PROPERTIES OF RESERVOIR GASES - Petroleum Reservoir Fluid Property Correlations - page 19

(2.12)
The units of B g resulting from the use of equation (2.12) are either
reservoir cubic feet per standard cubic feet (res cu ft/scf) or cubic meters
per cubic meter; appropriate conversion factors can be used to alter
the units. The z-factor at standard conditions, z sc , is usually assumed
to be 1.0, and the z-factor at p and T can be calculated with equations
(2.4)-(2.8).
Equation (2.12) is not a correlation. It was derived directly from the
definition of gas formation volume factor. Thus, the results of the use
of equation ( 2.12) are as accurate as the values of gas z-factors used in
the calculations.
Gas Viscosities
Gas viscosity , also called the coefficient of viscosity of a gas , is a measure of
the resistance to flow exerted by a gas. Viscosities in units of centipoise
are most often used in the petroleum industry. These are called dynamic
viscosities . The units of centipoise are grams of mass per 100 second-
centimeters (g mass/100 sec cm). Sometimes viscosity is reported in
centistokes . These are called kinematic viscosities and have units of square
centimeters per 100 seconds (cu cm/100 sec cm). Thus, the relationship
between kinematic and dynamic viscosity is
(2.13)
where the density in grams per cubic centimeters (g/cc) is determined at
the pressure and temperature of interest.
Carr, Kobayashi, and Burrows proposed graphical correlations
to estimate gas viscosities. 19 One graph allows calculation of the gas
viscosity at atmospheric pressure as a function of gas molecular weight
and temperature, with adjustments for nonhydrocarbon content of
the gas. Another graph relates the gas viscosity at the pressure and
temperature of interest to the gas viscosity at atmospheric pressure
using pseudoreduced temperatures and pseudoreduced pressures.