Modeling Diffusion Coefficients of Carbon Dioxide in Water for Carbon Sequestration

Abstract

The increase in atmospheric concentrations of carbon dioxide (CO₂) is believed to be a key player in global warming, necessitating identification of viable technological options for its capture and storage. Perhaps, the most promising of these options is injection into saline aquifers whose merits include large sink capacities, and enhanced storage integrity upon CO₂ dissolution. This growing interest in CO₂ injection calls for a better understanding of CO₂ diffusion – quantified by infinite dilution (denoted as D^∞) and Fickian (denoted as D) diffusion coefficients – into water and brine. However, there is neither sufficient experimental data for D^∞ and D, nor unified models for accurate predictions. We propose a new model for calculating D^∞ for CO₂-water mixtures. The model takes into account the temperature dependence of the dipole moment of water and the polarizability of CO₂, and fits experimental CO₂-water data at low and high pressures with an accuracy of 4.9%. Remarkably, the proposed model also accurately predicts D^∞ for binary mixtures of methane (CH₄) and hydrogen sulfide (H₂S) in water where solute polarizability is comparable in magnitude to that of CO₂. Both CH₄ and H₂S are often present as impurities in CO₂-injection streams. Moreover, we present – to the best of our knowledge – the first predictions of composition-based D for CO₂-water over the temperature range 298.15-413.15 K, and pressures up to 50 MPa.