CO2-H2O MIXTURES IN THE GEOLOGICAL SEQUESTRATION OF CO2. I. ASSESSMENT AND CALCULATION OF MUTUAL SOLUBILITIES FROM 12 TO 100°C AND UP TO 600 BAR

Abstract

Evaluating the feasibility of CO2 geologic sequestration requires the use of pressure-temperature-composition (P-T-X) data for mixtures of CO2 and H2O at moderate pressures and temperatures (typically below 500 bar and below 100°C). For this purpose, published experimental P-T-X data in this temperature and pressure range are reviewed. These data cover the two-phase region where a CO2-rich phase (generally gas) and an H2O-rich liquid coexist and are reported as the mutual solubilities of H2O and CO2 in the two coexisting phases. For the most part, mutual solubilities reported from various sources are in good agreement. In this paper, a noniterative procedure is presented to calculate the composition of the compressed CO2 and liquid H2O phases at equilibrium, based on equating chemical potentials and using the Redlich-Kwong equation of state to express departure from ideal behavior. The procedure is an extension of that used by King et al. (1992), covering a broader range of temperatures and experimental data than those authors, and is readily expandable to a nonideal liquid phase. The calculation method and formulation are kept as simple as possible to avoid degrading the performance of numerical models of water-CO2 flows for which they are intended. The method is implemented in a computer routine, and inverse modeling is used to determine, simultaneously, (1) new Redlich-Kwong parameters for the CO2-H2O mixture, and (2) aqueous solubility constants for gaseous and liquid CO2 as a function of temperature. In doing so, mutual solubilities of H2O from 15 to 100°C and CO2 from 12 to 110°C and up to 600 bar are generally reproduced within a few percent of experimental values. Fugacity coefficients of pure CO2 are reproduced mostly within one percent of published reference data.