What is the Productivity Index (PI)?

PI (also written J) is the ratio of a well's flow rate to its pressure drawdown: PI = q / (Pr - Pwf). Units are STB/D/psi. A higher PI means the well delivers more fluid per psi of drawdown — indicating better reservoir quality, near-wellbore conditions, or larger completion interval.

What is an IPR curve?

The Inflow Performance Relationship (IPR) is a plot of bottomhole flowing pressure (Pwf) on the y-axis versus flow rate (q) on the x-axis. For an oil well above bubble point, it is a straight line with slope = -1/PI. The x-intercept (Pwf = 0) gives the Absolute Open Flow (AOF), the theoretical maximum rate.

When is the linear IPR model valid?

The linear model (Darcy flow) is valid when reservoir pressure stays above bubble-point pressure. Below bubble point, a curved IPR (Vogel or Composite) should be used instead. This calculator implements the linear model.

What are typical PI values?

Tight reservoirs may yield PI < 0.5 STB/D/psi. Average permeability reservoirs range from 1–5 STB/D/psi. High-permeability or stimulated wells can exceed 10 STB/D/psi. Horizontal wells with multi-stage fracs can reach 20–50 STB/D/psi or more.

What units does this calculator use?

Flow rate in STB/D (stock-tank barrels per day), pressure in psi, and PI in STB/D/psi — the standard US oilfield unit system for liquid PI. All IPR points and the chart use the same units.

What is Absolute Open Flow (AOF)?

AOF is the theoretical maximum flow rate when Pwf = 0 (no backpressure). It equals PI × Pr. AOF is rarely achievable in practice but sets the upper deliverability limit used for well and facility design.

Well Productivity Index Calculator

Calculate well PI from flow rate and pressure drawdown, then plot the IPR curve showing reservoir deliverability across a range of bottomhole flowing pressures.

Well Test Data

Average Reservoir Pressure (Pr) (psi)

Static bottomhole reservoir pressure

Bottomhole Flowing Pressure (Pwf) (psi)

Measured during stabilized flow test

Stabilized Flow Rate (q) (STB/D)

Stock-tank barrels per day at test conditions

Sample wells

Enter well test data and click Calculate to see PI and the IPR curve.

PI (J)

—

STB/D/psi

Drawdown

—

psi

AOF (max rate)

—

STB/D

Inflow Performance Relationship (IPR) Curve

Flow Rate (STB/D) vs Bottomhole Flowing Pressure (psi)

IPR Data Points

Pwf (psi)	Drawdown (psi)	Flow Rate (STB/D)	% of AOF

Formulas (linear IPR — Darcy flow, above bubble point)

PI = q / (Pr − Pwf)

q(Pwf) = PI × (Pr − Pwf)

AOF = PI × Pr (when Pwf = 0)

Summary

The Well Productivity Index (PI) is a fundamental reservoir engineering parameter that measures how readily a well delivers fluid from the reservoir to the surface. Defined as PI = q / (Pr - Pwf), it relates flow rate (STB/D) to the pressure drawdown (psi) between average reservoir pressure and bottomhole flowing pressure. This calculator computes PI from a single flow test point and generates a full Inflow Performance Relationship (IPR) curve across 5 or more drawdown points, letting engineers estimate deliverability at any surface operating condition.

How it works

Enter the average reservoir pressure (static bottomhole pressure) in psi.
Enter the measured bottomhole flowing pressure (Pwf) during the flow test in psi.
Enter the stabilized flow rate measured during the test in STB/D.
The calculator computes PI = q / (Pr - Pwf) in STB/D/psi.
The IPR table and curve are plotted from Pwf = Pr (zero flow) down to Pwf = 0 (AOF) using the linear IPR model (q = PI × drawdown).

Use cases

Estimating Absolute Open Flow (AOF) potential of an oil well.
Selecting artificial lift equipment sized to the well deliverability.
Comparing well performance before and after stimulation or workover.
Forecasting production rates at various tubing intake pressures.
Screening candidate wells for gas lift or ESP installation.
Teaching reservoir engineering students the IPR concept.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu