Engineering lesson
Productivity index versus production rate
Productivity index is rate per pressure drawdown on a stated model and volume basis. Production rate also depends on the available drawdown; PI alone is not a rate forecast.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Keep coefficient and operating condition separate
For straight-line inflow, q = J(pr − pwf). A PI of 2 STB/day/psi gives 1,000 STB/day at 500 psi drawdown and 2,000 STB/day at 1,000 psi drawdown.
The same J can therefore correspond to several rates. Conversely, the same rate at different drawdowns implies different apparent productivity.
Use the appropriate inflow shape
A single constant PI presumes linear inflow over the interpreted range. Two-phase oil inflow can be nonlinear, and an apparent q/Δp ratio can change with operating point.
The linked straight-line calculator evaluates that specified model. The related Vogel lesson explains a different curve and its applicability rather than treating one PI as universal.
Add outflow before predicting an operating point
Tubing, surface pressure, lift and facility constraints determine which drawdown can actually be achieved. A large modeled PI does not guarantee a large installed-system rate.
Record pressure datum and stock-tank versus local volume basis with J so its units remain meaningful.
Explore the example
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Drawdown (psi) = 500. Oil rate (STB/day): 1000; PI (STB/day/psi): 2.
- Oil rate (STB/day)
Worked example
Fixed PI, changing drawdown
Linear PI J = 2 STB/day/psi; pressures use one compatible datum and fluid-volume basis.
Relation and unit basis
q = J Δp
This is an inflow relation. An operating point additionally requires an outflow model.
1. Define the baseline
Drawdown (psi) = 500
Linear PI J = 2 STB/day/psi; pressures use one compatible datum and fluid-volume basis.
2. Select the relation
q = J Δp
This is an inflow relation. An operating point additionally requires an outflow model.
3. Substitute the values
2 STB/day/psi × 500 psi = 1,000 STB/day
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Oil rate (STB/day) = 1000; PI (STB/day/psi) = 2
PI is a coefficient, while rate depends on the selected drawdown.
Result and interpretation
Oil rate (STB/day) = 1000; PI (STB/day/psi) = 2
PI is a coefficient, while rate depends on the selected drawdown.
Drawdown varies at fixed J. The linear response is an inflow model, not a complete producing-system solve.
Exact scenario data
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
| Drawdown (psi) | Oil rate (STB/day) | PI (STB/day/psi) |
|---|---|---|
| 100 | 200 | 2 |
| 250 | 500 | 2 |
| 500 | 1,000 | 2 |
| 750 | 1,500 | 2 |
| 1,000 | 2,000 | 2 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. What J corresponds to 600 STB/day at 300 psi drawdown?
Show the worked answer
2 STB/day/psi. The rate at another condition still requires that condition's drawdown and a valid linear model.
2. Reproduce the baseline Oil rate (STB/day) without rounding intermediate values.
Show the worked answer
2 STB/day/psi × 500 psi = 1,000 STB/day. Oil rate (STB/day) = 1000. PI is a coefficient, while rate depends on the selected drawdown.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Drawdown (psi) changes from 100 to 1000. Other controls remain as stated in the premise. Oil rate (STB/day) changes from 200 to 2000.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. This is an inflow relation. An operating point additionally requires an outflow model. PI is a coefficient, while rate depends on the selected drawdown.
Before using the result
- Keep J units visible.
- Match reservoir and flowing-pressure datums.
- Evaluate outflow separately.
PI is a coefficient, while rate depends on the selected drawdown.
Continue learning
Start with the broader workflow guide · Related method guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.