Experimental and numerical study of the effects of formation brine salinity and reservoir temperature on convection mechanism during CO₂ storage in saline aquifers

One of the most effective and safe mechanisms for CO₂ storage in saline aquifers is solubility trapping by which CO₂ will be dissolved into the brine. Since the brine contained CO₂ is denser than the surrounding fluids, it would sink to the bottom of the rock formation over time, trapping CO₂ more securely. However, the rate of CO₂ dissolution into the brine depends on active mass transfer mechanisms, such as convection and diffusion mechanisms, under storage conditions. Convective mixing significantly enhances the rate of CO₂ dissolution into the brine. In this paper, we are aiming to comprehensively study the effects of formation brine salinity and reservoir temperature on convection mechanism and on CO₂ storage in saline aquifers. For this objective, a series of high-pressure high-temperature experiments were performed in a specially designed PVT bead pack cell. Based on the pressure data, the effects of aforementioned parameters on CO₂ dissolution, diffusion coefficient, Rayleigh number and the onset time of convection were studied. To further investigate the effects of above parameters on convection mechanism and to quantify the effectiveness of convection mechanism on CO₂ dissolution, Sherwood number and the time (t_Shmax), when it reaches its maximum value, were obtained in each experiment. Finally, a commercial software was used to model the convection mechanism and to study the effects of above parameters on density-driven fingers and convection mechanism. The results of both experimental and numerical studies revealed that decrease in brine salinity and temperature lead to increase in the Rayleigh number and Sherwood number. Furthermore, at low brine salinity and temperature, density-driven fingers grow faster at earlier times; therefore, convection mechanism is stronger, and it reaches its maximum value at earlier stages which is favourable for a storage project. The results of this study will help us in selecting the most suitable saline aquifer for safe CO₂ storage.