Abstract

Conventional well test analyses typically assume that the wellbore skin factor is constant over the entire duration of the well test. For certain cases such as during acidizing treatments, however, the skin factor can change significantly and affect the pressure response. The concept of well test analysis has been used in real-time evaluation of well stimulation treatments. Numerical and analytical methods have been developed to estimate the evolution of skin during acidizing, but these methods can be time consuming and cumbersome when numerous pressure and rate data exist.

In this paper, we apply the flow rate normalization method used in well testing to estimate skin as a function of time. We demonstrate that the new technique is accurate, robust, fast, and easy to use. We apply the method to two matrix acidizing treatments performed in a water well and an oil well. The skin calculations for each example agree well with those calculated from current accepted methods. The new technique is a fast, simple and more accurate method for real-time interpretation of acidizing treatments where a multitude of pressure and flow rate data exists. We compare the results from the new method to current methods for several field examples.

Introduction

To control the quality of matrix acidizing, well test analysis has been adopted to monitor well response to injection during an acidizing treatment. Skin factor is calculated from pressure and rate, and from the skin evolution, the effectiveness of stimulation and diversion can be clearly identified. Current analytical methods to analyze the pressure responses of acidizing treatments are based on superposition of infinite-acting pressure solutions.1,2,3 Other numerical methods use simulated pressure responses to obtain the skin.4 Although the current numerical and analytical methods are reliable, these methods can be time consuming and cumbersome when numerous pressure and rate data exist. When used in real time, computation time can be a critical limitation on the application of a method.

The focus of this paper is to provide a new method for skin calculation based on flow-rate normalization used in well testing.5 We show that the method can be easily applied to determine skin evolution during matrix acidizing. Mathematically, it is much faster then the current analysis methods because there is no superposition calculation involved.

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