A thermodynamic model for mutual solubility of water and CO2 in brines
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This chapter focuses on the partitioning of CO2 and H2O components in different fluid phases. It reviews the numerically efficient thermodynamic model for phase partitioning in the CO2-H2O system without salinity. Modeling of CO2 disposal into saline formations requires accurate formulation for the Pressure-Volume-Temperature (PVT) properties, and mutual solubility of CO2 and water as a function of temperature, pressure, and salinity. The formulation includes the provisions to deal with the nonideality of CO2 at near-critical conditions that are shown to provide an excellent match to experimental data in the range 12-100C and up to 600 bars. The chapter also reports on some results obtained by extending this formulation to account for dissolved salts in the aqueous phase. The basic solubility model for the pure components is based on equating chemical potentials in the gas and aqueous phases, and using a modified Redlich-Kwong (RK) equation of state to calculate fugacity coefficients of CO2 and H2O in the compressed gas phase. The mixing rules implemented in this model have a standard form, but assume infinite H2O dilution in the CO2-rich phase, allowing the use of a non-iterative algorithm for computing mutual solubilities at given pressures and temperatures. 2005 Elsevier Ltd. All rights reserved.