Page 126 - Petrophysics 2E

POROSITY 99

An expression similar to Equation 3.6 may be derived for estimating the
initial gas-in-place. In this case it is convenient to express the gas volume
in cubic feet. At standard conditions-is., P,, = 14.7 psia and T,, = 6O"F,
the initial gas-in-place in a volumetric reservoir is given by:

(3.7)

where Bgi, the initial gas formation volume factor in ft3/SCF, is calculated
from:

(3.8)

The initial gas deviation (also called compressibility) factor, zi, is
calculated at the initial pressure pi of the gas reservoir. This factor
accounts for the difference between the actual and ideal gas volumes.

The reservoir temperature T is in degree Rankin (OR).
EXAMPLE

A volumetric gas reservoir has the following characteristics:

A = 1,320 acres T = 200°F
h=45ft Pi = 4,000 psia
@ = 0.175 zi = 0.916
Si, = 0.23

SOLUTION

The initial gas formation volume factor is:

0.916(460 + 200) ft3
Bgi = 0.02829 = 0.004276-
4,000 SCF

The initial gas in place is:

1
(1,320)(45)(0.175)( - 0.23)
G = 43,560 = 81.539 x lo9 SCF
0.004276