Experimental study of heterogeneity-induced capillary trapping in the context of leakage from geologic carbon sequestration sites

Leakage of CO₂ from geologic carbon sequestration sites could undermine the long-term goal of reducing emissions to the atmosphere. Despite this, leakage processes, especially the vertical transport of gases through geologic formations overlaying target repositories, are poorly characterized. The goal of this work was to experimentally assess how sub-basin scale heterogeneity in overlaying formations could reduce CO₂ leakage. High-pressure columns packed with sand and glass beads of different sizes were used to create a capillary barrier, which is an analog of low-permeability inter-beds. Transport of the resulting plume was recorded in real time using electrical resistivity. The effect of pressure, temperature, permeability, surface wettability, and CO₂ flow rate were all assessed.

Real-time monitoring and quantification of CO₂ saturation suggests that capillary heterogeneity trapping is primarily controlled by permeability contrast, pressure, temperature and water/CO₂ wettability on mineral surfaces. The amount of trapping was considerably enhanced when permeability contrasted increased, showing a good agreement with equilibrium capillary pressure - saturation analysis in the literature. Temperature and pressure controlled experiments demonstrated the sensitivity of capillary trapping to geothermal and pressure gradient. Wettability alterations also increased initial trapping when more CO₂-philic materials is presented and a much greater increase in residual trapping (defined as 10 pore volume water re-imbibition). Variation of leakage rate was not shown to result in significant difference in the overall saturation values, but the stability of the trapped plume was reduced at high CO₂ injection rates.

These results suggest that local capillary trapping could contribute to secondary trapping and slow the buoyancy-driven rise of CO₂. These measurements could have important implications for minimizing risk associated with leakage from carbon sequestration sites.