P/Z Plot Calculator
Estimate original gas in place (OGIP) from pressure/Z-factor vs cumulative production data. Cole plot diagnostics for water influx detection. Free alternative to MBAL.
Gas Properties
Pressure & Production Data
Enter reservoir pressure (psi) and cumulative gas production, Gp (Bcf). Z-factor is auto-calculated or manually entered.
| # | Pressure (psi) | Gp (Bcf) | Z-factor | P/Z (psi) |
|---|
OGIP (G)
--
Current Recovery Factor
--
Remaining Reserves
--
Abandonment Recovery
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P/Z vs Cumulative Production
Cole Plot (Water Influx Diagnostic)
Material Balance Equation
P/Z = (Pi/Zi) × (1 - Gp/G)
Where G = OGIP. The x-intercept of P/Z vs Gp gives G (OGIP). Deviations from linearity may indicate water influx or formation compaction.
How this was calculated
Gas Material Balance: For a volumetric gas reservoir (no water influx), P/Z vs Gp is a straight line. The x-intercept equals OGIP.
Z-factor (DAK): The Dranchuk-Abou-Kassem correlation computes gas compressibility factor from pseudoreduced pressure and temperature using the Standing-Katz chart correlation.
Cole Plot: Plots (P/Z)/Gp vs P/Z. For a volumetric reservoir, points cluster on a horizontal line. An upward trend indicates water influx support.
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Book a free strategy call →Gas Material Balance and P/Z Analysis
The P/Z plot is the most widely used method for estimating original gas in place (OGIP) in gas reservoirs. By plotting P/Z against cumulative gas production (Gp), a straight line emerges for volumetric reservoirs. Extrapolating to P/Z = 0 gives the OGIP. The Cole plot serves as a diagnostic for water influx. Commercial tools like Petroleum Experts MBAL ($20,000-50,000/yr) perform similar analyses. All calculations run in your browser. Built by Groundwork Analytics.