Page 88 - PVT Property Correlations

66 PVT Property Correlations

to define the wet gas and generate its PVT properties. Initial CGR between
10 and 66 STB/MMscf is associated with wet gases. The ranges suggested
by McCain are modified in this book based on analysis of many gas samples
and based on considering the economic value of condensate relative to the
produced gas. Details on the modified criteria are given in the Reservoir-
Fluid Classification (Chapter 2).

PVT PROPERTIES FOR WET GASES
The engineering of wet gases includes reservoir, production, and process cal-
culations. Almost all calculations required to forecast the reservoir behavior
or estimate production rates of a well require PVT properties. The required
PVT properties for engineering wet gases include the same set of PVT prop-
erties required for engineering dry gases (z-factor, gas formation volume fac-
tor, gas viscosity, gas density, and gas compressibility) and for estimating
dew point pressure and natural gas liquids (NGL) derived from gas plants.

HANDLING PVT PROPERTIES FOR WET GASES
As the wet gases lie between dry gases and gas condensates, the prediction of
wet gas properties can be approached in two different ways. Wet gases can be
treated as gas condensates or as dry gases. We recommend the latter approach
when condensation in the reservoir is not significant. Chapter 2 contains addi-
tional explanation as to which gases can be approximated as wet gas.
If the wet gas is treated as if it were a gas condensate, the best approach
will be to run complete gas condensate PVT experiments (including CVD
experiment) on a representative reservoir fluid sample. The laboratory obser-
vations should be used to tune an equation of state (EOS) model. The resul-
tant EOS model can then be utilized to generate a compositional or modified
black oil PVT model.
If the wet gas is treated as dry gas, then the composition or specific grav-
ity of the gas will be utilized in deriving all required PVT properties from
correlations. While the application of gas composition is straightforward (as
with dry gases), the application of specific gravity is not. In dry gases, the
composition of surface gas is similar to the composition of reservoir gas, and
therefore, the surface gas specific gravity can be used as input to the dry gas
PVT correlations. In wet gases, however, surface gas and reservoir gas do
not have the same composition. In wet gases, gas drops liquid condensate in
surface facilities. Therefore, if the reservoir gas specific gravity is to be
included in any PVT property calculations, surface gas specific gravity will
have to be adjusted to account for the condensate drop out on surface.
Several approaches have been developed to incorporate the surface conden-
sate into the calculation of reservoir gas specific gravity. The following sum-
marizes these different approaches.