With reference to the oil and gas production wells, the term ‘Oil Well Productivity Index’ is an indicator of ability of the reservoir to produce fluid flow in relation to the reservoir pressure. The productivity index can be mathematically expressed as the ratio of volumetric flow produced by a well (bbl/day) to the pressure loss between reservoir and bottomhole (psi).
PI = Q/(PR – PBH) bbl/day/psi
Over a longer timescale as the oil and gas reservoirs are depleted, the reservoir pressure drops down. Hence flow and pressure loss also change. But this depletion can only be considered over a timescale of a few years. For shorter time scales, the hydrocarbon fluid flow as well as the reservoir pressure can be considered to be constant. This is known as pseudo steady state for the producing well.