The Real Gas Equation
Natural gas departs from ideal behavior at reservoir pressure, and the z factor is the correction that keeps volumes honest.
The equation
pV = z n R T
- z is the gas deviation factor, near 1 at low pressure
- Dips to about 0.8 at moderate pressure, climbs above 1 when very high
- Read from Standing Katz charts using pseudo reduced properties
Worked example
At conditions where z is 0.85, a gas occupies only 85 percent of its ideal volume. For a reservoir holding gas at 3,000 psi, using z of 1 would overstate gas in place by 18 percent, an expensive rounding error.
When to use it
Every gas material balance runs on p/z: plot it against cumulative production and a straight line points at the initial gas in place. Departures from that line whisper about water drive or compartment boundaries.
Related Calculators and Reference
- Mud Weight and Pressure Gradient Calculator: convert between ppg, SG, and psi per foot.
- Probabilistic STOIIP Calculator: volumes with uncertainty ranges.
- PetroWiki by SPE: the industry reference for petroleum engineering.