Page 82 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 21

Z - Factor

0.95

0.85

0.75
0 1000 2000 3000 4000 5000
Pressure (psia)

Fig. 1.8 Isothermal Z− −− −factor as a function of pressure (gas gravity = 0.85;
temperature = 200° F)

V volume of n moles of gas at standard conditions
E = sc =
V volume of n moles of gas at reservoir conditions

and applying equ. (1.15) at both standard and reservoir conditions this becomes

V p T Z
E = sc = sc sc (1.24)
V p sc T Z

For the field units defined in connection with equ. (1.13), and for standard conditions of
p sc = 14.7 psia, T sc = (460+60) = 520°R and Z sc = 1, equ. (1.24) can be reduced to

p
E = 35.37 (vol/ vol) (1.25)
ZT

At a pressure of 2000 psia and reservoir temperature of 180°F the gas whose
composition is detailed in table 1.1 has a Z−factor of 0.865, as already determined in
sec. 1.5(b). Therefore, the corresponding gas expansion factor is

35.37 2000
×
E = = 127.8 (vol/ vol)
0.865 640
×
In particular, the gas initially in place (GIIP) in a reservoir can be calculated using an
equation which is similar to equ. (1.2) for oil, that is