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The pseudopressure function is well understood and widely used for analyzing gas well drawdown and buildup data. The usefulness of the pseudotime function for analyzing buildup data with drawdown type curves pseudotime function for analyzing buildup data with drawdown type curves has also been established. Unfortunately, the pseudotime concept is often incorrectly applied when drawdown data are analyzed by semilog methods.. From the practical viewpoint, the most important result is that two Horner time ratios exist-one based on a normalized shut-in time and one based on shut-in pseudotime-that yield an accurate analysis of buildup data, including the correct determination of average pressure using the liquid Matthews-Brons-Hazebroek (MBH) functions. Introduction . Thus. conceptually. pressure drawdown or buildup data can be analyzed accurately with pressure drawdown or buildup data can be analyzed accurately with the real-gas pseudopressure function for cases where large variations in the mu z product render the pressure-squared method of analysis tenuous or where the variation in pressure divided by mu z is too large to analyze data by the pressure method. For cases where non-Darcy flow effects are pseudo-pressure function. Transient drawdown data can be analyzed accurately by plotting P pwf vs. t. P pwf vs. t.Buildup data can be analyzed accurately by plotting P pws vs. the standard Horner time ratio (t + Delta)/Deltat provided that the producing rate before shut-in is not too high. If the producing rate producing rate before shut-in is not too high. If the producing rate before shut-in is high. the semilog slope on a Horner plot of buildup data may be approximately 10% higher than the "correct" slope `.]If the producing time is long, then the standard MBH functions based on liquid solutions cannot be used to obtain a reliable estimate of average pressure, however, if the producing time is short, the liquid MBH functions can be used to estimate the average reservoir pressure. Here, a long producing time t means t is greater than t pss, where is the time at which pseudosteady-state flow would begin for the corresponding liquid case if production were at a constant rate. . The chief disadvantage of these corrected MBH methods" is that they require knowledge of the initial reservoir pressure` The key advantage the MBH method has over material-balance calculations is that it does not require knowledge of the initial pressure. Recognizing this fact. Kazemi presented a procedure for obtaining an estimate of average reservoir pressure from procedure for obtaining an estimate of average reservoir pressure from gas well buildup data. His method does not require knowledge of initial pressure. . The main disadvantage of his method is that it requires an iterative procedure. As is well known, the pseudopressure function removes the non-linearity in the gas flow equation that is a result of variations in the mu z product.The pseudotime product. mu c, which is a function of pressure. Strictly speaking, this viewpoint is incorrect and it leads to the inference that pseudotime should always be used when pressure data from gas well tests are analyzed. pseudotime should not be used when drawdown data are analyzed by semilog methods. Agarwal showed that pseudo. time should be used when buildup data from fractured gas wells are analyzed with fractured-well type curves based on drawdown solutions. Lee and Hold itch established that if wellbore storage effects are significant, then gas well pressure data (either drawdown or buildup) can be analyzed accurately with liquid wellbore storage and skin type curves, provided that data are plotted in terms of pseudopressure and pseudotime. Scott has also presented a procedure for analyzing buildup data from fractured gas wells.Al though Scott's work on normalized time and Agarwal s work on pseudotime appeared in 1979, to the best of our knowledge no one has presented detailed results regard ing the applicability of these times to drawdown and buildup tests conducted under radial flow conditions. This results analysis of gas well drawdown and buildup tests by use of semilog methods. how the time scales should be defined to optimize the accuracy of the analysis. Most important, we show that two Horner time ratios exist that can be used to obtain a highly accurate analysis of gas well buildup data including an accurate determination of average pressure with the liquid MBH functions.
