Natural-gas Z-factor from specific gravity, temperature, and pressure (Dranchuk–Abou-Kassem / Standing–Katz), with real-gas density.
PV = ZnRT; Z from Dranchuk–Abou-Kassem (Standing–Katz fit)
How it works
The gas specific gravity sets the pseudo-critical temperature and pressure (Sutton), which reduce the operating temperature and pressure to pseudo-reduced values. The Dranchuk–Abou-Kassem correlation — an analytic fit to the Standing–Katz chart — then returns Z, and ρ = PM/(ZRT) gives the real-gas density.
Worked example
A 0.65-gravity natural gas at 40 °C and 10 MPa absolute. Pseudo-reduced to Tpr ≈ 1.54, Ppr ≈ 2.16, giving Z ≈ 0.83 — about 17 % more compressible than an ideal gas at these conditions.
Inputs
Inputs
Gas specific gravity (air = 1) — Methane ≈ 0.55; richer gas ≈ 0.7–0.9.
Temperature (°C)
Pressure (abs) (kPa)
Tpr (if selected)
Ppr (if selected)
Frequently asked questions
What is the compressibility factor Z?
The ratio of the real gas volume to the ideal-gas volume at the same temperature and pressure: PV = ZnRT. Z = 1 is ideal; natural gas dips well below 1 at high pressure and moderate temperature, so ignoring Z overestimates the volume and underestimates the mass held in a vessel.
How accurate is the Dranchuk–Abou-Kassem correlation?
It reproduces the Standing–Katz chart to within about 1 % over its valid range (1.0 ≤ Tpr ≤ 3.0, 0.2 ≤ Ppr ≤ 30), which is why it is the standard analytic substitute for reading the chart.
Does it work for sour or CO₂-rich gas?
The Sutton pseudo-criticals assume sweet natural gas. For significant H₂S or CO₂, apply the Wichert–Aziz correction to the pseudo-critical properties before computing Z.
Assumptions
Dranchuk–Abou-Kassem correlation (an analytic fit to the Standing–Katz chart), valid for 1.0 ≤ Tpr ≤ 3.0 and 0.2 ≤ Ppr ≤ 30.
Pseudo-critical properties from the Sutton (1985) specific-gravity correlation for natural gas; sour/CO₂-rich gases need a Wichert–Aziz correction.
Real-gas density from ρ = PM/(ZRT).
References
Dranchuk, P.M. & Abou-Kassem, J.H. (1975), J. Canadian Petroleum Technology.
Vapor Pressure (Antoine) — Saturation pressure of a pure component from Antoine coefficients. Pick a preset or enter your own A, B, C.
Ideal Gas Law — Moles, mass, and density of an ideal gas from P, V, and T.
Gas Compressor Power — Single-stage adiabatic compression power, head, and discharge temperature for an ideal gas.
Steam Tables (IAPWS-IF97) — Saturated and single-phase water/steam properties — saturation pressure/temperature, enthalpy, entropy, and specific volume — from the IAPWS-IF97 standard.