Calculating Low Temperature Phase Equilibrium of NaCl-KCl-CaCl₂ Completion Fluid Based on Pitzer Model

Crystallization point, a main performance index of composite brine completion fluids, can only be obtained through laboratory experiments because most of the articles and handbooks only give the crystallization point of the single salt. If the crystallization point of a composite brine can be calculated with mathematical model, then large amount of tedious experiment can be saved, and the optimization of composite brine completion fluid can be achieved using the calculated data. Using the Pitzer model and the Spencer model parameters, salts commonly used in completion fluids, such as NaCl, KCl and CaCl₂, were calculated for their solid-liquid phase equilibrium both as single salt and binary salts. The liability of the calculation was verified using documented data and experiment data. It was shown that at temperatures below 25℃, the relative standard deviation between the calculated values and the documented data was less than 0.1, satisfying the need for completion fluid study and design. At temperatures above 25℃, the relative standard deviation for KCl became larger, and the Pitzer parameters of KCl needed to be adjusted for phase equilibrium recalculation.