PROPERTIES OF RESERVOIR GASES - Petroleum Reservoir Fluid Property Correlations - page 18

Gas Densities and Pressure Gradients
Gas densities are calculated with equation (2.9).
(2.9)
If a value of 10.732 psia cu ft/lb-mole ºR is used for universal gas
constant, R , then pressure should be in psia, temperature should be in
ºR, and the molecular weight of the gas can be calculated with equation
(2.10).
(2.10)
Values of z-factor can be obtained with equations (2.4)-(2.8). The
resulting density will be in pounds per cubic feet (lb/cu ft).
Equation (2.9) is not a correlation; it is simply a rearrangement
of the real gas equation, equation (2.2). Also, equation (2.10) is not a
correlation; it follows from the definition of gas specific gravity. Thus,
the results of the use of equation (2.9) are as accurate as the values of
z-factor used in the calculations.
Gas pressure gradients are calculated with
(2.11)
where g and g c have the same numerical value at sea level (in the set of
units used here) and thus are not usually shown in the equation. The
resulting pressure gradient has units of pounds per square inch per
feet (psi/ft).
Gas Formation Volume Factors
Gas formation volume factor is defined as the volume of gas at reservoir
temperature and pressure required to produce one standard volume
of gas at the surface. The equation for gas formation volume factor is
derived using the real gas equation, equation (2.2), to calculate the gas
volumes for reservoir conditions and standard conditions.