The oil industry deals with wellhead fluids which are a mixture of hydrocarbons, water and impurities. In a two-phase separator design, the gas and liquid phases of a fluid stream are separated at a specific temperature and pressure. A proper separator design is important, because a separation vessel is normally the initial processing vessel in the surface facilities. Improper design can bottleneck and reduce the capacity of the entire facilities. The optimum separator pressure is defined as the pressure at which the maximum liquid volume is accumulated in the stock tank per volume of the feeded reservoir fluid.

The objective of this study is to find the optimum pressure in a single stage separation for the oil that was sampled from a well head at 300 psia by a "flash equilibrium separator". This type of sampling technique enables to determine gas-oil ratios and shrinkage by a flash liberation or vaporization method. A bottom hole or recombination sample is expanded from above its bobble point pressure into a chamber which is equipped with a back pressure and temperature controller. The volume of gas librated is measured with a gas meter at the atmospheric condition while the liquid volume is measured in the separator chamber. Then the liquid is drained and its gravity is measured.

In this paper, by using the "flash equilibrium separator", "gasometer" and "densimeter" apparatuses, the optimum pressure for an Iranian oil sample is obtained and also the gas-oil ratio, oil formation volume factor and residual oil gravity are depicted all as a function of the separator pressure at the desired temperature.

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