Assembly and Comparison of Different Equations of State for Multiphase Carbon Dioxide in the Subsurface

Sequestration of carbon dioxide in deep saline aquifers has been suggested as a possible means for reducing greenhouse gas concentrations in the atmosphere. For geological sequestration, an accurate equation-of-state (EOS) is critical for evaluating and estimating migration and ultimate diagenetic effects on permeability and porosity. We assembled, coded, and compared two different EOS algorithms. One is the modified Redlich-Kwong (MRK) EOS, which employs modification of the attractive term from van der Waals equation. The other is Span and Wagner's (SW) EOS, an empirical representation of the fundamental equation of Helmholtz energy. Using these fundamentally different EOS, the density, fugacity, and enthalpy of carbon dioxide in the gas and supercritical phases were simulated for several test case models. Results included some disparities between simulated and experimental data. Furthermore, for some simulations small errors were compounded and induced very significant impacts on ultimate carbon dioxide plume migration and associated subsurface chemical reactions. In particular, aqueous carbon dioxide solubility calculations by the two EOS algorithms reflected large differences, especially during longer time-scale simulations. In sum, results suggest that even small differences in thermodynamic properties of carbon dioxide, as calculated between the MRK and SW EOS, may produce magnified differences in ultimate projections of subsurface plume histories and associated chemical reaction processes.