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 as
33%. 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% at
431.6°F; for a 42°API kerosene, the solubility of water in oil is 34.97 mol% at
507.2°F; and for a 29.3°API lube oil, the solubility of water in oil is 43.44
mol% at 537.8°F. Nelson also showed that water solubility in oil is as high as
42 mol% at 540°F. Heidman et al. showed that the solubility of water in liquid
C8 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.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
12 January 2004
- Revised manuscript received:
27 July 2005
- Manuscript approved:
8 August 2005
- Version of record:
15 December 2005
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