Implementation of the methodology for calculation of the three-phase equilibrium of hydrocarbons and the aqueous phase

The paper discusses the development and implementation of the algorithm for calculation of the three-phase equilibrium in the hydrocarbon/aqueous system. The work is done as a part of PVT module improvement for the “d-Flow” hydraulic simulator which is under development at Novosibirsk R&D Center LLC. The numerical methods used in the presented three-phase algorithm are simple and available in the literature, however these methods are often need to be improved and tuned before an actual application. In order to increase phase composition calculation stability, a modification for stationary point selection algorithm is proposed. Moreover, unexpected behavior of the algorithm may be revealed during implementation, which is not described in the original articles. This work attempts to present the three-phase equilibrium calculation algorithm ready for direct use in the designed hydraulic simulator and provide the detailed description for different aspects and difficulties of the implementation. The numerical schemes for the phase stability test and the calculation of equilibrium compositions are described, and the general equilibrium search algorithm is given. To test the algorithm, simulations of different mixtures are done and phase diagrams are presented for several mixtures. The solubility of hydrocarbons in water for binary mixtures is also calculated and compared with experimental data from the literature.