Experimental and Modeling Study on Phase Equilibrium of CaCl2 Type Oilfield Brine from the Junggar Basin at 398.2 K

Abstract The stable phase equilibria of the quaternary system Na+, K+, Ca2+//Cl––H2O and its ternary subsystems were systematically investigated at 398.2 K using the isothermal dissolution equilibrium method. The ternary system Na+, Ca2+//Cl––H2O contains 1 invariant point, 2 univariant curves, and 2 crystallization regions, with a significant salting-out effect of CaCl2 on NaCl. The ternary system K+, Ca2+//Cl––H2O features 2 invariant points, 3 univariant curves, and 3 crystallization regions. The quaternary system exhibits 2 invariant points, 5 univariant curves, and 4 crystallization regions. Based on the Pitzer model, single-salt and mixed-salt parameters, together with the solubility products (lnK) of NaCl and KCl, were obtained using temperature-dependent formulas, while the lnK values of CaCl2·2H2O and KCl·CaCl2 were fitted to experimental data. The model predictions agree well with the experimental data, with small relative errors and highly consistent solubility curves. This study fills the gap in phase equilibrium data for this system at 398.2 K, confirms the applicability of the Pitzer model to high-temperature, high-salinity oilfield brines, and provides a thermodynamic basis for scaling prediction and brine chemistry management in deep hydrocarbon development.