Page 50 - PVT Property Correlations

30 PVT Property Correlations

PVT PROPERTIES FOR DRY GASES
In fact, the most widely available types of hydrocarbon gases can be consid-
ered dry gases. Although some of these gases can produce some condensate,
the dry gas concept can be used to handle them. The engineering of dry gas
reservoirs requires multiple calculations, including material balance, reser-
voir and well production forecasts, flow in pipes calculations, and surface
facilities. All such calculations require estimation of gas PVT properties at
different pressures and temperatures. The PVT properties necessary for engi-
neering dry gases include z-factor, gas formation volume factor, gas density,
gas viscosity, and isothermal gas compressibility.
Although all these gas PVT properties can be obtained in the laboratory
by performing experiments on a representative sample, the number of full
PVT reports for dry gases is not significant. It is usually sufficient to mea-
sure the composition of the dry gas. The composition can also be measured
with simple equipment at the well site. Because dry gases are not expected
to drop condensate at reservoir or surface, it is usually accurate to take the
sample from the wellhead or at any point in the surface production system
(e.g., separator).

Specific Gravity of Gas
Gas-specific gravity is defined as the ratio of the gas density to the density
of dry air. Both densities are measured at the same temperature and pressure.
This relation is given by the following equation:
ρ g
γ 5 ð3:1Þ
g ρ
a
At standard conditions, the densities of gas and air can be represented by
the ideal gas law, where the ideal gas law is given by the following
equation:
pV 5 nR T ð3:2Þ
Both temperature and pressure must be at absolute conditions (i.e., tem-

perature in R and pressure in psia are used instead of F and psig). The

number of moles (n) can be represented by the mass of the gas divided by
the molecular weight as in the following equation:
m
n 5 ð3:3Þ
M
Recall that density is given by mass over volume, then
m
pV 5 RT ð3:4Þ
M