Page 75 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 14

pV
Z =
14.7V o

By varying p and measuring V, the isothermal Z(p) function can be readily obtained.
This is the most satisfactory method of determining the function but in the majority of
cases the time and expense involved are not warranted since reliable methods of direct
calculation are available, as described below.

b) The Z-factor correlation of Standing and Katz

This correlation requires a knowledge of the composition of the gas or, at least, the gas
gravity. Naturally occurring hydrocarbons are composed primarily of members of the
paraffin series (C nH 2n+2) with an admixture of non-hydrocarbon impurities such as
carbon dioxide, nitrogen and hydrogen sulphide. Natural gas differs from oil in that it
predominantly consists of the lighter members of the paraffin series, methane and
ethane, which usually comprise in excess of 90% of the volume. A typical gas
composition is listed in table 1.1.
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In order to use the Standing-Katz correlation it is first necessary, from a knowledge of
the gas composition, to determine the pseudo critical pressure and temperature of the
mixture as

p pc n p ci (1.16)
=
i
i
and

T pc nT (1.17)
=
i ci
i
where the summation is over all the components present in the gas. The parameters p ci
th
and T ci are the critical pressure and temperature of the i component, listed in table 1.1,
while the n i are the volume fractions or, for a gas, the mole fractions of each
component (Avogrado's law). The next step is to calculate the so-called pseudo
reduced pressure and temperature

p
p = (1.18)
pr
p pc

and

T
T pr = (1.19)
T pc

where p and T are the pressure and temperature at which it is required to determine Z.
In the majority of reservoir engineering problems, which are isothermal, T pr is constant
and p pr variable.

With these two parameters the Standing-Katz correlation chart, fig. 1.6, which consists
of a set of isotherms giving Z as a function of the pseudo reduced pressure, can be
used to determine the Z−factor.