production · inflow
Composite IPR (Beggs)
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Inputs
m³/d/kPa
psi
psi
psi
Description
Computes inflow rate across the full producing life of an undersaturated oil well using Beggs' (1991) two-regime composite IPR — a linear (constant-PI) relationship above the bubble point, transitioning smoothly to a Vogel-shaped curve below it as free gas evolves near the wellbore. Automatically selects the correct regime from the target Pwf, so a single calculator covers a well from initial (above-Pb) conditions through depletion below the bubble point.
Variables
| Symbol | Unit | Description |
|---|---|---|
| qo | STB/d | Inflow Rate |
| AOF | STB/d | Absolute Open Flow |
| J | m³/d/kPa | Productivity index measured (or extrapolated) above the bubble point — the same quantity produced by the Productivity Index (PI) calculator. Always entered in m³/d/kPa regardless of the active unit toggle (see that calculator's own field for the same convention). |
| Pr | psi | Average reservoir (static) pressure. |
| Pb | psi | Bubble point pressure of the reservoir fluid — the boundary between the linear (above Pb) and Vogel-shaped (below Pb) regimes. |
| Pwf | psi | Flowing bottomhole pressure at which to evaluate the inflow rate — may fall above or below Pb; the calculator selects the correct regime automatically. |
Assumptions
- Flow above the bubble point is single-phase and undersaturated, following the same linear IPR assumption as the Productivity Index (PI) calculator
- Flow below the bubble point follows Vogel's empirical curve shape, the same solution-gas-drive assumption Vogel's own equation relies on
- J is a representative, currently-valid productivity index — not a stale value measured before significant near-wellbore damage or stimulation changed it
- Pr and Pb are both current reservoir values, not initial (pre-depletion) values
Limitations
- Explicitly single-phase oil only — this calculator does not model free water production. A well producing a meaningful water fraction needs the oil+water "Liquid Composite IPR" variant, a materially different formulation (Brown, 1984, The Technology of Artificial Lift Methods, Vol. 4) that is not built here and is out of scope for this calculator
- J is assumed constant above the bubble point — if the well has near-wellbore skin or damage not already reflected in a freshly-measured J, the linear (Case I) regime will misrepresent actual deliverability
- The below-Pb regime is an empirical curve-shape fit (Vogel's), not a mechanistic two-phase flow model — the same limitation Vogel's own equation carries
Use Cases
- → Full-life IPR from initial conditions through depletion: Model a single well's inflow performance both while it's still above the bubble point and after reservoir pressure has depleted below it, without switching between two separate calculators.
- → Nodal analysis across a well's producing life: Provide a single IPR curve that automatically transitions from linear to Vogel-shaped as the target Pwf or reservoir pressure changes, for pairing with tubing performance curves.
- → AOF estimation independent of current Pwf: Estimate absolute open flow potential directly from J, Pr, and Pb, without needing a separate below-Pb test point.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring wells that began above the bubble point commonly cross below it over their producing life — this calculator's regime-switching behavior avoids needing to swap calculators mid-life as depletion proceeds.
Global
Always confirm which regime a target Pwf actually falls into before comparing against a single-regime tool (Productivity Index above Pb, Vogel or Standing IPR below it) — this calculator's own regime label in its result note makes that explicit.
References
Primary source
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