Analysis of Short-Time Transient Test Data By Type-Curve Matching

Abstract

A new type-curve matching technique significantly easier than those previously published allows the estimation of permeability, skin factor, and wellbore storage coefficient from short-time transient test data. The method is explained here and is illustrated with several examples. Introduction Occasionally, insufficient transient test data are available for analysis using semilogarithmic plotting methods. This usually happens when data collection stops before wellbore storage (afterflow) has become negligible. Under those circumstances, the semilogarithmic straight line does not develop, and common semilogarithmic analysis methods cannot be used. When such methods cannot be used, the engineer either obtains no information from the test or must use the available, short-time data to estimate reservoir characteristics. This paper presents a technique for the approximate analysis of such short-time transient test data. The method applies to buildup, falloff, drawdown, and injectivity tests when wellbore storage effects are important. It should not be used if data can be analyzed by more conventional, semilogarithmic plotting methods.It has long been recognized that wellbore storage (afterflow) can impede pressure transient test analysis. Several ways have been suggested for determining when well known semilogarithmic techniques can be used for transient test analysis. Gladfelter et al. and Russell present calculational methods for analyzing the portion of transient test data influenced by wellbore storage. Curve matching, and regression techniques have also been proposed for accomplishing such analyses.All these methods have disadvantages. The techniques presented by Gladfelter et al. Russell, and Earlougher and Kersch, utilize either trial-and-error analysis or require that the afterflow schedule be calculated, or both. These approaches are tedious and not always successful.In spite of its disadvantages, curve matching seems to be the most promising of the methods, particularly for the engineer who does not have a computer available. Cooper et al. present type curves and an analysis technique for specific flow and injection tests with the well shut in before testing. At the start of the test, the pressure instantaneously changes to some new value. Then both pressure and flow rate vary during the test. The Cooper-Bredehoeft-Papadopulos type curves are useful for analyzing data taken during the flow period of a drillstem test. Agarwal et al. point out that neglect of the skin effect makes the Cooper-Bredehoeft-Papadopulos type curves of dubious value. In any case, those curves do not apply to the more common transient testing situations: buildup, falloff, injectivity, and drawdown. Ramey and Agarwal et al. suggest type-curve matching for these kinds of transient tests. They present applicable type curves that form several families of curves with skin factor and wellbore storage coefficient as parameters. Ramey's curve-matching method requires that the data plot be slid both horizontally and vertically to obtain a match. This feature and the fact that the curves have very similar shapes make the matching technique difficult to use unless there are data at least onto the start of the semilog straight line.McKinley uses a similar approach, but with a different kind of type-curve plot. JPT P. 793