Page 126 - PVT Property Correlations

104 PVT Property Correlations

G p 5 G pðsurfÞ 1 GE 3 N p 5 G p;ps 1 G p;ss 1 G p;st 1 GE 3 N p ð5:11Þ
For a two-stage separation system (primary separator and stock-tank), the
following equation applies:
G p 5 G pðsurfÞ 1 GE 3 N p 5 G p;ps 1 G p;st 1 GE 3 N p ð5:12Þ
In practice, the gas production from secondary separator and stock-tank
is not usually available. In this case, the following equation is used to esti-
mate the total reservoir gas production.

G p 5 G p;ps 1 V eq 3 N p ð5:13Þ
where V eq is the vapor equivalent of one barrel of primary separator liquid
and is estimated from the correlations given by Gold et al. (1989).

Problem 2—Calculation of Original Gas in Place for Gas
Condensate Reservoir Using Material Balance Equations

The gas condensate fluid of Problem 1 is contained in a volumetric reservoir
and is produced under depletion. Use the available CVD data to compare the
calculated OGIP with the following assumptions: (1) dry gas assumption,
and (2) gas condensate assumption. Compare the p/z versus cumulative gas
produced in the two cases.
Table 5.13 shows the CVD experiment data where the stock-tank conden-
sate has gravity of 58.6 API (specific gravity of 0.77).

Solution of Problem 2
Calculating the equivalent molecular weight: M o 5 5954=ðAPI 2 8:811Þ 5
5954=ð58:6 2 8:811Þ 5 119:6 lbm/lbm mol
Calculating the condensate equivalent: GE 5 133γ =M o 5 133 3 ð0:77=
o
119:6Þ 5 0.856381 scf/STB
We use the gas equivalent (GE) to calculate the GE to the surface con-
densate and add it to the total dry gas (i.e., primary, secondary and stock-
tank gas) in every pressure step. Table 5.14 summarizes the results of the
calculated wet gas values.
Fig. 5.10 shows the comparison between p/z versus cumulative gas
produced when single-phase z-factor and two-phase z-factor are used.
Extrapolation of the pressure over two-phase z-factor line reaches the correct
OGIP for the CVD experiment (1000 Mscf), while this is not the case if sin-
gle phase z-factor is used.

Plant Products (Gallons Per Thousand)

As explained above, the NGL can be of significant value in gas condensate
reservoirs. The economics of gas condensate reservoirs depend on the cost of