Inputs
Gas gravity: a measure of the density of a gas relative to air. Gas gravity is usually based on a gas composition analysis. Gas gravity must be between 0.55 and 1.10.
Average pressure in the reservoir. This value can be determined from pressure build up test, shut in bottom hole pressure or it can be estimated from mud weights during drilling. Bottom hole pressure can be estimated by the following formula: Bottom Hole Pressure (psig)= True Vertical Depth (ft) * Mud weight (ppg) *0.052. Reservoir pressure must be between 10 psia and 20,000 psia.
Average temperature in the reservoir. This value can be estimated from a well log header. Reservoir temperature can also be measured during a shut in bottom hole pressure measurement. Reservoir temperature must be between 50°F and 500°F.
Gas impurities such as carbon dioxide (CO2), hydrogen sulfide (H2S), and Nitrogen (N2) should be input as mole percent. These values are usually determined from a gas analysis. These values are used to adjust the critical temperature and pressure of the gas to account for these impurities. For the calculations to be valid, the sum of CO2, H2S and N2 must be less than 100% and the sum of CO2 and H2S must be less than 80%.
There are two options for gas type miscellaneous or condensate. This determines what correlation is used to calculate the critical pressure and temperature for the gas. Gas type will not greatly affect the results of the calculation at low gas gravity. However at higher gas gravity the effect can be significant.
Base pressure is the pressure at which gas is measured for sales or custody transfer. For Example: 14.65 psia in Texas and 15.025 psia in Louisiana.
Results
Z Factor: The code for this program is based on standard z factor
correlations.
Bg:
Compressibility: Estimates gas compressibility. This correlation can
give poor results for Tpr values less than 1.4 at Ppr between 0.4 and 3.0 (see
note on p. 513 in Ref. 1 and Fig. 1.7 in Ref. 2)
References
1) McCain, William D.: The Properties of Petroleum Fluids, second edition,
PennWell Publishing Co., Tulsa (1990) 512-513.
2) Craft, B. C., Hawkins, M., and Terry, R. E.: "Applied Petroleum Reservoir
Engineering", second edition, Prentice Hall, Englewood Cliffs (1991)26-28.
Viscosity: Calculates gas viscosity from pressure, temperature, z factor
and gas gravity.
References:
1) Lee, A.L., Gonzalez, M.H., and Eakin, B.E.: "The Viscosity of Natural
Gases," JPT (August 1966) 999.