production · nodal analysis
Nodal Analysis: Operating Point
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Inputs
STB/d
psi
psi
ft
—
in
cp
in
Description
Solves a well's operating point — the flow rate and flowing bottomhole pressure at which the reservoir's inflow capacity matches the wellbore's lift capacity — by plotting Vogel's IPR curve against a single-phase liquid Tubing Traverse VLP curve on a shared rate axis and finding where they cross. This is the same concept industry calls 'Nodal Analysis' or 'IPR/TPR Intersection' — not two separate ideas. v1 pairs one specific IPR model (Vogel) with one specific VLP model (single-phase Tubing Traverse); other combinations (Fetkovich, Standing, or Composite IPR against Multiphase Gradient/Beggs-Brill VLP) are a well-scoped future extension, not this calc's current scope.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Q_op | STB/d | Operating Rate |
| Pwf_op | psi | Operating Flowing BHP |
| qmax | STB/d | Absolute open flow potential for the IPR curve (Vogel) — the rate at Pwf=0, from a multi-point well test or extrapolation. Also sets the swept Q-axis's upper bound for both curves. |
| Pres | psi | Average reservoir (static) pressure, at or below the bubble point for Vogel's IPR equation to apply. |
| Pwh | psi | Flowing wellhead pressure — the VLP curve's starting point at the top of the tubing string. |
| TVD | ft | True vertical depth of the tubing string over which the VLP curve's pressure traverse is evaluated (vertical well assumed). |
| SG | — | Specific gravity of the flowing liquid (water = 1.0), used for both the VLP curve's hydrostatic and friction terms. |
| d | in | Tubing internal diameter. |
| μ | cp | Viscosity of the flowing liquid at tubing conditions. |
| ε | in | Absolute roughness of the tubing's internal surface. 0.0018 in (0.0457 mm) is the standard commercial steel value. |
Assumptions
- The reservoir is at or below the bubble point with solution-gas drive dominant (Vogel's own IPR assumption)
- Single-phase liquid flow throughout the tubing string — no free gas evolving or flowing alongside the liquid (the VLP side's Tubing Traverse assumption)
- Vertical well — tubing length is treated as equal to true vertical depth
- Exactly one crossing exists in the swept 0-to-qmax range; if IPR and VLP don't cross there, the calculator reports no operating point rather than guessing
Limitations
- v1 scope is fixed to Vogel IPR × single-phase Tubing Traverse VLP only — other IPR models (Fetkovich, Standing, Composite/Beggs) and VLP models (Multiphase Gradient/Beggs-Brill) are not selectable here; a future model-selection extension is a separate, larger effort
- Inherits single-phase-liquid-only from the Tubing Traverse side — not valid for a well producing meaningful free gas alongside liquid
- If VLP pressure exceeds IPR pressure across the entire 0-to-qmax range, no operating point exists and the calculator reports this explicitly (a real diagnostic result — the well cannot flow as configured — not a silent zero or an error)
- Deviated or horizontal completions need a true measured-depth-based traverse this calculator does not provide
Use Cases
- → Well deliverability check: Confirm whether a well can flow at all under a given completion (tubing size, wellhead pressure) and reservoir condition, and at what rate/pressure it will stabilize.
- → Tubing sizing sensitivity: Compare the solved operating point across different tubing IDs or wellhead pressures to evaluate how a completion change shifts producible rate.
Related Calculations
Region Notes
Global
This is the classic Nodal Analysis / IPR-TPR intersection technique used throughout the industry to size artificial lift and diagnose deliverability problems — the same concept previously deferred on this platform under 'Gas Lift Injection Rate' and 'Jet Pump Flow Rate' for lacking this exact dual-curve infrastructure.
References
Primary source
Primary source
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