Abstract: Two-phase flow and capillary trapping are crucial processes and key mechanisms in geological CO₂ storage within deep saline aquifers. Previous studies have extensively investigated the injection pattern and theoretical model in porous media, but research on two-phase flow characteristics and capillary trapping mechanism during cyclic injection is relatively limited. Through pore-scale water-oil two-phase cyclic injection visualization experiments, we investigated the effects of pore-scale disorder and cycle numbers on two-phase flow and capillary trapping characteristics. The experimental results show that cyclic injection disrupts the continuity of the non-wetting phase, breaking it into isolated “droplets” that are difficult to remobilize. After four cyclic injections, capillary trapping efficiency can reach up to 40%. As pore-scale disorder increases, the bypass mechanism significantly traps the non-wetting phase, with the capillary trapping efficiency in disordered porous media increasing by 32.5% compared to the homogeneous case. The experiments also reveal four typical types of capillary-trapped droplets during cyclic injection: bubble droplets, bridge droplets, ganglia droplets, and cluster droplets. Bubble droplets are the most numerous but contribute the least to trapping efficiency, whereas cluster droplets contribute the most. Our findings enhance the understanding of capillary trapping mechanisms during cyclic injection and provide theoretical support and practical insights for optimizing CO₂ storage efficiency in deep saline aquifers.

Key words: disordered porous media, two-phase flow, cyclic injection, capillary trapping, microfluidic test