Visualization of gaseous and dissolved CO₂ migration in porous media

Visualization experiments using a rectangular transparent acrylic cell, which contained glass beads saturated with alkaline solution, with a universal pH indicator, were conducted to investigate the migration behaviors of gas and dissolved CO₂ in water-saturated porous media, assuming that CO₂ exsolution occurs from the CO₂-saturated groundwater. Immediately after the CO₂ gas injection (t < 1 s), which is comparable to the period immediately following CO₂ exsolution, CO₂ gas moved upward, percolating into the pores through various pathways. During CO₂ gas injection (t > 1 s), CO₂ gas bubbles continued to move upward to the interface between the saturated porous medium and air, while dissolving in the pore water. When the CO₂ gas reached the interface, a thin, dense yellow layer was formed, followed by the gradual coalescing of the small fingers, forming a thick dense yellow layer, indicating density-driven convection. The fingers formed by the density-driven convective flow were clearer in the coarser glass beads. The migration of carbonated water was faster, as the glass bead size and the CO₂ gas injection rate increased, causing earlier CO₂ saturation in pore water. The temporal changes in the effluent CO₂ gas concentration, emitted from the saturated glass beads to air, varied with the grain size of the porous medium and the CO₂ gas injection rate.