reservoir · volumetrics
Average Reservoir Pressure (Volumetric)
P/Z = (Pi/Zi) × (1 − Gp/G)
click formula to derive ↑
Inputs
psi
—
scf
scf
Description
Estimates the current P/Z value for a volumetric (no water influx) gas reservoir from cumulative production as a fraction of GIIP. Plotting P/Z against Gp gives the classic straight-line material balance signature used to estimate GIIP and reservoir pressure.
Variables
| Symbol | Unit | Description |
|---|---|---|
| P/Z | psi | P/Z |
| Pi | psi | Initial reservoir pressure at discovery. |
| Zi | — | Gas z-factor at initial reservoir pressure. |
| Gp | scf | Cumulative gas produced to date, at standard conditions. |
| G | scf | Original gas in place, from volumetric estimation or a P/Z plot extrapolation to Gp=G. |
Assumptions
- The reservoir is volumetric — no significant water influx supporting pressure
- Gp and G are both measured/estimated at standard (surface) conditions on a consistent basis
- Reservoir temperature is constant over the production history (isothermal depletion)
Limitations
- A curving (non-linear) P/Z trend indicates water influx, compartmentalization, or measurement error — the simple straight-line formula no longer applies
- Requires an independent GIIP (G) estimate as an input; if G is highly uncertain, P/Z output uncertainty is correspondingly large
- Does not directly output absolute pressure — converting P/Z to P requires an iterative z-factor correlation calculation
Use Cases
- → GIIP back-calculation from production history: Use multiple (Gp, P/Z) historical pairs to fit the straight-line trend and extrapolate to P/Z=0 for an independent GIIP estimate.
- → Pressure depletion forecasting: Forecast future P/Z (and hence pressure, via z-factor correlation) at planned future cumulative production levels.
- → Volumetric drive verification: Compare the calculated P/Z trend against actual measured pressures — systematic deviation signals water influx not captured by this volumetric model.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring gas-condensate volumetric reservoirs commonly show good P/Z linearity once retrograde liquid dropout effects are corrected for in the z-factor.
Haynesville
High-pressure shale gas P/Z plots often show two slopes (early high-permeability fracture drainage, then slower matrix depletion) — a single straight line may not fit the full history.
Appalachian Basin
Marcellus shale gas reservoirs frequently deviate from simple P/Z linearity due to desorption of adsorbed gas, which adds an apparent "extra" gas source as pressure declines.
Global
Always plot P/Z vs Gp before trusting an extrapolated GIIP — a curving trend will give a misleading (usually overstated) intercept if forced through a straight line.
References
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