Experimental investigation of the impact of mixed-wettability on capillary trapping using a real-rock microfluidic chip

Wettability governs two-phase flow in porous media, thereby directly affecting the security of CO₂ geological sequestration and oil recovery efficiency, yet the role of mixed-wettability remains poorly understood. This study investigates the dynamics of capillary trapping using a real-rock microfluidic chip that preserves the authentic pore structures and mineral composition of natural rock. By employing high-resolution in situ imaging, we compared waterflooding behaviors under water-wet and stearic acid-induced mixed-wet conditions across a range of capillary numbers. In unaged water-wet media, residual oil is primarily trapped as isolated singlets due to the snap-off events driven by the wetting films and corner flows. High injection rates were found to suppress this mechanism by restricting the film development time, thereby significantly reducing residual oil saturation. Conversely, wettability alteration fundamentally shifts the trapping regime. In mixed-wet systems, residual oil saturation increased significantly, forming extensive multi-pore clusters anchored to oil-wet regions. This enhanced trapping results from the inability of water to form stable films on oil-wet walls, which suppresses snap-off events and promotes macroscopic bypassing. Furthermore, a direct correlation between local wettability distribution and trapping mechanics was established, identifying four distinct behaviors in mixed-wet porous media: classical snap-off, local bypassing in asymmetric pores, ganglion trapping, and macroscopic bypassing across continuous oil-wet zones. These findings demonstrate that the spatial distribution of wettability governs the transition from film-driven snap-off to wettability-induced bypassing, serving as the dominant factor controlling residual oil morphology in complex porous media.