Two-phase convective mixing under a buoyant plume of CO₂ in deep saline aquifers

The storage of carbon dioxide (CO₂) in deep saline aquifers has been suggested as a promising method for stabilizing the atmospheric concentration of CO₂. An accurate evaluation of the CO₂ trapping mechanisms, such as convective mixing, is crucial for estimates of storage capacity and security. We recently investigated the gravitational stability of the diffusive boundary layer underneath a capillary transition zone by performing a linear stability analysis, which provides a quantitative description of the onset of convection for the two-phase, buoyancy-driven flow in the presence of the capillary transition zone (Emami-Meybodi and Hassanzadeh, 2013). In this paper, we further examine the effect of the capillary transition zone on the onset of convection and subsequent convective mixing using direct numerical simulations. We describe key features of the two-phase convective mixing for systems with low Rayleigh numbers (Ra≤1000) and the measurement of several global quantities, such as the total CO₂ dissolution, Sherwood number, swelling factor, and interface velocity. We show that the commonly used assumption of a sharp CO₂-brine interface with constant CO₂ concentration at the top of an aquifer (i.e. single-phase system) may lead to erroneous estimates of not only the onset of convection, but also of the rate and magnitude of CO₂ dissolution. The significant effect of the capillary transition zone on the dissolution of CO₂ under a buoyant plume in saline aquifers is explained; and, the link between the capillary transition zone and the volume change, due to CO₂ dissolution and the interface velocity over the mixing process, is demonstrated. Compared to the single-phase system, a crossflow through the interface of the diffusive boundary layer with the capillary transition zone, as well as the upward advance of the interface motion, may enhance the convective mixing early in the period of natural convection. The decrease in the onset time and stronger mass flux may be more profound in the two-phase system than in the previously reported single-phase models. Furthermore, we report several scaling relationships that characterize the mixing process in the presence of the capillary transition zone. Our findings provide further insight into the understanding of the two-phase mixing features and the long-term fate of the injected CO₂ in deep saline aquifers.