Microfluidic study of CO₂-water-oil displacement in porous media with variable pore size and depth

AbstractCO₂ displacement is a key technique for enhancing oil recovery (EOR) and achieving carbon sequestration. However, the pore-scale flow mechanisms of CO₂-water-oil three-phase flows remain poorly understood. Here, high-temperature and high-pressure microfluidic experiments were conducted to investigate the pore-scale behaviors of CO₂-water-oil three-phase flow following water flooding. Glass micromodels with varying pore sizes (40–400 μm) and etching depths (20/40 μm) were employed to systematically analyze the effects of pore structure on flow pathways, interfacial stability, and displacement efficiency. The results reveal that: (1) under immiscible conditions, capillary resistance in smaller pores suppresses CO₂ migration, while viscous fingering tends to occur in larger pores; under miscible conditions, scCO₂ preferentially diffuses into oil within larger pores and subsequently invades smaller pores, resulting in an 18 percentage points higher oil recovery compared to immiscible displacement [(S_oi − S_or)/S_oi]; (2) the low viscosity and high diffusivity of scCO₂ facilitate oil mobilization, shifting the displacement mechanism from piston-like to a dissolution-extraction, governed by Taylor dispersion, extraction, and water-film shielding; (3) deep pores exhibit higher interfacial instability due to elevated capillary numbers, whereas shallow pores exhibit more stable displacement dominated by capillary forces, highlighting depth-induced velocity heterogeneity as a critical regulator of three-phase flow. This study clarifies pore-scale displacement mechanisms of three-phase flow and provides theoretical insights for optimizing CO₂-based EOR and storage strategies.