Summary
Water is usually considered insoluble in the oil phase; however, at the temperatures typically encountered in the steam-injection process, water may have higher than 40 mol% solubility in the oil phase. On a mass basis, experimental results from the literature indicate water solubility as high as33%. We developed a practical and robust algorithm for a 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 found 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 the steam-injection process. A simulation study shows that when water is soluble in the oil phase, it may increase oil recovery appreciably. We also found that the oil fluids should be characterized with at least three components for accurate compositional thermal reservoir-simulation study.
Introduction
Steam injection is used widely as an improved-oil-recovery method for the production of heavy oil and many light-oil resources. Conventional reservoir simulation of the steam-injection process simplifies the computations by ignoring water solubility in the oil phase. However, as temperature increases, water solubility in the oil phase increases significantly. Griswold and Kasch studied water/oil mutual solubilities at elevated temperatures. Their data show that for a 54.3°API naphtha, the solubility of water in oil is 16.18 mol% at431.6°F; for a 42°API kerosene, the solubility of water in oil is 34.97 mol% at507.2°F; and for a 29.3°API lube oil, the solubility of water in oil is 43.44mol% at 537.8°F. Nelson also showed that water solubility in oil is as high as42 mol% at 540°F. Heidman et al. showed that the solubility of water in liquidC8 is 38.7 mol% at 500°F. Glandt and Chapman obtained up to 33.3 wt% of water dissolved in crude-oil mixtures and analyzed its effect on oil viscosity. This high solubility will dramatically change the viscosity, density, and thermal expansion of the hydrocarbon phase and, consequently, affect the production performance. Therefore, a rigorous and efficient multiphase flash algorithm is needed to evaluate the phase equilibrium of water/hydrocarbon systems. Also, fully compositional thermal reservoir simulations, which consider water-in-oil solubility, are necessary to evaluate the extent to which the water-in-oil solubility affects oil recovery in the steam-injection process.
Robust Flash Calculation Algorithm for Microemulsion Phase Behavior
