Direct observation of anomalous CO₂ dispersion in multi-scale porous media

CO₂ dispersion dynamics in multi-scale porous media is critical for various environmental applications. The heterogeneity of natural porous media leads to immobile zones and preferential flow paths, resulting in a broad distribution of local velocity and giving rise to anomalous dispersion. Existing experimental methods find it challenging to establish multi-scale porous media for observing the anomalous dispersion of CO₂ at high temporal-spatial resolution. To address this gap, we developed a novel visualization method based on micromodels and a colorimetric method. The visualization of CO₂ dispersion during CO₂ drainage revealed that the reduced connectivity of porous media causes CO₂ sub-dispersion, while high CO₂ injection rates promote super-dispersion. In the fractured porous media, capillary force balance further induced anomalous multi-stage CO₂ dispersion. The preferential flow of CO₂ solute in large fractures significantly influences CO₂ dispersion in the matrix. Our study reveals the synergistic effects of microscope heterogeneity of porous media and solute injection rate on dispersion dynamics, with important implications on CO₂ geological sequestration and transport of subsurface contaminants.