Page 109 - PVT Property Correlations

Chapter 5

Gas Condensates

Gas condensate fluids (also called retrograde gas condensates) are usually
found in deeper reservoirs. From the phase behavior point of view, gas con-
densates lie between wet gases and volatile oils reservoir fluids. They repre-
sent reservoirs that predominantly contain gas, but produce significant
amounts of liquids when the gas reaches the surface. The liquids are origi-
nally vaporized in the gas phase, and condense on surface due to the inability
of the gas to retain those liquids when it reaches a surface pressure and tem-
perature below reservoir pressure and temperature. Fig. 5.1 represents a typi-
cal gas condensate phase diagram. Both reservoir temperature and separator
conditions lie within the two-phase envelope.
Gas condensate reservoirs are found either as under-saturated (initial res-
ervoir pressure is higher than dew point pressure) or saturated (initial reser-
voir pressure is equal to dew point pressure). The reservoir temperature is
higher than the critical temperature. Above the dew point pressure, the gas
condensate fluid has constant composition, and the composition of both the
produced gas and condensate is constant. When production occurs below
the dew point pressure, gas and condensate change composition. Whether the
initial pressure lies above or on the dew point curve, the reservoir pressure
decreases (at the reservoir temperature) with reservoir depletion. This
decrease causes some condensate to fall (drop out) from the gas phase in the
reservoir. With further reduction in reservoir pressure, the condensate (liq-
uid) saturation in the reservoir increases until it reaches a pressure point
(varying according to the gas condensate fluid) after which liquid saturation
decreases. This phenomenon is termed “retrograde condensation.” At these
low pressures, the reservoir gas (vapor) composition becomes leaner and
can accept revaporized condensate. In general, the liquid that drops out in
the reservoir does not move and can be recovered only by means of ex-
pensive gas cycling operations. Fig. 5.2 shows the surface (producing)
condensate gas ratio (CGR) with reservoir pressure for a typical gas
condensate fluid.
According to McCain (1994), reservoirs that initially produce with CGR
between 66 and 312 STB/MMscf are considered gas condensate reservoirs
(equivalent to GOR between 3200 and 15,000 scf/STB). Another criterion

PVT Property Correlations. DOI: https://doi.org/10.1016/B978-0-12-812572-4.00005-9
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