- Qo = oil flow rate, STB/day
- J = productivity index, STB/day/psi
- avg(Pr) = volumetric average drainage area pressure (static pressure)
- pwf = bottom-hole flowing pressure
- Deltap = drawdown, psi
The productivity index is generally measured during a production test on the well. The well is shut-in until the static reservoir pressure is reached. The well is then allowed to produce at a constant flow rate of Q and a stabilized bottom-hole flow pressure of pwf. Since a stabilized pressure at surface does not necessarily indicate a stabilized pwf, the bottom-hole flowing pressure should be recorded continuously from the time the well is to flow. The productivity index is then calculated from the above Equation.
It is important to note that the productivity index is a valid measure of the well productivity potential only if the well is flowing at pseudosteadystate conditions. Therefore, in order to accurately measure the productivity index of a well, it is essential that the well is allowed to flow at a constant flow rate for a sufficient amount of time to reach the pseudosteady-state as illustrated in the Figure. The figure indicates that during the transient flow period, the calculated values of the productivity index will vary depending upon the time at which the measurements of pwf are made.
Since most of the well life is spent in a flow regime that is approximating the pseudosteady-state, the productivity index is a valuable methodology for predicting the future performance of wells. Further, by monitoring the productivity index during the life of a well, it is possible to determine if the well has become damaged due to completion, workover, production, injection operations, or mechanical problems. If a measured J has an unexpected decline, one of the indicated problems should be investigated.