![]() The drawdown and buildup curves appear to be identical for all times. 3 is the diagnostic plot that results when the derivative is taken with respect to radial equivalent time and the time-plotting function is radial equivalent time. The longer the producing time, the longer the flattening is delayed and the longer the time the buildup diagnostic plot is essentially identical to the drawdown diagnostic plot.įig. The time at which the flattening of the pressure change curve (and corresponding downward movement of the derivative) becomes apparent is a function of the producing time before shut-in. Notice the significant difference in the shapes of both the derivative and pressure change curves for buildup and drawdown tests, with the pressure change curves flattening for buildup tests and the derivatives moving downward with an ultimate slope of –1. Wellbore storage distortion is not included in any of the diagnostic plots in this section.įig. ![]() Both pressure and time are in terms of dimensionless variables. For these plots, the derivative was taken with respect to shut-in time and derivative and pressure change curves are plotted vs. Infinite-acting, radial flow reservoirs are described on this page. Some pressure transient test analysis software allows the user a choice in the time function used in taking the derivative and another choice in time plotting function for other software, the time functions used are "hard-wired." The results can be bewildering. either of these time functions, and the shape differs for each plotting function. The derivative of pressure change may be taken with respect to the logarithm of either shut-in time or equivalent time. To further complicate matters for buildup test analysis, the shape of the derivative curve depends on how the derivative is calculated and plotted. (There are different equivalent time functions for radial flow, linear flow, and bilinear flow, as discussed in more detail in Fluid flow in hydraulically fractured wells.)īasically, equivalent time functions apply rigorously only to situations where either the producing time and the shut-in time both lie within the middle-time region or, as is commonly the case, the shut-in time is much less than the producing time before shut in. This problem is augmented by the common use of "equivalent time" functions to analyze buildup tests on drawdown type curves. However, boundary effects can cause quite different shapes for a given reservoir model at late times in buildup and drawdown tests. The shapes of the diagnostic plots for a buildup test and a drawdown test are essentially identical during the early- and middle-time regions for most tests. Once the proper reservoir model has been determined, test analysis may be relatively straight-forward type-curve matching or regression analysis using modern well-test analysis software. The model selected to interpret the test quantitatively must be consistent with geological and geophysical interpretations. However, a problem in recognition is that many reservoir models may produce similar pressure responses. Recognizing the influence of boundaries can be as important as analyzing the test quantitatively.
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