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.