Page 89 - PVT Property Correlations

Wet Gases Chapter | 4 67

Option 1—From Reservoir Gas Composition
If the reservoir gas composition is available, we can use the standard proce-
dure (explained in the Dry Gases Chapter) to calculate the pseudocritical
properties and follow the dry gas procedure to calculate z-factor and other
PVT properties. This option is essentially the same as calculating PVT prop-
erties for dry gases from reservoir gas composition. The main difference will
be that both the mole fraction for C7 1 and C7 1 molecular weight in wet
gases are higher than that of dry gases. Practically, since the C7 1 mole
fraction is usually significant in wet gases, it is necessary to have a reason-
ably accurate estimate for its molecular weight.

Option 2—From Reservoir Gas Specific Gravity

This calculation option requires that we find out the specific gravity of the
reservoir gas (not the surface gas). The specific gravity of reservoir gas is
calculated from the composition of reservoir gas. If separator gas and liquid
compositions are the only compositions available, then recombination calcu-
lations are required to compute the reservoir gas composition. If separator
gas, stock-tank gas, and stock-tank liquid compositions are available, recom-
bination calculations are required to first calculate the reservoir gas composi-
tion. Once the reservoir gas composition is available, the specific gravity of
reservoir gas can be computed. The PVT properties calculation procedure is
the same as the calculation procedure for dry gas properties from specific
gravity of the gas (explained in the Dry Gases Chapter). The main difference
is that we can consider the measured specific gravity on surface for dry gases
to represent the reservoir gas specific gravity. However, for wet gases, the
surface gas does not generally represent reservoir gas. The reservoir gas spe-
cific gravity for wet gases must be calculated from reservoir gas
composition.

Option 3—From Knowledge of Surface CGR
When either reservoir gas composition or surface fluids (gas and condensate)
compositions are not available, we can use measured surface GOR or CGR
to estimate the reservoir gas specific gravity. Eq. 4.1 is used to estimate the
specific gravity of reservoir gas. The equation requires the surface GOR (or
CGR), specific gravity of separator gas, and API gravity of stock-tank
condensate.
Rγ 1 4600γ
γ gw 5 gSP o ð4:1Þ
o
R 1 133; 300 γ =M o