Calculating Low Temperature Phase Equilibrium of NaCl-KCl-CaCl2 Completion Fluid Based on Pitzer Model
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摘要: 结晶点是混合盐水完井液的主要性能指标，绝大部分文献和手册只提供了单盐的结晶点数据，混合盐数据只有通过实验才能得到。通过数学模型计算混合盐水的结晶点可省去大量的实验工作，并为完井液组成优化提供依据。采用Pitzer模型和Spencer模型参数进行了完井液常用的NaCl、KCl和CaCl2的单盐及二元混合盐体系的固液相平衡计算。通过文献数据和实验数据验证了计算结果的可靠性。验证结果表明，在25℃以下时，计算值与文献值的相对标准偏差均小于0.1，能够满足完井液研究和施工设计的需要；25℃以上时，KCl的单盐体系计算偏差变大，需调整KCl的Pitzer参数再进行相平衡计算。Abstract: 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.
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