Abstract
Geological storage of CO 2 in deep saline aquifers relies critically on capillary trapping to immobilize buoyant CO 2 and limit post-injection migration. Despite extensive pore-scale studies, the combined influence of wettability and capillary number on residual CO 2 trapping after a complete drainage–imbibition sequence remains insufficiently resolved. Here, systematic pore-scale experiments were conducted to isolate the coupled effects of wettability and injection rate on CO 2 invasion dynamics, connectivity, and ultimate capillary trapping. Three wettability states were imposed via vinyltrimethoxysilane surface modification, yielding contact angles of approximately 20° (strongly water-wet), 47° (moderately water-wet), and 80° (weakly water-wet). CO 2 –water displacement experiments were performed at injection rates from 1 to 12 mL min⁻ 1 , corresponding to capillary numbers of 6.43 × 10⁻ 6 to 7.72 × 10⁻ 5 , spanning capillary-dominated to viscous-influenced flow regimes. High-resolution optical imaging and quantitative image analysis were used to measure residual CO 2 saturation and classify trapped-phase morphology into channels, clusters, and snap-off ganglia. At low capillary numbers (1 mL min⁻ 1 ), residual trapping was dominated by large clusters, with trapped saturations of ∼30–31% across all wettability states. Increasing the capillary number to high injection rates (12 mL min⁻ 1 ) caused a sharp reduction in residual trapping for strongly and moderately water-wet systems, decreasing to 4.85% and 3.94%, respectively, an 84–86% reduction relative to low-rate conditions. In contrast, weakly water-wet systems retained 10.40% residual CO 2 , approximately 2.5 times higher than water-wet cases, primarily as compact clusters stabilized by reduced water-film continuity. These results demonstrate that trapping efficiency alone does not determine storage security; instead, pore-scale connectivity and spatial organization of trapped CO 2 govern long-term stability.