Phase stability analysis for tight porous media with the Helmholtz free energy

Phase stability analysis is commonly used to determine whether a mixture splits into two or more phases at equilibrium. Compositional reservoir simulators use it to initiate phase equilibrium calculations which are necessary to evaluate the amount of oil and gas present in a reservoir. However, conventional methods for stability analysis are not robust when applied to modeling phase behavior in tight reservoirs where equilibrium phase pressures can be different. Capillary forces in tight reservoirs are strong enough to alter the equilibrium phase compositions and pressures. Phase stability analysis in tight porous media is challenging because it involves predicting the appearance of an additional phase, the pressure of which is initially unknown. This causes common failures of conventional stability analysis algorithms based on the Gibbs free energy. Due to the lack of robust algorithms, the effects of capillary forces on phase behavior have not yet been included in any commercial software. This is a substantial problem that makes it difficult to accurately model and optimize the production of hydrocarbons from tight reservoirs. This report presents a new method of phase stability prediction for multicomponent mixtures in tight formation based on the minimization of the Helmholtz free energy. Case studies demonstrate why conventional methods based on the Gibbs free energy are not robust in the presence of capillary pressure. The new method can correctly identify the pressure-temperature region for a given mixture, outside of which the mixture is stable for any capillary pressure