Water is usually considered insoluble in the oil phase, however, at the typical temperatures encountered in steam injection process, water may have higher than 40 mole% solubility in the oil phase. We developed a practical and robust algorithm for water/oil/gas three-phase flash calculation. The algorithm is based on the well-developed vapor-liquid two-phase flash calculation algorithm and avoids trivial or false solutions commonly occurred in multiphase flash calculations. We also developed a fully compositional, thermal reservoir simulator, considering water/oil mutual solubility, to study the effect of water-in-oil solubility on oil recovery in steam injection process. Simulation study shows that when water is soluble in the oil phase, it may appreciably increase oil recovery. We also found that the oil fluids should be characterized with at least 3 components for accurate compositional thermal reservoir simulation study.

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