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Multicomponent-Mulitphase Equilibria in a CO2 Sequestration Project

Renee J. Perez and Robert A. Heidemann
University of Calgary, Calgary, AB

We propose a thermodynamic model for partitioning gases, such as CO2, H2S, N2, noble gases, and light alkanes in mixtures of aqueous sodium chloride solutions and liquid hydrocarbons. The model couples a modified Henry's Law for the solubility of gases in brines, and the Soave-Redlich-Kwong EoS for the solubility of gases in petroleum liquids and vapor-rich hydrocarbons. The model reproduces experimental data and critical phase behavior in all possible phases with less than 5% error, and is valid for temperatures up to 500 K, pressures to 2 kbars, and ionic strength solutions to 5 m. The set of non-linear equations relating gas solubility is solved with a Gibbs free energy minimization method, commonly described as the Rachford-Rice routine, using a standard multivariable Newton-Raphson method with a successive-substitution procedure. We applied the model to understand the distribution of gases in the IEA-GHG Weyburn CO2 Monitoring and Storage Project, in Saskatchewan, Canada, throughout the first and second year after commencement of CO2 injection. Results indicate that a free-gas phase formed in the south-east of the field ~363 days after injection, increased in size and volume with time, but did not correlate with zones showing excess CO2 production. We also observed overlaps between areas of 13C depleted HCO-3 and areas with free-vapor phase. We provide mathematical-physical demonstration of CO2 dissolution in the aqueous-rich and oil-rich phase, and conclude that CO2 dissolution in liquids is a viable short-term storage method, prior to the onset of long-term mineral trapping resulting from silicates-CO2 interactions.