One of the main techniques to achieve phase behavior calculations of reservoir fluids is the equation of state. Soave - Redlich - Kwong equation of state can then be used to predict the phase behavior of the petroleum fluids by treating it as a multi-components system of pure and pseudo-components. The use of Soave – Redlich – Kwon equation of state is popular in the calculations of petroleum engineering therefore many researchers used it to perform phase behavior analysis for reservoir fluids (Wang and Orr (2000), Ertekin and Obut (2003), Hasan (2004) and Haghtalab (2011))

This paper presents a new flash model for reservoir fluids in gas – oil separation stations. The proposed model uses Soave - Redlich - Kwong equation of state for calculating vapor and liquid mole fraction after each separation stage of heavy oils. The form of Soave - Redlich - Kwong equation of state that used in this paper is the cubic equation for Z-factor. The solution of this cubic equation depends on using Newton's and false position methods to find the three roots of the equation. This model finds the initial guess of Z-Factor using the false position technique and calculates the exact value of Z-Factor by applying the Newton-Raphson technique. The new model distinguishes between liquid and vapor phases through choosing the minimum root as the Z factor for the liquid phase and the maximum root as the Z factor for the vapor phase.

Many empirical relationships have been applied to calculate the related parameters which are utilized to perform the flash calculation. AL–Dulaimy's correlation (2002) is utilized to calculate the critical properties for the plus fraction (C7+). Wilson's correlation (1968) is used to guess the initial value of equilibrium ratio. Whitson's correlation (1984) is employed for calculating boiling point for plus fraction (C7+).

The new model has been tested using Iraqi oils data for several degasing stations. Calculated results of vapor mole fractions are found to be in excellent agreement with the field data. The graphical demonstration of the calculated results and field data shows credit index to utilize the new model in separation calculations of heavy oils under different temperatures. Two more Iraqi separation stations have been used to check the efficiency of the proposed model.

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