The variation of fluid composition with depth in a reservoir is called compositional grading. As depth increases, the mole fraction of light components decreases, density increases andGOR decreases. In the case of these reservoirs prediction of phase behavior and accurate characterization of reservoir fluids becomes more critical. The reservoir under study is a saturated oil reservoir with a large initial gas cap and a tall column of oil. This fractured carbonate reservoir contains intermediate oil and is produced under natural depletion mechanism at the all of its life time. In this work several complete PVT reports from different depths of Asmari reservoir that show definite variations in composition and physical properties (especially saturation pressure), and some other reservoir/field data were used. Two fluid samples were chosen as the most representative samples, one for light samples and another for heavy samples. Also an optimum regression method was applied using a PVT package to characterize the reservoir fluid system. Finally results were prepared to use in Asmari reservoir simulation.
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