Gas Compressibility Factor
Typical: 0.55-0.70 (sweet gas), 0.70-0.90 (rich gas)
DAK: Most widely used, 11-coefficient Standing-Katz fit
HY: Starling-Carnahan equation, better at some extremes
Both: Compare methods for quality assurance
Understand real gas behavior, Standing-Katz chart, pseudo-critical correlations, and equations of state
The Z-factor (gas compressibility factor) is a dimensionless correction that accounts for the deviation of real gas behavior from the ideal gas law. Z = 1.0 for an ideal gas. For natural gas at typical pipeline pressures (500-1500 psia), Z ranges from 0.65 to 0.92, meaning the gas is 8-35% denser than ideal gas predictions.
The Z-factor is calculated in two steps: (1) determine pseudo-critical properties (Tpc, Ppc) from gas specific gravity using Sutton correlations, then calculate pseudo-reduced temperature and pressure; (2) solve the Dranchuk-Abou-Kassem or Hall-Yarborough equation iteratively using Newton-Raphson method. For sour gas, Wichert-Aziz correction adjusts pseudo-critical properties before step 2.
Always use Z-factor correction when pressure exceeds 100 psia. At low pressure (< 50 psia), Z is approximately 1.0 and ideal gas is adequate. For custody transfer, pipeline hydraulics, compressor sizing, and relief valve calculations, accurate Z-factor is essential. At 1000 psia, ignoring Z can introduce 15-25% error in density and flow calculations.
Both methods are mathematical fits of the Standing-Katz Z-factor chart. Dranchuk-Abou-Kassem (DAK) uses an 11-coefficient equation solved for reduced density, while Hall-Yarborough uses the Starling-Carnahan equation of state. Both are accurate to within 1% for typical natural gas conditions. DAK is more widely used in the gas industry; HY sometimes converges better at extreme conditions.