In compositional simulation of oil and gas reservoirs, equation-of-state (EOS) methods are seeing increasing usage for phase equilibrium calculations. EOS compositional reservoir simulation is an accurate and powerful means to model complicated phase and flow behavior when the displacement process depends on pressure and fluid composition. Many field development projects have strong composition dependence, such as miscible gas injection for oil reservoirs or liquid recovery under lean gas injection for condensate reservoirs or water-alternating-gas injection.

This paper presents characterization of some crude oil samples from an Iranian reservoir using the Peng-Robinson Equation of State (PR-EOS) to arrive at one EOS model that accurately describes the PVT behavior of crude oil produced from the different wells in the reservoir. The multi-sample characterization method is used to arrive at one consistent model for crude oil for the whole reservoir.

However, uncertainties still exist that are associated with the use of an EOS to model petroleum reservoir fluids because of the complicated nature of such fluids and limitations of the most commonly used EOS's. Tuning the adjustable EOS parameters to match experimental fluid PVT data is a common practice. The tuning procedure for the EOS is done systematically by matching the volumetric and phase behavior results with laboratory results. Also, a consistent C7+ pseudo component split using the Whitson splitting method is used for all samples to arrive at a consistent model for crude oil for the whole reservoir.

Results showed a good agreement between model predictions and laboratory data. Also the results demonstrate that multi-sample method has high ability to provide one EOS model for the crude oil using PVT test results from different wells. The EOS model developed for this particular field may be used in reservoir simulation studies to optimize hydrocarbon recovery.

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