Experiment and Calculation of Phase Equilibrium in Quaternary System NaBr–KBr–CaBr2–H2O at 273.15 K Containing in Gas Field Brine of Sichuan Basin

Abstract In this work, the isothermal dissolution equilibrium method was used to investigate the solid-liquid equilibrium of the quaternary system NaBr–KBr–CaBr 2 –H 2 O at 273.15 K. The isothermal solubility of three bromides in aqueous solution was measured, and the dry salt phase diagram and water diagram were constructed. The determination results indicate that the system is a simple co-saturation type, and the equilibrium crystals are all simple salts. The isothermal phase diagram of the quaternary system NaBr–KBr–CaBr 2 –H 2 O at 273.15 K contains three isothermal dissolution lines intersecting at an invariant point. The saturated salts at this point are: NaBr⋅2H 2 O, KBr, and CaBr 2 ⋅6H 2 O. The entire phase space of the dry salt phase diagram is divided into three parts by three isotherms, that is, the crystalline region of NaBr⋅2H 2 O that occupies most, the larger crystalline region of KBr and the narrow crystalline region of CaBr 2 ⋅6H 2 O. The invariant point characteristics of the system at different temperatures were compared, and the Pitzer model was used to theoretically predict the isothermal solubility of the quaternary system at 273.15 K. Since the selected parameters in the calculation program come from the temperature correlation equation reported in the literature, the reliability of the parameters is relatively strong. The predicted value of solubility has a small deviation from the experimental value, and the result proves that the reliability of the experimental data is strong. The results of the phase equilibrium study not only reveal the dissolution and crystallization rules of bromine salts in the brine of oil and gas fields in the Sichuan Basin, but also verify the applicability of the Pitzer model in predicting the solubility of low-temperature bromine-containing system.