Abstract

Through the use of a casing corrosion monitoring and evaluation program, production costs associated with casing leaks can be budgeted for and a workover program scheduled. Three basic services are currently available: Casing Potential Profile, Magnelog and Vertilog. The Potential Profile, Magnelog and Vertilog. The Casing Potential Profile aids in evaluating the predictive metal losses as well as the need for predictive metal losses as well as the need for and effectiveness of cathodic protection. The Magnelog records the nominal weight and size of the casing string and detects the gross metal losses which are the result of drill pipe wear, tubing wear or corrosion. Detection and recording of small-diameter pitting and differentiation between internal and external pitting is provided by the Vertilog. Establishing pitting is provided by the Vertilog. Establishing base logs and a periodic re-evaluation program make it possible to estimate the life of the casing string. The estimated life of the casing string versus the predicted production life of the well provides the data necessary to evaluate the available corrosion mitigation programs. The need for re-work can therefore be programs. The need for re-work can therefore be evaluated and budgeted for prior to the actual occurrence of a casing leak. The theory and application of these casing evaluation services will be discussed in this paper.

Introduction

The problem of well casing corrosion first became apparent shortly after the discovery of well casing. Causes of well casing corrosion have been previously discussed.

These causes include, but are not limited to, the following:

External Corrosion

  1. Stray current electrolysis

  2. Varying mineral content of formations

  3. Bacterial attack

  4. Dissimilar metals

  5. Metallurgical defects

Internal Corrosion

  1. Erosion

  2. Corrosive fluids

  3. Oxygen intrusion

  4. Mechanical damage in drilling and completion

  5. Acidizing operations

One of the most common methods of determining if a corrosion problem exists is to generate a leak-rate profile. When this profile indicates that the problem is severe, then mitigation methods are investigated.

At one time, it was believed that production in a field would be completed prior to the occurrence of a serious corrosion problem. With the recent increase in the value of production and the resulting advent of improved secondary and enhanced recovery methods, this belief is no longer valid.

Ideally, corrosion considerations are evaluated even before a well is drilled. Occasionally the corrosion is designed into a well, though not intentionally. During the drilling and completion process, the compatibility of mud, cement and process, the compatibility of mud, cement and casing systems relative to corrosion should be examined carefully. Errors in judgement at this time will plague the well through its entire life.

The above considerations are not always made. As a result, well casing corrosion and leaks occur with devastating results.

The cost of repairing leaks has spiraled from $535 per well in 1948 to a recent figure of as much as $250,000 to replace the casing, assuming the well is not lost in the process.

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