Onset of convection controlled by N₂ impurity during CO₂ storage in saline aquifers

This paper presents experimental and theoretical studies of the onset of natural convection induced by the dissolution of carbon dioxide- nitrogen (CO₂-N₂) mixtures into brine. Using a high- pressure Hele-Shaw cell, we conducted four sets of experiments under high-pressure conditions (with initial pressure range of 3375 to 3691 kPa) where 0%, 10%, 20% and 100% (molar base) N₂ at different brine salinities and permeability (Rayleigh number of 3120 to 4840) were conducted to obtain quantitative and qualitative data from pressure decay and image analysis respectively. Furthermore, we performed Linear Stability Analysis (LSA) based on the quasi-steady-state approximation and Direct Numerical Simulation (DNS) to theoretically examine the effect of N₂ on the onset of convection. We calculated effective diffusion coefficients from experimental data, and then used them in the LSA and DNS analyses. The experimental results confirmed by theoretical analysis revealed that the onset of convection occurred earlier in 10% N₂ - 90% CO₂ cases than in the pure CO₂ cases. On the other hand, the onset of convection for an impure CO₂ mixture with 20% N₂ was close to that of the pure CO₂ system. These findings suggest that, depending on the mole fraction of N₂, the presence of N₂ as an impurity may impede or promote the onset of natural convection. Since both the solubility and diffusion of CO₂ in brine are nonlinear functions of N₂ concentration, increasing N₂ concentration reduces the solubility and diffusion coefficient, which have opposing impacts on the stability of the diffusive boundary layer.