The productivity index (PI) conveys information on the reservoir productive capacity and can be measured during well tests. It is defined as the ratio of the production flowrate q and the pressure drawdown. As a result, the PI dynamic behavior is dominated by a transient period (henceforth referred to as transient PI) which is followed by a pseudo-steady state regime. This works deals with the former, since the latter is rarely witnessed during well tests.

Usually, the pressure drawdown employed in PI calculations is measured as the difference between a constant reservoir pressureand the downhole flowing pressure pwf(t). In this paper, it is shown that, by coupling the under-saturated material balance equation with the deconvolved pressure response ?pDEC (t) and its associated flowrate qDEC, it is possible to account for a time-varying average pressure . The algorithm is valid for reservoirs whose behavior during a well test is modelled by a closed geometry, but has not yet reached the pseudo-steady state flow regime.

Subsequently, a comparison will be drawn regarding the transient PI evaluated under a constant average pressure against the transient PI evaluated with a decreasing average pressure. We will also study the conditions where the assumption of constant reservoir pressure will lead to a reliable estimate of the transient PI. The procedure will, then, be applied to a field case, where the constant pressure assumption leads to a significantly undervalued PI.

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