Gravity-assisted hydrocarbon miscible gas flooding for enhanced oil displacement efficiency and CO2 trapping

Gravity-assisted effects are a critical controlling factor in enhancing oil displacement efficiency during gas flooding enhanced oil recovery (EOR). This study establishes a pore-scale mathematical model for hydrocarbon mixture gas displacement in oil-bearing porous media based on the Navier–Stokes equations, mass conservation principles, and porous media seepage theory. The model quantitatively characterizes the synergistic mechanisms of gravity assistance, dynamic velocity field evolution, molecular diffusion, and injection angle on EOR efficiency and CO2 sequestration. The results demonstrate that in heterogeneous porous media, the coupling of gravity and diffusion significantly mitigates non-uniform fluid flow by reducing the velocity disparity between main flow channels and corner regions, thereby expanding gas flooding sweep efficiency and enhancing CO2 storage performance. Under gravity-driven segregation, increasing the gas injection angle amplifies the vertical potential gradient between injectors and producers, favoring horizontal-dominated seepage of mixed gas. This increases oil recovery by 26.6% and the CO2 trapping amount by 2.15 × 10−3 mol. A larger oil–gas density difference strengthens gravity-assisted horizontal displacement, boosting recovery by 18.2% and the CO2 trapping amount by 4.71 × 10−3 mol. Additionally, a lower gas injection rate (2.0 × 10−5 m/s) improves oil recovery by 23.9% and increases the amount by 2.96 × 10−3 mol compared to a higher rate (8.0 × 10−5 m/s), as it promotes oil migration from secondary pores to main flow pathways. This research provides theoretical foundations and optimization strategies for synergistically improving oil recovery and geological CO2 sequestration via hydrocarbon mixture gas flooding.