fluid_properties · gas properties
Gas Z-Factor (Compressibility Factor)
Z = f(Tpr, Ppr) — Hall-Yarborough (1973) or Dranchuk-Abou-Kassem (1975), both fit to the Standing-Katz (1942) chart
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Inputs
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Description
Computes the gas compressibility (deviation) factor Z from pseudo-reduced temperature and pressure using a selectable equation-of-state fit to the Standing-Katz (1942) chart — Hall-Yarborough (1973) or Dranchuk-Abou-Kassem (1975). Z is the single largest source of uncertainty in gas FVF, gas-in-place, and material-balance calculations, and this closes the loop on gas_fvf (which otherwise requires Z to be entered by hand).
Variables
| Symbol | Unit | Description |
|---|---|---|
| Z | — | Gas Z-Factor |
| Tpr | — | Pseudo-reduced temperature = reservoir absolute temperature ÷ pseudo-critical temperature (T/Tpc). Compute Tpc from gas gravity (Sutton or Standing correlation) first. Typical gas reservoirs fall in 1.3–2.2. |
| Ppr | — | Pseudo-reduced pressure = reservoir absolute pressure ÷ pseudo-critical pressure (P/Ppc). Compute Ppc from gas gravity first. Typical gas reservoirs fall in 1–15. |
| Method | — | 1 = Hall-Yarborough (1973) — Starling-Carnahan EOS fit, the most widely used, but not valid for Tpr < 1.0; 2 = Dranchuk-Abou-Kassem (1975) — 11-constant fit with a slightly wider stated range. Both reproduce the Standing-Katz chart to within chart-reading accuracy. |
Assumptions
- Sweet natural gas — no significant non-hydrocarbon content. CO2, H2S, and N2 shift the pseudo-critical properties (apply a Wichert-Aziz correction before computing Tpr/Ppr if sour)
- Pseudo-reduced properties were computed from a valid pseudo-critical correlation for the gas gravity in question
- Single gas phase — no retrograde condensate drop-out that would change composition across the pressure range
Limitations
- Hall-Yarborough (method 1) is explicitly not valid for Tpr < 1.0 (the authors' own stated limit) — the calculator warns and Dranchuk-Abou-Kassem should be used near or below the pseudo-critical temperature
- Both correlations are curve-fits to the Standing-Katz chart and inherit its ~1–2% accuracy; they are not a substitute for a laboratory-measured Z on a real gas sample
- For very high pressure (Ppr > 30) or high-molecular-weight/sour gases, neither fit is reliable — use a compositional EOS with appropriate mixing rules
Use Cases
- → Gas FVF and gas-in-place: Compute Z at reservoir Tpr/Ppr, then feed it to gas_fvf (Bg) and gas volumetrics (giip) — the dominant uncertainty in both is the Z-factor.
- → P/Z material balance: Evaluate Z across a depleting reservoir's pressure history to build the P/Z-vs-cumulative-production plot used to estimate gas reserves and detect aquifer support.
- → Correlation QC: Cross-check a lab-reported Z, or compare Hall-Yarborough against Dranchuk-Abou-Kassem to gauge fit uncertainty at a given Tpr/Ppr before committing to one.
Related Calculations
Region Notes
Global
Dry to moderately wet natural gases (γg 0.6–0.8) at typical reservoir conditions give Tpr 1.4–2.2 and Ppr 2–12, squarely inside both correlations' fitted ranges. Near-critical (Tpr ≈ 1.05–1.15) or very high-pressure (Ppr > 20) gases are where the two methods diverge most and where a lab Z is worth obtaining.
Haynesville
High-pressure, high-temperature dry gas (initial pressures 10,000–12,000+ psi, 300–400°F). Ppr can exceed 10–12; Dranchuk-Abou-Kassem is generally preferred at these elevated pseudo-reduced pressures, and both should be sanity-checked against PVT data given the extreme conditions.
References
Primary source
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