nonideal gas, Oil & Gas Glossary

A nonideal gas, also called a real gas, is one whose pressure, volume, and temperature relationship departs measurably from the ideal gas law and must instead be described by an equation of state of the form pV = znRT, where z is the gas deviation factor, or compressibility factor, that accounts for the difference. The ideal gas law assumes molecules occupy no volume and exert no attractive forces on one another, assumptions that hold reasonably at low pressure and high temperature but collapse at the elevated pressures found in producing oil and gas reservoirs. In the Western Canadian Sedimentary Basin, a Montney gas well at a reservoir depth of roughly 2,500 m can carry an initial pressure near 35,000 to 45,000 kPa, equivalent to about 5,000 to 6,500 psi, and at those conditions natural gas behaves as a strongly nonideal fluid with a z-factor that can fall well below unity before climbing back above it at very high pressure. The z-factor is dimensionless: a value of 1.0 means the gas behaves ideally, a value of 0.85 means the gas occupies only 85 percent of the volume an ideal gas would at the same pressure and temperature, and that compression has a direct and large effect on how much gas a reservoir actually holds. Because z depends on pressure, temperature, and gas composition, it is not a constant but a function evaluated at every condition along the production path. The dominant engineering tool for estimating z for natural gas mixtures is the Standing-Katz correlation, published by Marshall Standing and Donald Katz in 1944, which plots z against pseudo-reduced pressure and pseudo-reduced temperature. An engineer first computes the pseudo-critical pressure and temperature of the mixture from its composition, often using the Sutton or Kay mixing rules, then divides the actual pressure and temperature by those pseudo-critical values to obtain the pseudo-reduced coordinates, and finally reads z from the chart or from a numerical fit such as the Dranchuk-Abou-Kassem or Hall-Yarborough equation that reproduces the Standing-Katz curves for computer calculation. The z-factor flows straight into the gas formation volume factor and therefore into every original-gas-in-place estimate, every material-balance calculation, and every reserves disclosure an operator files. Getting z wrong by even a few percent propagates directly into a few percent error on booked gas reserves, which is why nonideal gas behavior is not an academic refinement but a core input to the economics of any WCSB gas play, sour or sweet, conventional or unconventional.

Key Takeaways

The equation of state pV = znRT: The single z term carries all the deviation from ideal behavior. At reservoir conditions z is typically below 1.0 because attractive forces dominate, meaning real gas compresses into less volume than the ideal law predicts. At very high pressure z rises above 1.0 as molecular volume dominates. For a Montney gas at roughly 40,000 kPa and 90 degrees C, z commonly sits near 0.85 to 0.95 depending on composition.
Pseudo-reduced coordinates drive the chart: Standing-Katz reads z as a function of pseudo-reduced pressure, p divided by pseudo-critical pressure, and pseudo-reduced temperature, T divided by pseudo-critical temperature, both in absolute units. Pseudo-critical properties come from gas composition via the Sutton or Kay mixing rules, with corrections for hydrogen sulphide and carbon dioxide that matter greatly for sour WCSB gas from formations such as the Nisku or Leduc.
Numerical correlations replace the chart: The Dranchuk-Abou-Kassem and Hall-Yarborough equations fit the Standing-Katz curves so reservoir simulators and PVT software can compute z iteratively at every timestep. These fits are valid across pseudo-reduced pressures up to about 15 and pseudo-reduced temperatures from 1.05 to 3.0, covering nearly all WCSB reservoir conditions.
Direct impact on gas-in-place: The gas formation volume factor Bg is proportional to zT divided by p, so original gas in place scales inversely with z. A z of 0.85 instead of an assumed 0.90 increases calculated reservoir gas density and changes booked reserves by several percent. Material balance, a workhorse of WCSB reserves estimation, depends on accurate z at each measured reservoir pressure.
Sour and high-CO2 gas needs correction: Hydrogen sulphide and carbon dioxide, common in Devonian WCSB carbonates, shift the pseudo-critical properties and require the Wichert-Aziz correction before the Standing-Katz chart is read. Skipping the correction for a 10 percent H2S Nisku gas can produce z errors large enough to misstate deliverability and reserves, which is why sour-gas PVT work always applies it.

From Composition to Z: The Calculation Path

Estimating z for a real WCSB gas begins with a compositional analysis from a recombined separator sample. The engineer computes pseudo-critical pressure and temperature as composition-weighted averages of the component criticals, applies the Wichert-Aziz correction if acid gases are present, then forms pseudo-reduced pressure and temperature at the condition of interest. Feeding those into a Dranchuk-Abou-Kassem solver returns z by iteration. For a sweet Cardium gas at 18,000 kPa and 65 degrees C, z might land near 0.91; for a deeper, sourer Slave Point gas the same workflow with the acid-gas correction could give a markedly different value. The discipline of doing this consistently keeps deliverability forecasts and reserves on a defensible footing.

Why Z Changes Through Field Life

Because z depends on pressure, it is not fixed but tracks the reservoir as it depletes. Early in life a high-pressure Montney gas sits in the part of the Standing-Katz chart where z is well below 1.0; as cumulative production draws pressure down, the pseudo-reduced pressure falls and z climbs back toward unity. A p/z material-balance plot exploits exactly this behavior: plotting reservoir pressure divided by z against cumulative gas produced yields a straight line for a volumetric reservoir, and extrapolating it to atmospheric pressure gives original gas in place. The straightness of that line is one of the most trusted reserves tools in WCSB gas engineering, and it works only because z is handled correctly.

Fast Facts

The Standing-Katz chart that still anchors gas engineering today was built in 1944 from laboratory measurements on natural gas mixtures, decades before digital computers existed. Marshall Standing and Donald Katz at the University of Michigan generalized scattered experimental data by introducing pseudo-reduced coordinates, and the resulting single chart proved so robust that the numerical correlations engineers run in modern PVT software are simply curve fits to those original 1944 isotherms, still accurate to within one to two percent across the pressure range of nearly every producing reservoir.

The nonideal gas concept underpins the gas formation volume factor, which converts surface gas volumes to reservoir conditions and depends directly on z. It connects to equation of state, the broader family of pressure-volume-temperature relationships of which pV = znRT is the simplest real-gas member. It also links to sour gas, because hydrogen sulphide and carbon dioxide force the Wichert-Aziz correction that keeps z accurate, a routine necessity for Devonian WCSB carbonate gas.

Real-World WCSB Scenario

A Tourmaline-style operator evaluating a deep Montney gas pool near Grande Prairie measures an initial reservoir pressure of 42,000 kPa at 95 degrees C on a gas analyzing 4 percent CO2 and trace H2S. Treating the gas as ideal would overstate the in-place density and inflate booked reserves; applying the Wichert-Aziz correction and a Dranchuk-Abou-Kassem solve returns z near 0.88, lowering the volumetric gas-in-place estimate by roughly 4 to 6 percent versus the ideal assumption. On a pool that might hold several billion cubic metres, that correction shifts the net present value by several million CAD.

The reservoir team carries the corrected z through the p/z material-balance plot as production accumulates, and the straight-line fit confirms the volumetric estimate within the first two years of depletion. The exercise shows that nonideal gas treatment is not a refinement applied at the end but the foundation the entire reserves and economics case rests on.