Page 22 - Fundamentals of Enhanced Oil and Gas Recovery

10 Amirhossein Mohammadi Alamooti and Farzan Karimi Malekabadi

where P is pressure, V is volume, R is universal gas constant, and T is temperature.
The term n is numbers of moles of gas. On the other hand, molecular weight is
defined as the weight of the gas divided by numbers of moles:
m
MW 5
n
So the pressure volume temperature relationship for ideal gases can be rewritten as:
m
PV 5 RT
MW
Density is defined as the ratio of mass to the corresponding volume:
m
ρ 5
V
So,
PUMW
ρ 5
RT
where ρ is density, m is mass, V is volume, R is universal gas constant, and T is
temperature.

1.9.3.3 Specific Gravity
The term specific gravity is expressed as the gas density to the air density under the
same condition (pressure and temperature).
ρ
γ 5 gas
ρ
air
where γ is specific gravity, ρ is gas density, and ρ is air density.
gas air
In the case of standard conditions, the behavior of gases is close to ideal gases;
therefore the specific gravity can be rewritten as:
ðP sc UMW gas Þ=ðRT sc Þ
γ 5
ðP sc UMW air Þ=ðRT sc Þ

MW gas MW gas
γ 5 5
MW air 28:96

1.9.4 Compressibility Factor
The ideal gas relationship can be used for real gases at very low pressure. Evaluation
of the relationship proves it works under low pressure conditions with error less than
2 3%. In spite of that, for high pressure, especially the pressure usual in the petro-
leum industry, this relationship cannot be used anymore. Volumes of molecules, the