The Role of Water Flow and Dispersive Fluxes in the Dissolution of CO₂ in Deep Saline Aquifers

One of the important mechanisms in CO₂ storage is dissolution trapping. The dissolution of CO₂ in aquifer brines increases the brine density and leads to hydrodynamic instabilities, formation of CO₂-rich fingers, and a desirable acceleration of the CO₂ dissolution. In recent decades, there has been an intensive effort to identify suitable deep aquifers for CO₂ sequestration. Despite reports that background horizontal flow exists in many of these aquifers, few numerical studies have addressed whether background flow affects the dissolution process. These studies had no available measurements to support their results. Here, we report on laboratory experiments, using a dyed mixture of methanol and ethylene-glycol (MEG) as a CO₂ analog. The effect of an imposed horizontal water flow was investigated by injecting MEG from above into a cell filled with glass beads. An imaging system was used to provide concentration maps, which were analyzed to calculate dissolution rates and to evaluate the characteristics of the convective fingers. The results show that background flow leads to suppression of the fingers' formation, a fivefold decrease of the fingers' wave number, and a twofold decrease in their propagation rate. Therefore, it was expected that the dissolution rate would also be suppressed, consistent with previous numerical results. However, our results show that the dissolution rate was hardly affected by the background flow. We postulate that the horizontal flow results in a trade-off between the suppression of the convective flux and the enhancement of dispersive fluxes, resulting in negligible net influence on the dissolution rate.