Monte Carlo Simulations of CO\(_{2}\) in NaCl and CaCl\(_{2}\) Aqueous Solutions

Abstract

Monte Carlo simulations were performed to obtain the phase behavior of binary H\(_{2}\)O- CaCl\(_{2}\) and ternary H\(_{2}\)O- CaCl\(_{2}\)- CO\(_{2}\) mixtures over a range of conditions. The solubility of CO\(_{2}\) in brines plays a key role in determining the amount that can be trapped via geological carbon storage. Carbon sequestration technologies could play an important role in reducing greenhouse gas emissions, so there is a need to understand the key processes that govern the long-term fate of CO\(_{2}\) underground. While alkali halide solutions such as NaCl(aq) have been well-studied by experiments and molecular simulations at high temperatures and pressures, there is a lack of data for alkaline-earth halides under reservoir conditions. We investigated thermodynamic and structural properties of H\(_{2}\)O- CaCl\(_{2}\) and H\(_{2}\)O- CaCl\(_{2}\)- CO\(_{2}\) through isobaric-isothermal and Gibbs-ensemble Monte Carlo simulations with several fixed-point charge force field models. We used the SPC and SPC/E models for water; the Åqvist, Deublein et al., and Smith- Dang parameterizations for CaCl\(_{2}\); and the EPM2, Murthy et al., and TraPPE models for CO\(_{2}\). In particular, we obtained liquid densities, vapor pressures, and pair-correlation functions of the binary H\(_{2}\)O - CaCl\(_{2}\) system and gas solubility data of the ternary H\(_{2}\)O - CaCl\(_{2}\) - CO\(_{2}\) system. As part of our goal of improving understanding of brines, we also verified and complemented existing data on H\(_{2}\)O- NaCl - CO\(_{2}\). While none of the CaCl2 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 CO\(_{2}\) solubility in aqueous CaCl\(_{2}\), 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. The combinations with adequate representation of liquid structure tend to match experimental thermodynamic property data better than those that are less accurate in the description of liquid structure. These simulation results are broadly consistent with calculations for the H\(_{2}\)O- NaCl- CO\(_{2}\) system; CaCl\(_{2}\) is found to have a stronger salting-out effect than NaCl at the same molality.