Diffusion of CO2 in Fluids Relevant to Carbon Capture, Utilisation and Storage

Diffusion coefficients of carbon dioxide (CO2) in liquids are necessary for the accurate design and optimal control of various processes, including carbon capture, utilization and sequestration and enhanced oil recovery. 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.

The work discussed extended previous measurements where available, and produced new measurements where not, of diffusion coefficients at infinite dilution of CO2 in H2O, and a selection of hydrocarbons at high temperatures (T < 423 K) and pressures up to 69 MPa using the Taylor dispersion method. A technique based on nuclear magnetic resonance was used to measure effective diffusion coefficients of CO2 in several brines, encompassing monovalent and divalent salt solutions, 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. No relationship between pressure or brine salinity and the hydrodynamic radius was found. In contrast to aqueous systems, the diffusion coefficient of CO2 in hydrocarbons was found to be strongly dependent on pressure, decreasing D by up to 50% over the pressure range investigated at a given temperature. A single set of parameters proved sufficient to correlate the diffusivity of CO2 in a homologous series of n-alkanes, ranging from n-hexane to n-hexadecane.