Applied Ultrasonic Technology in Wellbore Leak Detection and Case Histories in Alaska North Slope Wells

Abstract

When operators are faced with well integrity problems, a variety of methods may be used to detect the source of annular communication. Methods for detecting downhole leak points include spinners, temperature logs, down-hole cameras, thermal decay logs, and noise logs. However, many of these methods are ineffective when dealing with very small leaks and can result in collecting data that requires a significant amount of logging finesse to interpret.

Ultrasonic listening devices have been used for a number of years to effectively detect leak sources in surface production equipment. Ultrasonic energy has some properties, when compared to audible frequency energy, which make it ideal for accurate leak detection. Like audible frequency energy, ultrasonic energy can pass through steel.  However, ultrasonic energy propagates relatively short distances through fluids when compared to equal energy audible frequency sound. 

On this premise, an ultrasonic leak detection tool was developed for downhole applications to take advantage of the unique properties of ultrasonic energy propagation through various media.  Data acquisition equipment and filtering algorithms were developed to allow continuous logging conveyed on standard electric line at common logging speeds. Continuous logging has proven to be significantly more efficient in locating anomalies than static logging techniques commonly used in noise logging operations.

During development, the tool was shown to be effective in measuring leaks as small as 0.026 gallons per minute (gpm) with an accuracy of 3 ft. in production tubing, casing, and other pressure containing completion equipment.  Leaks have also been detected through multiple strings of tubing and casing.

This paper will compare conventional leak detection methods, describe a downhole ultrasonic leak detection tool, discuss development testing, and provide case histories where the ultrasonic leak detection tool was used to successfully locate leaks that other diagnostic methods were unable to locate.

Introduction

Well integrity continues to be an issue of critical importance for all forward-thinking operators.  When faced with well integrity problems, the prudent operator understands the value of gaining a complete understanding of the communication source prior to embarking on a remediation campaign. Comparing the process to that of a doctor diagnosing a patient, it is difficult to prescribe a remedy without fully understanding the root cause of the ailment. Symptoms of wellbore integrity issues are usually fairly evident. The root cause of these problems, however, can be rather elusive. In particular, when leaks are of a nature that precludes the use of conventional logging tools, it is often difficult, if not impossible, to detect their source.

Detection of very small tubing leaks, (less than one gpm) is difficult using conventional logging techniques such as spinners and temperature logs. Small leaks result in velocity and temperature changes that may be below the resolution of any logging tools, or may result in data that is virtually impossible to interpret. Conventional noise logs can detect fluid or gas movement, but must be utilized in a stationary mode and more distant noise sources may confuse interpretation.  Downhole cameras can be useful in finding a variety of leaks and diagnosing other problems but require that the wellbore contain optically clear fluid or gas.  Most of these tools, along with various mechanical methods of leak detection, are ineffective when trying to locate leaks that occur behind multiple casing strings.