PVT Experiments: Separator Test
production-technology.org/pvt-experiments-separator-test/
By Islam Fetoui
October 13, 2017
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 rst separator is let into a second separator at a lower pressure and
temperature than the rst one. At which conditions, more gas will be liberated as
sketched in the figure below. As with the gas from the rst 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.
NB: Often the composition of the separator gas and stock tank gas are measured.
Example:
The separator test experiment results are presented in the following table. It includes a
Bo-factor for the reservoir uid at the saturation point (199.7 bar) and reservoir
temperature (97.8°C). This Bo-factor expresses how much saturated-oil at the reservoir
temperature will shrink through a four-stage separation with separator temperatures and
pressures as presented in Table. The remaining Bo-factors in the Table express the
shrinkage of the oil from the current stage of the separation down to stock tank
conditions. The separator gas/oil ratio equals the ratio between the volume of the gas
liberated from the current stage taken to standard conditions and the volume of the oil
from the last separator stage, which is at standard conditions.
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* This example is from the book “Phase Behavior of Petroleum Reservoir Fluids”.
Using the separator test data, for a given set of separator conditions, in conjunction with
the differential liberation data, will provide a means of obtaining the PVT parameters
required for field use. It is considered that the differential liberation data can be used to
describe the separation in the reservoir while the separator flash data account for the
volume changes between reservoir and stock tank.
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