Gas reservoirs with abnormally high-pressure have been encountered all over the world. For these reservoirs, a straight line plot of P/Z Vs. Gp for the early production data and extrapolation to zero reservoir pressure project incorrect, IGIP, initial gas in place. The P/Z plot is based on the assumption that gas compressibility is the "Sole" reservoir drive mechanism. In abnormal pressure gas reservoirs, however, grain expansion, formation water expansion, and water influx from shale or small associated aquifer, in addition to gas expansion contribute significantly to the gas production. Several material balance models have been proposed to calculate the initial gas in place for abnormally high-pressure gas reservoir. The present study is concerned with analyzing the different material balance models used to estimate the IGIP for these kind of reservoirs. Therefore, it reviews the bases and assumptions on which these models have been developed, as well as discusses the strength and weakness of every model. In addition, the present study comprises calculations of the IGIP by analytical and numerical models of the material balance equations for eight case histories. The study shows that solution plot of Havlena and Odeh can be used to estimate the IGIP for abnormal pressure gas reservoirs without prior knowledge of the aquifer size or the formation compressibility. Moreover, the present investigation reveals that most of the material balance models analyzed in this study are sensitive to the value of the initial reservoir pressure and the early data. Unfortunately, this is the time when reliable estimate for the IGIP is vital for economic decision regarding the development of such gas reservoirs.

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