Page 71 - Introduction to Petroleum Engineering

OIL PROPERTIES 55
The real gas equation of state can be written in the form

pV
Z = (3.12)
nRT
where Z is the dimensionless gas compressibility factor, R is the gas constant, and n
is the number of moles of gas in volume V at pressure p and temperature T. The gas
is an ideal gas if Z = 1 and a real gas if Z ≠ 1. Gas FVF for a given temperature
and pressure is calculated from the real gas equation of state as
p ZT reservoir volume
B = sc = (3.13)
g
ZT p standard volume
sc sc
The subscript sc denotes standard conditions (typically 60°F and 14.7 psia).
Viscosity. The viscosity of gases at reservoir conditions usually ranges from
0.02 to 0.04 cp. Correlations are available for more precise estimates. Viscosities of
gases are rarely measured for oil and gas applications—they are normally estimated
with correlations.
Heating Value. The heating value of a gas can be estimated from the composition
of the gas and heating values associated with each component of the gas. The heating
value of the mixture H is defined as
m
m ∑
H = N c y H i (3.14)
i
i=1
where N is the number of components, y is the mole fraction of component i, and H i
c
i
is the heating value of component i. Heating values of individual components are
tabulated in reference handbooks. The heating value of a natural gas is often between
1000 and 1200 BTU/SCF where BTU refers to energy in British thermal units and
SCF refers to standard cubic feet of gas.

3.5 OIL PROPERTIES

Examples of correlations for estimating three properties of oils are provided in this
section: BP pressure, FVF, and viscosity. Many correlations have been published.
They often represent a particular geographic region or selection oil. The selection of
a correlation should take into account the source of the data that was used to prepare
the correlation. Correlations based on McCain (1990) are used here.
bubble‐Point Pressure. If a container is filled partly with oil and the remainder
with gas, the amount of gas dissolved in the oil increases as pressure in the container
increases. As long as some gas phase remains in the container, the applied pressure
is the saturation pressure, and it is often called a BP pressure even though there may
be more than a tiny bubble of gas in the container. At pressures higher than
that needed to dissolve all the available gas in the container, the oil is considered
undersaturated. The BP pressure, or P in psi, can be related to the amount of gas in
bp