The productivity index is defined as the flow rate per unit pressure drop and serves as an indication of the production potential of a well. It is defined as:

For example, if a well flows at a 1000 stbbls/d with a flowing (sandface) pressure of 1500 psi and at an average reservoir pressure of 2000 psi, then the productivity index is:

PI = 1000 / (2000 - 1500) = 2 stbbls/d / psi

Once the *PI* is know the equation
can be re-arranged to determine the deliverability rate as follows:

This equation is only valid for a well in an undersaturated reservoir. For wells in saturated reservoirs or for gas wells, the relationship is not as straight forward, and the simple relationship described above does not apply. In these situations either an IPR (for oil) or AOF (for gas) analysis should be performed.