Page 85 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 24

GAS
COMPRESSIBILITY

psi 1
-1
16 ×10 c g = p
-4
14
1 1 δZ
c g = p - Z δp
12

10

8

6

4

2

0
0 1000 2000 3000 4000 5000
Pressure (psia)

Fig. 1.9 Isothermal gas compressibility as a function of pressure (gas gravity = 0.85;
temperature = 200° F)

M 19.91
γ g = = = 0.687 (air = 1)
M air 28.97

The density at standard conditions can be evaluated using equ. (1.27) as
Mp 19.91 14.7
×
sc = sc = = 0.0524 Ib / cu.ft
Z RT sc 1 10.732 520
×
×
sc
or, alternatively, using equ. (1.30) as
ρ sc = 0.0763 γ g = 0.0524 lb / cu.ft

2) The density of the gas in the reservoir can be directly calculated using
equ. (1.27), or else by considering the mass conservation of a given quantity of
gas as

(ρV) sc = (ρV) res

or

ρ res = ρ scE

o
which, using equ. (1.25), can be evaluated at 2000 psia and 180 F as
35.37ρ sc p 35.37 0.0524 2000
×
×
ρ res = = = 6.696 Ib / cu.ft
ZT 0.865 (180 + 460)
×