Reservoir engineers usually work under much uncertainty. Among the several sources of uncertainty in reservoir management, calculation of the water influx from adjacent aquifers is undoubtedly one of the most important. Several models have been proposed for quantifying the influx of water accumulated into the reservoir. Among the classic models, the most used are van Everdingen & Hurst, (1949), Hurst modified (1958), Fetkovich approximate (1971), Carter & Tracy (1960), Allard & Chen (1984), and Leung (1986). This work will deal, in a comparative way, with the total influx of water by the aquifer performance as a function of time, taking as the base case the model proposed by van Everdingen & Hurst (1958), because this is the best in terms of solution of the diffusive equation. All analyses in this work were done in computer spreadsheets using the equations proposed by the authors of each model. In this comparison it is possible to analyse the water influx into the reservoir as a function of time, and the relative error of each model against the base model. Besides the instructional value of summarizing in a single paper several classic models of aquifers, one additional purpose of this work is to alert against the inadequate use of the Fetkovich model, as commonly found in flow simulators and in material balance programs, when the ratio between the radius of the aquifer and the radius of the reservoir is below 12. In the conclusions, some corrections of mistakes found in tables and formulations published in the original papers are presented, as well as alternative formulations for the calculation of the aquifer productivity index, specifically with regard to the original publications of Fetkovich, 1971 and of Leung, 1986.
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