reservoir · volumetrics
Gas Recovery Factor
RF = 1 − (Pa × Zi) / (Pi × Za)
click formula to derive ↑
Inputs
psi
psi
—
—
Description
Estimates gas recovery factor for a volumetric (depletion-drive) gas reservoir from the pressure decline between initial and abandonment conditions, expressed through the P/Z material balance relationship.
Variables
| Symbol | Unit | Description |
|---|---|---|
| RF | fraction | Recovery Factor |
| Pi | psi | Initial reservoir pressure at discovery. |
| Pa | psi | Reservoir pressure at economic abandonment — the pressure at which the well no longer produces economically. |
| Zi | — | Gas compressibility (z) factor at initial pressure, from a PVT correlation (e.g., Standing-Katz) or lab measurement. |
| Za | — | Gas z-factor at abandonment pressure — typically closer to 1.0 at the lower abandonment pressure. |
Assumptions
- The reservoir is volumetric (no significant water influx) — a water-drive gas reservoir requires a different material balance approach
- Z-factors are accurate for the gas composition at each pressure (from a calibrated PVT correlation or lab measurement)
- Abandonment pressure reflects the actual economic limit for the well/facility, not an arbitrary low pressure
Limitations
- A water-drive gas reservoir will show abnormally high apparent RF early and then stall — this formula assumes no water influx and will overstate ultimate RF if water drive is present
- Reservoir compartmentalization or poor lateral continuity can leave the volumetric assumption invalid in heterogeneous reservoirs
- Does not account for non-recoverable gas trapped by relative permeability effects in tight or unconventional reservoirs
Use Cases
- → Conventional gas reserve estimation: Estimate ultimate recovery factor for a volumetric gas reservoir directly from pressure history, without needing a separate GIIP estimate.
- → P/Z plot calibration: Cross-check RF implied by a P/Z plot extrapolation to P=0 against the explicit pressure-ratio RF formula.
- → Abandonment pressure planning: Evaluate how lowering the practical abandonment pressure (e.g., via compression) would increase ultimate recovery factor.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring gas-condensate intervals are typically volumetric with RF of 60-80% before liquid dropout complications reduce ultimate gas recovery.
Haynesville
High-pressure volumetric shale gas reservoirs often achieve RF of 40-60% due to permeability limitations and slow pressure equilibration, lower than conventional gas despite a true volumetric drive mechanism.
Appalachian Basin
Marcellus shale gas RF is commonly 20-40%, well below the conventional 70-90% range, due to nanodarcy matrix permeability limiting drainage even under volumetric depletion.
Global
Always check the P/Z trend for linearity — curvature suggests water influx or compartmentalization, invalidating the simple volumetric RF formula.
References
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