RESERVOIR ENGINEERING - NATURAL GAS PHYSICAL PROPERTIES - II

Gas Z-Factor, Viscosity & Formation Volume Factor for known Specific gravity

INPUT   DATA

Title  

Gas Gravity   air = 1.0

Component Mole Fraction
N2
CO2
H2S

Reservoir Temperature ° F
Reservoir Pressure psig

     Reset

OUTPUT   VARIABLES   &   GRAPH

Pressure        psig
Z-factor    
Viscosity   cp
Bg   rbbl/scf

Pressure  Z-factor   Viscosity    Bg 
psig centipoiserbbl/scf
0      
250      
...      
5000      
THEORY  &   FORMULAE

Physical Properties Of Natural Gas - II

This calculator takes in the specific gravity of the natural gas (and molar fractions of gaseous impurities, if any) and outputs estimates of 3 physical properties of natural gas - Real gas deviation factor (Z), Gas viscosity (μ) and Formation volume factor (Bg). The 3 properties are listed and plotted for the specific temperature and pressure entered as well as for pressures in the range zero to 5000 psig.

Z-factor is computed using the Hall-Yarborough direct method. This consists of 2 equations:

        

where:
    ppr = pseudo-reduced pressure
    t = reciprocal of pseudo-reduced temperature
    y = "reduced" density (an unknown).

The specific gravity and gas composition values are first used to determine the constants in the equations (critical and pseudo-reduced pressures and temperatures). The second non-linear equation is solved for "y" using the Newton-Raphson iterative technique, and the result is plugged into the first equation.

Gas formation volume factor Bg is derived from Z according to the equation:

Bg = [Z*psc*TR]/[5.615pR*Tsc] ... reservoir bbls/scf

where:
    psc = pressure at standard conditions (14.7 psia)
    pR = pressure at reservoir condition (psia)
    Tsc = temperature at standard conditions (60 + 460 ° R)
    TR = temperature at reservoir condition (° R)

Finally, Gas viscosity is computed from the correlations by Carr et al.

BIBLIOGRAPHY