Traditionally, the p/z plot has been used to describe the behavior of volumetric depletion gas reservoirs. Several authors1,2,3, 4 have pointed out, though, that this technique can lead to serious misinterpretations of the drive mechanism and overestimation of the original gas in place (OGIP) if the reservoir produces under the influence of an encroaching aquifer.

Although regression analyses have been presented5,6 to obtain a more accurate aquifer description, a solution to the non-uniqueness problem still relies on a prior knowledge of the aquifer properties and geometry.

The approach proposed here makes use of aquifer influence functions. A general pattern of the normalized absolute error function allows the simultaneous determination of the OGIP of a reservoir and a description of the aquifer performance. No assumptions regarding aquifer properties or geometry need to be made.

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