Reservoir Fluid Properties - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 28

Question #3.3 ʼ wwf
Temperature = 190 °F
Water salinity = 100,000 ppm NaCl
Find ʼ wwf = ___ cp

Gas

Gas Formation Volume Factor B g . The behavior of natural gases is non ideal. Thus,
the ideal gas law needs a “fudge factor” to make it truly reflect the behavior of natu-
ral hydrocarbon gases. This fudge factor is given the symbol Z and is known as the
supercompressibility factor .
The ideal gas law can then be re-written as:

V
VZ

T
T

p
p

1 1

==´´

sc sc wf

B

If the Z factor is known for the conditions encountered and the gas in question,
then B g may be found by direct solution of this equation. However, Z is not normally
known. A short-cut method uses Fig. 3.12 , which requires only temperature, pres-
sure, and gas gravity as inputs; 1 / B g may be read directly as output.
An example of how to use the “quick look” chart to find 1 / B g and hence V gwf is
straightforward.
If V gsc = 400 cubic ft, ʳ g = 0.70, T wf = 200 °F and P wf = 2,000 psia.
Then select the ʳ g = 0.70 section and enter the abscissa at 2,000 psia. From there
go vertically to 200 °F and then left to 1 / B g = 125.
Remembering that 1 / B g = 125 = V gac / V gwf it follows that V gwf = 3.2 cubic ft.

g wf wf sc

Question #3.4 B g
Pressure = 3,000 psi
Temperature = 200 °F
Gas gravity = 0.7
Find 1/ B g = ___

When more accurate results are required, the Z factor must be determined. This
is a more laborious task and involves:

1. finding the pseudo-critical pressure ( p pc )
2. finding the pseudo-critical temperature ( T pc )
3. finding the pseudo-reduced pressure ( p pr )