Convective Dissolution of Carbon Dioxide in Deep Saline Aquifers: Insights from Engineering a High-Pressure Porous Visual Cell

We present the first experiments of dissolution-driven convection of carbon dioxide (CO2) in a confined brine-saturated porous medium at high pressures. We designed an alternative cell that allows for both visual and quantitative analyses, and address the effects of free-phase CO2 and brine composition on convective dissolution. The visual examination of the gas volume combined with the measurement of pressure, which both evolve with dissolution, enables us to yield insights into the dynamics of convection in conditions that more closely reflect the geologic conditions. We find and analyze different dissolution events, including diffusive, early, and late convection and shutdown regimes. Our experiments reveal that in intermediate regimes, a so-called "quasisteady" state actually never happens. Dissolution flux continuously decreases in this regime, which is due to a negative feedback loop: the rapid reduction of pressure following convective dissolution (allowed by gas expansion), in turn, decreases the solubility of CO2 at the gas-brine interface and thus the instability strength. We introduce an alternative scaling factor that not only compensates the flux reduction but also the nonlinearities that arise from different salt types. We present robust scaling relations for the compensated flux and for the transition times between consecutive regimes in systems with NaCl (Ra similar to 3271-4841) and NaCl CaCl2 mixtures (Ra similar to 2919-4283). We also find that NaCl + CaCl2 mixtures enjoy a longer intermediate period before the shutdown of dissolution, but with a lower dissolution flux, as compared to NaCl brines. The results provide an alternative perspective into how the presence of two separate phases in a closed system as well as different salt types may affect the predictive powers of our experiments and models for both the short- and long-term dynamics of convective dissolution in porous media.