Pore-scale investigation of the effects of wetting phase re-imbibition on gas capillary trapping in porous media

Capillary trapping of the non-wetting phase in porous media is vital for long-term CO2 sequestration and underground gas storage. While injection strategies have received extensive research attention, the pore-scale mechanisms controlling residual gas stability during wetting phase re-imbibition under varying injection directions coupled with buoyancy remain unclear. This study used high-resolution micro-focus X-ray computed microtomography imaging and quantitative analysis to investigate gas trapping in a hydrophilic glass bead pack across multiple capillary numbers. Both downward (gravityaligned) and upward (gravity-opposing) re-imbibition were tested. The results demonstrate that downward injection promotes bubble fragmentation and stabilization, sustaining higher residual saturation, increased populations of small bubbles, and greater specific surface area even under elevated capillary numbers. Upward injection, in which buoyancy aligns with flow, enhances bubble coalescence and mobilization, lowering residual saturation and trapping efficiency. These pore-scale trends highlight the critical interplay of capillary, viscous, and buoyancy forces in shaping gas trapping behavior. The findings of this study provide valuable experimental insights for optimizing injection direction and flow rate, in order to improve long-term CO2 storage security and underground gas storage operations. Document Type: Original article Cited as: Zhang, C., Zhang, K., Yoshida, S., Li, Z., Zhao, W., Suekane, T. Pore-scale investigation of the effects of wetting phase re-imbibition on gas capillary trapping in porous media. Capillarity, 2025, 17(1): 16-26. https://doi.org/10.46690/capi.2025.10.02