Page 313 - Principles of Applied Reservoir Simulation 2E

298 Principles of Applied Reservoir Simulation

temperature of the oil decline from reservoir to surface conditions. Consequently,
measurements of the change in volume as a function of pressure are desirable,
especially for the oil phase.
The determination of gas formation volume factor provides an interesting
contrast to the determination of oil formation volume factor. Gas formation
volume factor is often determined with reasonable accuracy using the real gas
equation of state PF= ZnRT where n is the number of moles of gas in volume
Fat pressure P and temperature T. The gas compressibility factor Z equals one
if the gas is an ideal gas. For real gases, Z * 1 for most pressures and tempera-
tures,

Specific Gravity
Specific gravity is defined as the density of a fluid divided by a reference
density. Gas specific gravity is calculated at standard conditions using air density
as the reference density. The specific gravity of gas is defined by

g
M a(air) 29
where M a is apparent molecular weight. Apparent molecular weight is calculated
as the mole fraction weighted average

M = Yy.M.
a L-t J i i
i=\
is the number of components,^, is the mole fraction of component /,
where N c
and M { is the molecular weight of component i.
Oil specific gravity is calculated at standard conditions using fresh water
density as the reference density. Oils are often characterized by specifying their
API gravity, which is related to oil specify gravity y 0 at standard temperature
and pressure by the equation

To

Heavy oils are oils with a relatively large J 0 and a relatively low API gravity.
Heavy oils typically do not contain much gas in solution. By contrast, light oils