Risk-Based Statistical Approach To Predict Casing Leaks
Authors: Wilson Adam 
Published: June 2018

This article, written by Special Publications Editor Adam Wilson, contains highlights  of paper SPE 183948, “Risk-Based Statistical Approach To Predict Casing Leaks,” by Mohammed D. Al-Ajmi, SPE, Saudi Aramco; Dhafer Al-Shehri, SPE, King Fahd University of Petroleum and Minerals; and Nasser M. Al-Hajri, Abdullrahman T. Mishkes, Muhammad A. Al-Hajri, SPE, and Nayef S. Al-Shammari, SPE, Saudi Aramco, prepared for the 2017 SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, 6–9 March. The paper has not been peer reviewed.

Currently, the predominant technology for measuring casing integrity is electromagnetic corrosion logging. While this technology has improved the ability to measure and monitor corrosion, the findings usually are not conclusive and need to be integrated with other data to enable qualitative assessments. A probabilistic approach was introduced to improve interpretation of data from electromagnetic corrosion logs. This paper presents a statistical risk-based approach to predicting casing leaks using electromagnetic corrosion logs.

Introduction

Sound well-integrity-management strategy in mature fields, where wells can sustain economic production for 30 to 50 years, is vitally important. Failing to implement this strategy can lead to a catastrophic loss of both assets and human life. One example of such loss is surface leaks that are caused by downhole multiple casing impairments caused by active shallow aquifer corrosion.

Corrosion logging provides the most-direct measurement of casing integrity and can be used as a predictive measure as well. Mechanical, ultrasonic, and electromagnetic tools are three main types of corrosion-logging tools. A mechanical multifinger tool uses multiple high-resolution calipers to measure slight changes in the internal diameter of tubing and casing strings. The tool deploys an array of hard-surfaced fingers that monitors the inner pipe wall. Each of the sensors generates an independent signal that is recorded against depth.

Electromagnetic Induction Tool (EMIT)

This paper focuses on the interpretation challenges associated with EMITs (Fig.  1), a casing-corrosion-logging technology. The tool uses three types of noninvasive electromagnetic measurements to characterize well casings, using low-, medium-, and high- frequency induction currents that are related to the casing-wall thickness, inside diameter, and permeability or conductivity. The lower the frequency, the deeper the penetration is to the outer casings.

Each parameter is averaged around the pipe circumference. The tool has multiple transmitters and receivers to send and receive electromagnetic signals. It detects average metal loss and changes in casing geometry irrespective of fluid type.

Despite its ability to assess metal loss in multiple casings, the tool can only read an azimuthal average loss across multiple strings. Consequently, wells with casing failures definitely will show average-metal-loss values of less than 100% unless the failure occurs around the 360° circumference. In other words, 50% average metal loss could mean a failure if one part of the casing is completely lost and another is intact.