PVT Experiments: Separator Test

Separator Test experiments are carried out for both oil and gas condensate mixtures. A sample of reservoir liquid is placed in the laboratory cell and brought to reservoir temperature and bubble-point pressure. Then the liquid is expelled from the cell through a number of stages of separation.  Usually, two or three stages of separation are used, with the last stage at atmospheric pressure and near-ambient temperature (60 to 80°F).

The gas is let out of the separator through the top and is transferred to standard conditions, where its volume is measured. As for the differential liberation experiment, liquid dropping out from the gas is converted to an equivalent gas volume at standard conditions.

The liquid from the first separator is let into a second separator at a lower pressure and temperature than the first one. At which conditions, more gas will be liberated as sketched in the figure below. As with the gas from the first separator, this gas is transferred to standard conditions.

The oil remaining after gas removal is brought to the conditions of the next separator stage. The gas is removed again and quantified by moles and specific gravity. Oil volume is noted, and the process is repeated until stock-tank conditions are reached. Final oil volume, Vo, and specific gravity, SGo, are measured at 60°F.

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PVT Experiments: Constant Composition Expansion

Once we have a sample of reservoir fluid, either directly or by recombination, we need to do reservoir fluid measurement, often called PVT analysis. There are mainly 6 fluid properties measured: oil density, gas density, solution GOR, bubble point pressure, formation volume factor, and viscosity. This post will discuss how the bubble point pressure is measured and what the Constant Composition Expansion Experiment is?

Experimental apparatus:

The bubble point pressure is determined by an experiment called Constant Composition Expansion (CCE). It’s also called Constant Mass Expansion (CME) experiment or Pressure-Volume (PV) relationship.

The apparatus used to perform this experiment is the PV cell, as shown in the figure below. The fluid is charged in the PV cell after recombining the oil and gas in the correct proportions. The temperature, controlled by a thermostat, is maintained constant throughout the experiment (the measured reservoir temperature). The cell pressure is controlled by a positive displacement pump and recorded on an accurate pressure gauge.

Bubble Point Pressure:

The bubble point pressure is the pressure, at some reference temperature, that the first bubble of gas is liberated from the liquid phase. It is also known as the saturation pressure.
The reference temperature is usually the reservoir temperature, but any temperature can be used.
Note that the bubble point pressure is a function of temperature and changing the reference temperature will change the bubble point pressure.

Bubble Point Measurement:

A schematic of a Constant Composition Expansion experiment steps is shown in the figure below. The PV cell pressure is initially raised to a value greater than the bubble point pressure. the pressure is subsequently reduced in stages, and on each stage, the volume of the cell is recorded.

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