production · inflow
Fetkovich IPR (Oil Wells)
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Inputs
psi
—
STB/d
psi
psi
Description
Computes oil well deliverable rate at a target flowing bottomhole pressure using Fetkovich's empirical backpressure (deliverability) equation for oil wells, and estimates the absolute open flow (AOF) potential. Shares the same functional form C·(Pr²−Pwf²)ⁿ as the Gas Well Deliverability (Rawlins-Schellhardt) calculator, but is Fetkovich's own distinct empirical finding — he showed via multi-rate testing of oil wells below the bubble point that the gas-well backpressure equation form also fits oil-well IPR data, with a narrower exponent range (n = 0.568–1.000) than typically seen in gas wells, reflecting oil's much lower compressibility and correspondingly smaller non-Darcy turbulence contribution.
Variables
| Symbol | Unit | Description |
|---|---|---|
| q | STB/d | Oil Rate at Pwf |
| AOF | STB/d | Absolute Open Flow |
| Pr | psi | Average reservoir (static) pressure. |
| n | — | Empirical exponent from a multi-rate backpressure well test. n = 1.0 is pure Darcy (laminar) flow; Fetkovich's own multi-rate oil-well testing found n ranging 0.568–1.000, narrower than the gas-well range because oil is far less compressible. |
| q_test | STB/d | Stabilized oil production rate from a backpressure test point, at which Pwf_test was measured. |
| Pwf_test | psi | Flowing bottomhole pressure measured at the test rate q_test. Used with Pr and q_test to back-calculate the deliverability coefficient C. |
| Pwf | psi | Flowing bottomhole pressure at which to evaluate the deliverable oil rate q — the design operating point. |
Assumptions
- The test rate q_test and pressure Pwf_test represent stabilized (pseudo-steady-state) conditions, not transient flow
- Reservoir pressure Pr is the current average static pressure, not an initial or extrapolated value
- The well is producing below the bubble point (two-phase, solution-gas-drive flow) — Fetkovich's power-law form was fit to this regime, matching the same reservoir condition Vogel's equation addresses
Limitations
- A single-point C determination (one test rate) is less accurate than a multi-rate isochronal test, which fits both n and C simultaneously with less uncertainty
- C changes as reservoir pressure depletes — a deliverability test performed at one stage of reservoir life does not reliably predict AOF at a significantly lower Pr
- Like the gas-well backpressure equation, this is an empirical curve fit rather than a mechanistic model — it does not itself explain why a given well shows a particular n
Use Cases
- → Oil well deliverability from limited test data: Estimate deliverable rate and AOF from a single stabilized test point, without needing the multi-rate isochronal test a rigorous n/C fit would otherwise require.
- → Nodal analysis inflow for oil wells below the bubble point: Provide an alternative IPR curve — q vs. Pwf — to Vogel's equation, for pairing with tubing performance curves.
- → Cross-check against Vogel's equation: Compare Fetkovich's power-law IPR against Vogel's equation for the same well as a reasonableness check on either method.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring oil wells below the bubble point sometimes use Fetkovich's power-law IPR as a cross-check against Vogel's equation, particularly where multi-rate test data is available from early flowback.
Global
Fetkovich's equation and Vogel's equation are both empirical fits to solution-gas-drive IPR behavior; neither is universally more accurate, and comparing both against available test data is good practice.
References
Primary source
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