reservoir · material balance
P/Z Plot (Gas Material Balance)
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Inputs
Paste one row per line — separate columns with a comma or tab: Cumulative Gas Produced (scf), P/Z (psi)
⚠ Needs at least 3 valid rows to calculate — 0 so far.
Description
Back-solves original gas in place (G) from a history of cumulative gas production and average reservoir p/z, using the classic volumetric gas material balance straight line: p/z declines linearly with Gp for a closed (no water influx), volumetric gas reservoir. A least-squares regression on the pasted (Gp, p/z) points recovers both the intercept ((P/Z)ᵢ = pi/zi) and the slope, from which G follows directly — the dynamic, multi-point counterpart to reservoir_pressure_volumetric's single-point forward calculation, which instead takes G as a given input.
Variables
| Symbol | Unit | Description |
|---|---|---|
| G | scf | Gas in Place (from P/Z trend) |
| (P/Z)ᵢ | psi | Initial P/Z (intercept) |
| Gp, P/Z | — | One row per pressure-survey/production-history point: cumulative gas produced to date and the corresponding p/z at that time. At least 3 points are needed for a meaningful straight-line fit — the standard volumetric gas material balance plot. |
Assumptions
- The reservoir is volumetric (closed, no water influx) — the straight-line p/z-vs-Gp relationship only holds under this assumption; a curving trend signals it doesn't apply
- Every (Gp, p/z) row represents the same reservoir or connected fault block at successive points in its depletion history, not different compartments
- Z-factors used to compute each p/z point are consistent (same correlation/gas composition) across the full production history
Limitations
- A curved (rather than straight) P/Z trend — common with water influx, compartmentalization, or abnormally-pressured gas reservoirs — invalidates the simple linear form; the R² diagnostic flags a poor fit but does not distinguish its cause
- Early-life points, before enough pressure depletion has occurred, carry disproportionate leverage on the fitted intercept/slope and can bias G — more reliable once several widely-spaced-in-Gp points are available
- Requires reliable average reservoir pressure surveys at each Gp — measurement or averaging error propagates directly into the fitted intercept and slope
Use Cases
- → Dynamic GIIP determination: Establish gas in place directly from production and pressure history, as a cross-check against (or in the absence of) a volumetric GIIP estimate.
- → Water-drive screening: A P/Z trend that curves upward relative to the straight-line fit (R² degrading as more points are added) is the classic diagnostic for water influx — prompting a switch to a full water-drive material balance instead of the simple volumetric form.
- → Recovery factor / abandonment planning: Feed the fitted (P/Z)ᵢ and G directly into the Gas Recovery Factor calculator's Pi/Zi inputs for a data-consistent abandonment-pressure recovery estimate, rather than assuming a separately-sourced Pi/Zi.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring gas-condensate intervals are typically volumetric — a clean straight-line P/Z fit is the expected and common result here.
Haynesville
High-pressure, over-pressured shale gas can show apparent P/Z curvature from pore-volume compaction (not water influx) at very high initial pressures — distinguish this from true water drive before concluding a non-volumetric reservoir.
Gulf of Mexico
Many GOM shelf gas reservoirs have at least partial aquifer support — watch for early P/Z curvature and cross-check against a Water Drive Index calculation before trusting a purely volumetric G.
Global
Always plot the residuals (actual vs. fitted p/z) visually, not just the R² value, before accepting the fitted G — a systematic curved pattern in the residuals is a stronger water-influx signal than R² alone.
References
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