Page 92 - Petrophysics

66 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

mixed hydrocarbons and to evaluate their fluid flow behavior in the
reservoir.

Gas Density
The density of gases may be calculated from the equation of state for
real gases (Equation 2.15), which is corrected for non-ideal behavior by
a compressibility factor 2. The factor 2 is the ratio of the actual volume
occupied by a real gas to the volume it would occupy if it behaved like
an ideal gas where Z = 1.0:

pV = ZmRT/M (2.15)
or
p = m/V = pM/ZRT (2.16)

where: p = pressure, psi.
v = volume, ft3.
2 = real gas deviation factor.
m = mass of gas, lb.
R = gas constant (10.73 psi-ft3/lbrnol-"R).

T = temperature, OR.
M = molecular weight of the gas.

Gravitational units are used because, to date, engineering charts in the
United States have not been converted to SI units.
The compressibility factor, or real gas deviation factor, is obtained
from the reduced temperatures and pressures and the compressibility
charts for pure and mixed gases (Figure 2.16). The reduced temperature
and pressure are calculated from the gas pseudocritical temperatures and
pressures as follows:

where: Tpr and ppr = pseudo reduced temperature and pressure,
respectively.
Tpc and ppc = critical temperature and pressure, respectively
(Table 2.5).

Viscosity of Gases
Gas viscosity varies with respect to temperature, pressure, and
molecular weight. The exact mathematical relationships have not been
developed; however, Carr et al. developed two charts that may be used