Sensitivity analysis of key influencing factors for CO2 residual trapping within porous media

Abstract

In this study, a pore-scale two-phase flow model is developed by coupling the Navier–Stokes equations with the phase-field method to investigate the effects of wettability, viscosity ratio (M), and drainage–imbibition cycles on CO₂ residual trapping. Numerical simulations reveal that under unfavorable viscosity ratios (M < 1), the CO₂ drainage front becomes unstable, while wettability significantly influences the two-phase drainage patterns. When the CO₂ front breakthrough, strongly water-wet media exhibit lower initial CO₂ saturation and higher inlet drainage pressure. After breakthrough, the inlet pressure in these media shows multiple peaks, with corresponding fluctuations in CO₂ saturation at the pressure maxima. During forced imbibition, CO₂ in the strongly water-wet medium undergoes initial residual trapping followed by transport and escape due to water phase expansion, resulting in a stepwise decline in residual CO₂ saturation and indicating potential secondary transport behavior. The two-phase M in the reservoir environment is not the main factor dominating the drainage pattern and CO₂ residual trapping efficiency. Strongly water-wet systems tend to form sparse, large-scale residual CO₂ clusters, while weakly water-wet systems favor dense, small-scale distributions. An increased M suppresses jamming by enhancing viscous resistance in the non-wetting phase, thereby increasing the average droplet size. Simulations of the drainage-imbibition cycle show that residual CO₂ droplets reduce the effective permeability of the porous medium, elevate the inlet pressure, and delay breakthrough phenomenon, which is beneficial for geological CO₂ sequestration. However, the cycling process increases the escape risk of residual CO₂ phase at the outlet, and the residual CO₂ saturation tends to decrease with the number of cycles. The research results provide theoretical support for the capacity evaluation and risk management of geological CO₂ storage.