Phase stability analysis for tight porous media by minimization of the Helmholtz free energy

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. (C) 2020 Elsevier B.V. All rights reserved.