Coupled phase and aqueous species equilibrium of the H 2O–CO 2–NaCl–CaCO 3 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite

Abstract

A model is developed for the calculation of coupled phase and aqueous species equilibrium in the H 2O–CO 2–NaCl–CaCO 3 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite. The vapor–liquid–solid (calcite, halite) equilibrium together with the chemical equilibrium of H +, Na +, Ca 2+, CaHCO 3 + , Ca(OH) +, OH −, Cl −, HCO 3 - , CO 3 2 - , CO 2(aq) and CaCO 3(aq) in the aqueous liquid phase as a function of temperature, pressure, NaCl concentrations, CO 2(aq) concentrations can be calculated, with accuracy close to those of experiments in the stated T– P– m range, hence calcite solubility, CO 2 gas solubility, alkalinity and pH values can be accurately calculated. The merit and advantage of this model is its predictability, the model was generally not constructed by fitting experimental data. One of the focuses of this study is to predict calcite solubility, with accuracy consistent with the works in previous experimental studies. The resulted model reproduces the following: (1) as temperature increases, the calcite solubility decreases. For example, when temperature increases from 273 to 373 K, calcite solubility decreases by about 50%; (2) with the increase of pressure, calcite solubility increases. For example, at 373 K changing pressure from 10 to 500 bar may increase calcite solubility by as much as 30%; (3) dissolved CO 2 can increase calcite solubility substantially; (4) increasing concentration of NaCl up to 2 m will increase calcite solubility, but further increasing NaCl solubility beyond 2 m will decrease its solubility. The functionality of pH value, alkalinity, CO 2 gas solubility, and the concentrations of many aqueous species with temperature, pressure and NaCl (aq) concentrations can be found from the application of this model. Online calculation is made available on www.geochem-model.org/models/h2o_co2_nacl_caco3/calc.php.