The common approach of using refined grids and small time steps usually does not provide accurate and efficient numerical models to simulate horizontal-well pressure-transient tests. In most cases, this approach requires impracticably long computational times or creates artifacts on pressure and derivative responses that may be confused with the characteristics of certain flow regimes. The results of this study indicate that the conventional well index should be used with log-distributed grid and time steps to obtain closer match to the analytical solution at early times. There is, however, a limit to grid refinement beyond which the results do not improve. This paper introduces the transient well index that allows the use of larger grids while improving the accuracy of simulations especially at early times.

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