Use of an Automatic History-Matching Technique To Analyze Pressure Buildup Data Affected by Wellbore Phase Segregation: Pressure Buildup Data Affected by Wellbore Phase Segregation: Case Histories

Abstract

An automatic history-matching technique has been developed for analyzing pressure buildup data affected by wellbore phase segregation. This automated type-curve analysis method uses a Gauss-Newton least-squares procedure to match observed field pressures and pressures simulated with Fair's pressures and pressures simulated with Fair's phase segregation model. From this analysis phase segregation model. From this analysis we determine not only estimates of wellbore skin factor and formation permeability but also indications of the magnitude of the phase segregation effects.

The basis of conventional semilog analysis techniques for buildup tests is the presence and correct identification of a presence and correct identification of a middle-time line whose slope is related to formation permeability. However, wellbore phase segregation may delay or completely phase segregation may delay or completely suppress the development of this line, so semilog analysis methods often cannot be used. Fair has developed type curves for analyzing buildup data distorted by wellbore phase segregation, but unique solutions are difficult to obtain from manual type-curve matching. Since our automatic history-matching technique does not rely upon the presence of a middle-time line, this new presence of a middle-time line, this new method becomes especially useful when conventional semilog analysis techniques cannot be applied. In addition, comparisons of results from our automatic history-matching and Fair's manual type-curve fitting suggest that more reliable solutions can be achieved with this new automated procedure.

We present analyses of pressure buildup data taken from the petroleum literature and a previously unpublished field test. Comparisons of the results from our automatic history-matching, Fair's manual type-curve fitting, and the Horner semilog plotting method illustrate the applicability of this new method for analyzing field buildup tests distorted by wellbore phase segregation.

Introduction

Pressure buildup tests are widely used by petroleum production engineers to evaluate the fluid flow characteristics of oil and gas reservoirs. The analysis techniques for buildup tests and the factors affecting the pressure response during these tests are well pressure response during these tests are well documented in the petroleum literature. One of these factors is wellbore phase redistribution, which refers to the segregation of the liquid and gas phases in the production string of a well that has been shut in at the surface. During a buildup test, the process of the segregation of the fluid phases causes the measured bottomhole pressure response to deviate from behavior pressure response to deviate from behavior predicted by conventional theory. predicted by conventional theory. An example of the effects of wellbore phase redistribution is shown in Figure 1, phase redistribution is shown in Figure 1, which is a Horner plot of a field pressure buildup test. The basis of semilog analysis techniques is the presence and correct identification of a middle-time line whose slope is related to formation permeability. When early-time effects (i.e., wellbore storage) cease to dominate the pressure response, this middle-time line will usually develop if the pressure transient is not affected by any reservoir boundaries (i.e., late-time effects).

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