Thermodynamic modeling of CO2 solubility in saline water using NVT flash with the cubic-Plus-association equation of state

Abstract

The accurate estimation of CO2 sequestration potential in deep saline aquifers requires the knowledge of CO2 solubility in brine, thus placing importance on reliable thermodynamic models that account for the effect of different salts and their mixtures over wide ranges of pressure, temperature and salt concentration. Most literature investigated CO2 solubility in a single-salt solution as a replacement of real saline water, which may significantly overestimate CO2 sequestration potential through solubility trapping. In order to accurately estimate CO2 sequestration potential over geological conditions, the Peng-Robinson Cubic-Plus-Association (PR-CPA) equation of state (EOS) is used in this study to model both aqueous and nonaqueous phases. A promising flash technique at given moles, volume and temperature, known as NVT flash, is employed and the salting-out effect is reproduced by correcting the chemical potential of aqueous nonelectrolyte components. To represent real saline environments, five salts are considered, including sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2), magnesium chloride (MgCl2) and sodium sulfate (Na2SO4). With taking into account the electrostatic contribution caused by salts, the combination of the salt-based PR-CPA EOS and NVT flash accurately models the solubility behavior of CO2 in mixed-salt solutions and the numerical results agree with experimental data very well. Moreover, the proposed CPA model exhibits neck-to-neck accuracy to the more sophisticated electrolyte CPA EOS, thus making it promising to accurately estimate carbon sequestration potential in saline aquifers through solubility trapping.