PROPERTIES OF RESERVOIR GASES - Petroleum Reservoir Fluid Property Correlations - page 27

(2.19)
This equation can be combined with the definition of pseudoreduced
pressure, equation (2.3), to result in equation (2.20) 37
(2.20)
where the product on the left side is called pseudoreduced compressibility .
(2.21)
Mattar et al. combined equation (2.20) with equation (2.8) to arrive
at equation (2.22). 38
(2.22)
Since the DAK equations have been selected as the most nearly
accurate source of z-factors, they will be used to evaluate the derivatives
in equation (2.22).
(2.23)
The accuracies of the values of gas compressibility, c g , calculated
with equations (2.21)-(2.23) depend on how well the DAK equations
represent the slopes of the isotherms of the graphical z-factor correlation
of Standing and Katz.