Page 79 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 18

700
PSEUDO CRITICAL PRESSURE, psia 650

600

CONDENSATE WELL FLUID

550 MISCELLANEOUS GASES

PSEUDO CRITICAL TEMPERATURE, degrees Rankine 500

450

400

350

300
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
GAS GRAVITY (Air = 1)

Fig. 1.7 Pseudo critical properties of miscellaneous natural gases and condensate
well fluids 19

c) Direct calculation of Z-factors

The Standing-Katz correlation is very reliable and has been used with confidence by
the industry for the past thirty-five years. With the advent of computers, however, there
arose the need to find some convenient technique for calculating Z−factors, for use in
gas reservoir engineering programs, rather than feeding in the entire correlation chart
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from which Z−factors could be retrieved by table look-up. Takacs has compared eight
different methods for calculating Z−factors which have been developed over the years.
These fall into two main categories: those which attempt to analytically curve-fit the
Standing-Katz isotherms and those which compute Z−factors using an equation of
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state. Of the latter, the method of Hall-Yarborough is worthy of mention because it is