An algorithm for modeling three-phase equilibrium in multicomponent petroleum mixtures containing water is presented. The relevance of this work is defined by the requirement for accurate prediction of the phase behavior of water-containing reservoir fluids at all field development stages. It is important for preventing flow assurance issues associated with hydrate formation. Existing methods (direct Gibbs energy minimization) are often computationally expensive and complex to implement, while simplified algorithms may not be stable enough, especially with a significant increase in the number of components. The proposed approach combines several previously developed techniques: the generation of initial equilibrium constants based on stability analysis; the application of a simplified model for hydrocarbon solubility in water (reduces the three-phase equilibrium problem to a pseudo-two-phase problem); the use of the Newton method for solving the system of equations. The algorithm is implemented using the Soave-Redlich-Kwong equation of state and tested on several mixtures, including simple ternary systems and multicomponent gas-condensate mixtures with compositions corresponding to real petroleum reservoir fluids. A comparison of the calculation results obtained using the developed software module with data from the PVTSim software was conducted. It is shown that even the simplified free-water flash (FWF) model provides good agreement with the commercial software data, while solving the equilibrium problem improves the accuracy of the results, reducing the average absolute relative error to less than one percent. The algorithm demonstrates high stability across the entire range of thermobaric conditions, making it a practical tool for PVT modeling.
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