmorillonite, implying that the injection pressure of CO2 should be low enough to achieve an efficient CH4 recovery and CO2 sequestration. However, the ideal injection pressure of CO2 should be around the pressure where it has the maximum adsorption selectivity for kaolinite-rich shale. This study may achieve a deeper understanding of the adsorption behavior of CO2/CH4 in clay-rich gas-bearing reservoirs; furthermore, it may provide the guidelines for future optimization design of CO2 injection for CH4 recovery and CO2 sequestration in the field applications.