Improved pressure decay method for measuring CO₂-water diffusion coefficient without convection interference

Carbon dioxide (CO₂) storage in deep aquifers is a promising solution to mitigate anthropogenic CO₂ emissions. CO₂ solubility in brine results in a non-buoyant phase providing an effective trapping mechanism. However, experimental work and numerical simulation results have shown that this diffusion-driven mechanism is a relatively slow process. Accurate determination of CO₂ diffusion coefficient is, therefore, essential. The pressure decay method is a widely employed technique for measuring diffusion coefficients of gases in bulk liquids or porous media. It involves introducing a volume of gas on top of the liquid in a closed system and monitoring the pressure decay over time. While the method is generally simple and accurate, artifacts from natural convection can significantly influence the measured diffusion for liquids that exhibit an increase in density due to gas dissolution. This work presents an improved experimental approach for measuring CO₂ diffusion coefficients in water in a convection-fee system. Our setup consisted of single open-ended borosilicate capillary tubes filled with water inside a high-pressure vessel filled with CO₂ gas. The water-filled capillary tubes were placed with their open ends facing down. This configuration exhibits bottom-top diffusion leading to gravity-stable CO₂ diffusion in water free of gravity-induced convection and viscous fingering. The effects of pressure and salinity variations confirm the agreement between our results and values reported in the literature. We also performed additional analysis to determine the effective diffusion coefficient of CO₂ in a porous medium. The proposed technique can be used to measure the diffusion coefficients for other gas-liquid systems.