A straight line, easy to use form of the mass balance equation, the "p/Z plot", is frequently used to determine both Original-Gas-In-Place and initial reservoir pressure of conventional gas wells and reservoirs. A coal well p/Z plot bends to the right, similar to that of a conventional gas well subject to water influx. The theoretical foundations to linearize this plot for coal wells were laid in a simulation study by King (ref. 1) with the introduction of his Z* variable and the resulting "p/Z* plot". However, when King's method was applied to actual coal wells, wildly erroneous results were obtained. Investigation of this failure resulted in a modified, more robust method. As did the original method, the modified method presented here allows determination of OGIP, initial reservoir pressure, and drainage area. This paper documents mathematical development of the modified method, application of both the original and modified methods to simulated coal well production data, and, lastly, application of the modified method to the actual production data which initiated this study.

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