Diffusion of CO2 at Reservoir Conditions: Models and Measurements

Abstract

Summary Mathematical models are developed to describe the transport of dissolved CO2 in a liquid phase, and results of measurements of the diffusivity of CO2 in hydrocarbons and water at reservoir conditions are reported. The measurements were made with novel techniques based on the direct observation of the motion of an interface caused by the diffusion of CO2 through oil or oil shielded by water. Diffusion coefficients were determined by fitting the mathematical models to the observed motion of the interfaces. This method allows the measurement of diffusion coefficients without the need to determine phase compositions and is therefore suited to measurements at elevated pressures (reservoir conditions). Measured diffusion coefficients are reported for CO2 in pentane, decane, and hexadecane at 25°C [77°F] and pressures up to 6000 kPa [870 psia]). Limited measurements of CO2 diffusion in Maljamar crude oil are also described. In addition, results of measurements for the diffusion of CO2 in water are presented. These are the first such measurements at high pressures (up to 6000 kPa [870 psia]). Correlations of diffusion coefficients in liquids at atmospheric pressure are shown to give reasonable estimates of diffusion coefficients for CO2 in fluids at reservoir conditions. Finally, the measured diffusion coefficients and mathematical models are used to assess the impact of diffusive mixing on CO2 floods at various length scales to examine the relationship between laboratory-scale corefloods and field-scale displacements.