Abstract This paper presents a new method of phase stability analysis in the presence of capillary pressure by minimization of the Helmholtz free energy. The thermodynamic consistency of phase stability is rigorously preserved between the Helmholtz and Gibbs free energy. Case studies demonstrate the main advantages of the new method over the conventional methods using the Gibbs free energy. The effect of capillary pressure on phase stability is inherently considered in the new method using the Helmholtz free energy. The most fundamental reason for various issues associated with using the conventional methods is that the Gibbs free energy in composition space requires a pressure to be specified; i.e., the conventional methods involve two Gibbs free energy surfaces and their relative location changes during the iterative solution with capillary pressure. Case studies further show that there exist indefinite solutions in phase stability analysis with capillary pressure, in which the fluid is unstable, but no two-phase solution exists. Also, it is demonstrated that the shadow-phase region in the presence of capillary pressure can be defined with the Helmholtz free energy, but not with the Gibbs free energy.

中文翻译：

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通过最小化亥姆霍兹自由能来分析致密多孔介质的相稳定性

摘要 本文通过最小化亥姆霍兹自由能，提出了一种在毛细管压力存在下进行相稳定性分析的新方法。亥姆霍兹自由能和吉布斯自由能之间严格保持相稳定性的热力学一致性。案例研究证明了新方法相对于使用吉布斯自由能的传统方法的主要优势。在使用亥姆霍兹自由能的新方法中，固有地考虑了毛细压力对相稳定性的影响。与使用传统方法相关的各种问题的最根本原因是组成空间中的吉布斯自由能需要指定压力；IE，传统方法涉及两个吉布斯自由能面及其在毛细管压力迭代求解过程中的相对位置变化。案例研究进一步表明，在具有毛细管压力的相稳定性分析中存在不定解，其中流体不稳定，但不存在两相解。此外，还证明了存在毛细管压力时的阴影相区域可以用亥姆霍兹自由能来定义，但不能用吉布斯自由能来定义。