Abstract

Efficient development of hydrocarbon zones requires the knowledge of average formation permeability, skin factor, reservoir pressure, formation continuity, reservoir boundaries, drive mechanism, and fluid properties (PVT relationship). The technique most respected and most widely used to determine the first five of these seven parameters is called transient pressure testing. This technique also allows one to make pressure testing. This technique also allows one to make qualitative inference of the last two parameters. Yet, the conventional transient pressure technique, at times, is unable to provide accurate and detailed answers and quite often may not even assure a valid test. Recent publications have suggested that inclusion of transient rate data along with the pressure data may allow one to eliminate some inadequacies associated with conventional pressure testing. In this paper, we extend the study by pressure testing. In this paper, we extend the study by presenting several field examples of oil and gas wells, illustrating that presenting several field examples of oil and gas wells, illustrating that inclusion of transient rate information in transient pressure testing (1) eliminates inaccuracies; (2) allows correct interpretation in the presence of fluid segregation within the wellbore; (3) allows determination of near wellbore skin condition and overall skin factor; (4) makes analysis possible when data are influenced by boundary effects; (5) allows more definite differentiation between matrix reservoir behavior and naturally fractured, multilayer flow effects and other flow patterns; and (6) assures a valid test. Apart from these aspects, this technique allows one to perform a well test of relatively duration.

Introduction

Transient pressure analysis has been one of the most respected and widely used conventional techniques of calculating formation permeability, skin factor, reservoir pressure, formation continuity, and reservoir boundaries. This technique also allows one to make qualitative inference of reservoir drive mechanisms and fluid properties (PVT relationship). These important parameters are essential to the efficient development and parameters are essential to the efficient development and exploitation of hydrocarbon formations. The wide application of the technique is also evident when one considers the large number of such tests being performed throughout the world by the oil and gas industry. Transient pressure testing involves producing and shutting in a well, measuring the pressure behavior in the wellbore, then analyzing for results. Such a process can take an expensively long time to complete, and problems may be encountered in interpretation because of the presence of wellbore storage effects, fluid segregation, and crossflow. For example, during the buildup period after shut-in, wellbore pressure increases very rapidly, which affects the wellbore storage continuously and, consequently, the duration of the test and the accuracy of the analysis. Earlougher has documented that conventional analysis of wellbore storage dominated data invariably results in inaccurate estimates of formation properties. Type-curve techniques have been proposed as a viable solution; however, results from these analyses have not provided adequate solutions. These type curves were developed for constant wellbore storage coefficient, whereas, in most transient data, the wellbore storage coefficient is continuously changing. In the early eighties, Ehlig-Economides and Ramey and Uraiet and Raghavan presented new methods for the analysis of pressure buildup tests from wells produced at constant pressure. pressure buildup tests from wells produced at constant pressure. In general, these methods are only applicable if pressure buildup data have the correct semilog straight line period and/or the wellbore storage coefficient remained constant during the test. Quite often, when the wellbore storage duration is too long, the semilog straight line portion is overshadowed with boundary effects. Effects of such phenomena on the distortion of data, extension of test duration, and at times making data unanalyzable are well documented for matrix formation and also for hydraulically fractured wells.

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