Pore‐Scale Imaging and Analysis of Phase Topologies and Displacement Mechanisms for CO2‐Brine Two‐Phase Flow in Unconsolidated Sand Packs

CO2 storage in saline aquifers is considered a potential solution for CO2 mitigation, owing to its significant capacity and worldwide distribution capability. It is therefore becoming more important to understand the underground CO2/brine flow mechanisms. CO2 migration is primarily controlled by the pore-scale subsurface flows in different saline aquifer sites with variable reservoir formation compositions and reservoir conditions. Variations occur in the state of CO2 phase (gas versus supercritical), brine salinity, and rock wettability, under different reservoir conditions, and may result in different subsurface CO2/brine migration phenomena. In this study, we investigate the drainage and imbibition procedures of CO2 and brine by injecting fluids into unconsolidated sand packs under different conditions of CO2 phase states, brine salinity, and wettability of sand packs. The pore-scale fluid distribution is visualized using micro X-ray computed tomography (micro-CT). It is found that the phase states of CO2, brine salinity, and wettability have low impacts on CO2 distribution during drainage. However, the increase in brine salinity significantly damages the connectedness of the water phase in pore structures and further decreases the CO2-brine interfacial areas. In addition, a pore-scale event called the droplet fragmentation of nonwetting phase is found to occur in the imbibition procedure, which is considered to be beneficial to the dissolution trapping in CO2 geological storages. It is experimentally demonstrated that the pore structure of rock cores is a factor that significantly contributes to droplet fragmentation.