Effect of brine composition on the onset of convection during CO2 dissolution in brine

We study the effect of brine composition on CO2 dissolution in brine. In particular, we address the effect of brine composition on the onset of convection through experiments, numerical simulations, and theoretical analyses. We use two brine solutions-one containing sodium chloride (NaCl) and one containing mixtures of NaCl and calcium chloride (CaCl2)-to study their differences in terms of the onset of convection. We perform experiments in isothermal conditions (similar to 50 C) with pressure range of 500-535 psi for different salinities and permeabilities (Rayleigh number of 2900 to 4900). Our experimental conditions and set-up design allow us to avoid problems associated with analogue fluids as well as with blind cells. We also conduct linear stability analysis (LSA) and high-resolution direct numerical simulations (DNS). We analyze pressure data and calculate other parameters such as diffusion coefficient, viscosity, and solubility. Specifically, we obtain the onset of convection from pressure decay curves and critical wave number from image analyses. We show that the onset of convection occurs earlier with higher wave number in brine solutions containing NaCl. Pressure results show that using mixture of NaCl and CaCl2 results in a higher CO2 diffusion coefficient, which in turn damps convective instabilities. Thus, the onset time of instabilities is later and finger growth rate is smaller for brines with NaCl and CaCl2. Our DNS results show that deviation between the cumulative dissolved CO2 as well as the dissolved CO2 due to only diffusion process occurs earlier for NaCl solution. We found a dimensionless Rayleigh-dependent onset of instability with parameters that are close for two mixtures. However, differences in the CO2 diffusivity result in smaller Rayleigh numbers for NaCl and CaCl2 containing mixtures. Our results have practical implications for CO2 geological sequestration in saline aquifers.