Fundamental investigation of reactive-convective transport: Implications for long-term carbon dioxide (CO2) sequestration

The density-driven convection coupled with chemical reaction is the preferred mechanism for permanently storing CO₂ in saline aquifers. This study uses a 2D visual Hele-Shaw cell to evaluate and visualize the density-driven convection formed due to gravitational instabilities. The primary goal of the experiments is to understand the various mechanisms for the mass transfer of gaseous CO₂ into brine with different initial ionic concentrations and flow permeability. Moreover, the impact of CO₂ injection locations, reservoir dipping angle, and permeability heterogeneity is also investigated. We observed that the presence of salts resulted in earlier onset of convection and a larger convective finger wavelength than the case with no dissolved salts. Additionally, a higher lateral mixing between CO₂ fingers is observed when dipping is involved. The CO₂ dissolution, indicated by the area of the pH-depressed region, depends on the type and concentration of the ions present in the brine and is observed to be 0.38–0.77 times compared to when no salt is present. Although convective flow is slowed in the presence of salts, the diffusive flux is enhanced, as observed from both qualitative and quantitative results. Moreover, the reduced formation permeability, introduced by using a flow barrier, resulted in numerous regions not being swept by the dissolved CO₂, indicating an inefficient dissolution. We also investigated the effect of discrete high-conductivity fractures within the flow barriers, which showed an uneven vertical sweep and enhanced flow channeling. Lastly, the parameters regarding CO₂ leakage risk during storage are identified and discussed.