Page 111 - PVT Property Correlations

Gas Condensates Chapter | 5 89

PVT PROPERTIES FOR GAS CONDENSATES
Gas condensates carry significant economic value over dry gas reservoirs.
They also pose challenges in performance management. The two main chal-
lenges in producing gas condensate reservoirs are (1) significant loss of
economic value due to condensation of liquids in the reservoir and (2) loss
of well productivity due to the formation of high liquid saturation around the
wellbore (condensate banking).
In general, prediction of reservoir behavior can be challenging in gas con-
densate reservoirs. Understanding phase changes requires the use of appro-
priate PVT properties that are capable of capturing the variations that occur
in solubility and revaporization in addition to volume changes of both oil
and gas (El-Banbi et al., 2000a; Fevang et al., 2000). The phase behavior
changes can also lead to production problems such as condensate banking
(El-Banbi et al., 2000b).
Several PVT properties are required for engineering gas condensate reser-
voirs. These PVT properties include the same set of PVT properties required
for engineering dry gases (specific gravity of gas, z-factor, gas formation vol-
ume factor, gas viscosity, gas density, and gas compressibility), in addition
to dew point pressure, two-phase z-factor, CGR, and natural gas liquids
(NGL).
PVT properties for gas condensate fluids are required in almost all reser-
voir, production, and surface facilities calculations. The economic value of
gas condensate reservoirs depends largely on the richness of the gas (how
much condensate drops out from the gas when it reaches the surface) and the
management of the reservoir. In some cases, additional value can be obtained
when the gas stream is further processed in a gas plant to produce additional
liquids. The following sections summarize the PVT properties that are
needed in calculations related to management of gas condensate reservoirs.

Specific Gravity of Gas Condensates
At dew point pressure and above, gas composition is constant. The specific
gravity of the gas above the dew point pressure is defined and obtained in
the same way as for wet gas fluids. Chapter 4, Wet Gases, contains the
details on the options available to calculate the specific gravity of the reser-
voir gas.

Dew Point Pressure
Dew point pressure is defined as the pressure at which the first droplet of liq-
uid appears at a specific temperature. The phase envelope for a typical gas
condensate shows that the dew point pressure is a function of the