Effectiveness of Well Casing Cathodic Protection-An Analysis
Authors: Kirklen Charles A. 
Published: December 2007

This study analyzes the effectiveness of cathodic protection as a means of alleviating external corrosion on well casings. The history of leaks is analyzed for seven groups of wells in which cathodic protection has been applied for 4 to 10 years. The 934 wells involved in the study are in South Texas, West Texas, and the Oklahoma Panhandle.

Introduction

In 1958, shortly after assuming operation of a major unitized field in South Texas, Sun Oil Co. encountered its first major external casing corrosion problem. Cathodic protection for a portion of this field was installed when it became apparent that the external corrosion could not be arrested by other means. This proved to be highly successful and appeared to be proved to be highly successful and appeared to be the answer to a very costly operational problem.After the success of the South Texas installation became apparent, casing cathodic protection was applied in other major fields throughout Sun's operation wherever frequent leaks signified the existence of a major external corrosion problem. However, many of these subsequent installations did not effect the drastic reduction in leaks that had been experienced in our initial venture. They appreciably slowed the frequency of leaks, but the fact that some leaks continued in a more or less regular pattern prompted a more detailed study of the cathodic protection process as it relates to well casings. process as it relates to well casings. The casings in a large unitized field in West Texas were cathodically protected in 1967, 2 years after an apparently successful pilot system had been installed. This 1967 expansion proved to be a corrosion engineer's nightmare since it soon became clear that the effectiveness of leak reduction was at or approaching zero. Even the once successful pilot system wells resumed leaking at an alarming rate. It appeared that we had come full circle from a highly successful first system to a dismal failure with a later one. In retrospect, we recognize that we had encountered a classic case of mutual interference of such magnitude that the current applied to each well was in effect nullified by interference from adjacent systems.

Predicting Casing Leaks Predicting Casing Leaks When steel is exposed to a corrosive medium, the cumulative number of leaks that occur tends to be logarithmic with respect to time as a linear function. This is valid where each corrosion perforation is counted as one leak. Obviously, the exact number of separate corrosion-induced holes in a casing cannot be counted, nor is that necessary from a practical standpoint. Each casing leak, regardless of the number of holes occurring at the time, is counted as a single leak in normal curve construction. An extrapolation of the data can be used to predict, sometimes with uncanny accuracy, the number of leaks that will occur in any year. This applies whether or not the wells are receiving protective current as long as conditions are the same as those existing at the time the base-line data are recorded.One of the major problems in constructing an accurate curve of casing-leak frequency is the lack of reliable information on exactly when some leaks occurred. Leaks in flowing wells where tubing is set on a packer or wells that have been shut in for some time are examples of cases in which the probable leak date must be estimated.

JPT

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