Mechanistic Insights into CO₂ Slug Displacement Assisted by N₂ and CH₄ in Wedge-Shaped Pores

The supply demand imbalance of energy facilitates the development of unconventional oil and gas, particularly shale reserves. Herein, the mechanism of N₂- and CH₄-assisted CO₂ slug flooding in wedge-shaped pores is studied by molecular dynamics simulations. The results show that in the process of N₂-assisted CO₂ flooding, CO₂ can form a sufficient mutual solubility effect with the oil phase in the pore II (pore size 3 ∼ 8 nm) when CO₂ is preflooded, and the postinjected N₂ has a strong driving ability, so the mutual solubility of the CO₂/oil phase can be better displaced. For N₂ preflooding, the gas channel is preferentially formed in the pore II, and the vortex-like movement is formed at the entrance of the pore I (pore size 8 ∼ 3 nm). In the process of CH₄-assisted CO₂ flooding, the preflooding of CO₂ has a better displacement effect. However, in the preflooding of CH₄, the phenomenon of gas channeling easily occurs, which leads to the decrease in the displacement effect in the pore I. In addition, the siphon phenomenon can be observed for all flooding. CO₂ flooding and its slug flooding would produce the phenomenon of CO₂ migration between pores. Overall, N₂ and CH₄ assistance in CO₂ flooding improves oil displacement and recovery efficiency in reservoir pores, offering a superior method for enhanced oil recovery in nanopore structures. The slug displacement method demonstrates a greater efficiency than the single gas methods, highlighting its potential for improving oil recovery in intricate nanopore structures.