The p/z vs. Gp material balance approach is an established, simple, quick and convenient means of surveying a reservoir's performance and determining gas reserves and gas-in-place (G). The x-intercept is G. Unfortunately, this approach may be an independent analysis, and not confirmed with a separate method.

Theory requires that the "p" in the p/z term to be the average reservoir pressure at which a cumulative production of Gp occurs. A source of average reservoir pressure is a pressure buildup test in which the MBH method is used to find average reservoir pressure. The MBH method requires an estimate of drainage area (A), which is directly related to G (∞Ah); this is termed GMBH. The average reservoir pressure that was calculated using the estimated A are plotted as p/z in the material balance plot. An extrapolation of the trend to the x-axis is Gp/z. The GMBH and Gp/z are used iteratively until the two estimates of G converge.

This proposed method is tested on synthetically derived pressure data simulated with a commercially available, analytical well testing program for two different reservoir shapes and three different well locations. One single-well gas reservoir is analyzed for the case study.

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