PROPERTIES OF RESERVOIR GASES - Petroleum Reservoir Fluid Property Correlations - page 8

temperature and pressure, usually 60ºF and atmospheric pressure.
Several authors have proposed equations for estimation of pseudocritical
properties using gas specific gravities. The Standing and the Sutton
equations require modification when nonhydrocarbon components
(hydrogen sulfide, carbon dioxide, and nitrogen) are present. 9
Both recommended the Wichert-Aziz correlation for adjusting the
pseudocritical properties for hydrogen sulfide and carbon dioxide; no
adjustment for nitrogen is available for these correlations. 10 Elsharkawy
et al. did not mention adjustments for presence of nonhydrocarbons;
however, Elsharkawy and Elkamel did take the compositions of the
nonhydrocarbons into account. 11 The Piper et al. correlation equations
include adjustments for all three nonhydrocarbons. 12 Londono et al.
did not offer any adjustments for nonhydrocarbon components. 13
Evaluation of proposed correlation equations for gas z-factors based
on gas specific gravities and nonhydrocarbon contents
Values of gas z-factors from a large set of petroleum industry fluid
property reports (PVT reports) were assembled for use in evaluating
the several correlations. The z-factors were measured at reservoir
temperatures for various pressures. All gases were samples of reservoir
gases; no surface gases, artificial gas mixtures, or pure gases were
included in the data set. The gases were gas condensates, gases liberated
from oils at pressures below the bubblepoint pressure of the oil, and
some dry gases. The z-factors of the gas condensates were measured
in constant composition expansion and constant volume depletion
experiments. The z-factors of solution gas were measured in differential
liberation experiments of the oils. The dry gas z-factors were measured
in constant composition expansion experiments. The samples were
obtained from reservoirs throughout the world.
Table 2-1 gives the ranges of the variables in the data used to evaluate
the correlations that use gas specific gravity to estimate the gas z-factors.
There are 6,000 lines of data that include the variables listed in the
table. The ranges of temperatures, pressures, and gas specific gravities
cover nearly all of the reservoir situations encountered in the petroleum
industry. More than three-quarters of the gases contained some
nonhydrocarbon components; 1,214 gas samples had 5 mole percent
(mol%) nonhydrocarbons or more, and 145 samples contained more
than 50 mol% nonhydrocarbons.