Predicting Phase Behavior of Water/Reservoir-Crude Systems With the Association Concept
- A.A. Shinta (Reservoir Engineering Research Inst.) | Abbas Firoozabadi (Reservoir Engineering Research Inst.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1997
- Document Type
- Journal Paper
- 131 - 137
- 1997. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 5.2.1 Phase Behavior and PVT Measurements, 5.2.2 Fluid Modeling, Equations of State, 5.4.6 Thermal Methods, 4.6 Natural Gas
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An association model has been incorporated in the Peng-Robinson equation of state (PREOS) to use the combined model for water/reservoir crudes phase-behavior calculations. The interaction coefficients between H2O and hydrocarbons follow a simple relationship. Such a relationship makes the use of the association equation of state (AEOS) both accurate and simple. The computed high-pressure crude/steam distillation results by the AEOS are in excellent agreement with the data.
The mutual solubilities of water/reservoir fluids depend on temperature, pressure, and the composition of the crude. The solubility of hydrocarbons in water is often small and may be neglected. The solubility of water in a hydrocarbon-rich liquid phase at high temperatures can be as high as 50%. There is a strong effect of water presence on the phase behavior of reservoir crude oils at high temperatures. As an example, consider the steam-distillation process. Depending on oil gravity, 15 to 30 vol% of oil could vaporize in the presence of steam at a pressure of approximately 500 psia and a temperature of 467°F. In the absence of steam, there may be no vaporization at the same temperature and pressure. Another example is the effect of water on bubblepoint of water/hydrocarbon mixtures. It is a common practice that the bubblepoint measurements are conducted on an H2O-free basis. It will be revealed in this paper that the three-phase bubblepoint pressure of some separator oil streams could decrease by 10% for certain compositions in the presence of water.
Equations of state (EOS's) have been used successfully to describe the phase behavior of reservoir crude and gas condensates. The phase behavior of H2O/reservoir crude oils both at reservoir temperatures and steamflooding conditions has not yet been successfully predicted by an EOS. Recently, we have demonstrated that incorporation of the association concept into a simple cubic equation such as the PREOS significantly improves phase behavior prediction of water/hydrocarbon and nonhydrocarbon systems. Data on binary, ternary, and quaternary mixtures of H2O and hydrocarbons from C1 to n-C10, and nonhydrocarbons such as CO2 and H2S were examined in Ref. 4. The investigation was, however, limited to simple systems in the two-phase region. To use the AEOS approach for H2O/reservoir-crude systems, (1) interaction coefficients of H2O/hydrocarbons should be provided, and (2) numerical difficulties in the three-phase flash and three-phase bubblepoint calculations have to be resolved.
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