Monte Carlo simulations of H₂O-CaCl₂ and H₂O-CaCl₂-CO₂ mixtures

Monte Carlo simulations were performed to obtain the phase behavior of binary H₂O-CaCl₂ and ternary H₂O-CaCl₂-CO₂ mixtures over a range of conditions. The solubility of CO₂ in brines plays a key role in determining the amount that can be trapped via geological carbon storage. Isobaric-isothermal and Gibbs ensemble Monte Carlo simulations with several fixed-point charge force field models were used for the calculations of liquid densities and vapor pressures for the binary, and compositions of both phases for the ternary system. We used the SPC and SPC/E models for water; the Åqvist, Deublein et al., and Smith-Dang parameterizations for CaCl₂; and the EPM2, Murthy et al., and TraPPE models for CO₂. While none of the model combinations were able to reproduce all the properties of interest, we found that some combinations produce accurate descriptions of individual properties. For the binary system, liquid densities are well represented by the SPC/E and Åqvist model combination, and vapor pressures are best described by the SPC and Åqvist model combination. For CO₂ solubility in aqueous CaCl₂, the combination of SPC, Smith-Dang, and TraPPE models gives the best predictions, but all the models studied show good predictive capabilities, given that no intermolecular potential parameters were optimized in the present study. These results are broadly consistent with previous calculations for the H₂O-NaCl-CO₂ system; CaCl₂ is found to have a stronger salting-out effect than NaCl at the same molality.