Diffusion of CO2 in fluids relevant to carbon capture, utilisation and storage

In this work, molecular diffusion coefficients of carbon dioxide (CO2) in liquids relevant to carbon capture, utilisation and sequestration and enhanced oil recovery are reported. These parameters are necessary for the accurate and optimal design and control of such processes. Knowledge of these values is required to fully describe the migration of CO2 away from the injection wells and also for calculating the rate of absorption of CO2 into the formation fluids. However, diffusion coefficients are amongst the least studied of thermophysical properties, especially at high pressure, high temperature conditions. This work extended previous measurements where available, and produced new measurements where not, of diffusion coefficients at infinite dilution of CO2 in H2O, and several relevant brines and hydrocarbons at high temperatures (< 423 K) and high pressures (< 69 MPa). The Taylor dispersion method was used to determine diffusion coefficients for CO2 in water and selected hydrocarbons. The hydrocarbons chosen as representative of major crude oil components were n-heptane, nhexadecane, squalane, cyclohexane, and toluene. A technique based on nuclear magnetic resonance was used to measure effective diffusion coefficients of CO2 in several brines, encompassing monovalent and divalent salts, and a mixed brine. The diffusion coefficients of CO2 in water were correlated using the Stokes-Einstein equation in which the Stokes-Einstein number was assigned a value of 4 and the hydrodynamic radius was treated as a linear function of temperature. No relationship between brine salinity and the hydrodynamic radius was found. The results indicated pressure did not have an observable impact on the diffusivity in aqueous systems. The experimental uncertainty was found to be 2.3% with a coverage factor of 2 for the CO2-water system and 1.5% with a coverage factor of 2 for the CO2-brine systems. In contrast to aqueous systems, the diffusion coefficient of CO2 in hydrocarbons was found to be strongly dependent on pressure. At a given temperature the diffusion coefficient decreased by up to 50% over the pressure range investigated (1 to 69 MPa). A correlation based on the Stokes-Einstein equation and a two-parameter correlation based on the rough hard sphere theory was used to model the experimental results. The experimental uncertainty was found to be 1.5% with a coverage factor of 2.Open Acces