Detecting Ultra-small Leaks With Ultrasonic Leak Detection-Case Histories From the North Slope, Alaska

Abstract

A new ultrasonic leak detection logging tool conveyed on electric line, and recently on wireline in memory mode, has been introduced which can detect leaks as small as 1/2 cup per minute. This revolutionary tool has been used to accurately identify leaks in tubing and behind pipe. Wells that otherwise would immediately be slated for a rig workover (RWO) have been repaired with non-rig solutions. Ultrasound energy has very rapid attenuation and the ability to transmit through various media and behind pipe. These attributes allow pinpoint accuracy for leaks as small as 0.0024 gallons per minute (gpm). The tool incorporates data acquisition equipment and filtering algorithms which allow continuous logging. The technology is far superior to old-style noise logs which require time consuming stationary counts. To date, BP has run 21 ultrasonic leak detection logs in Alaska fields with an 81% success rate. The recent ability of this tool to be conveyed in memory mode has opened up additional logging opportunities. This has led to the development of a new technique using nitrogen to identify wells that leak only to gas. Application of this tool has great significance for any operator concerned with well integrity, and particularly, in areas where rig workovers are expensive including remote, offshore, and arctic locations. Introduction BP operates several enhanced oil recovery and waterflood oil field which have experienced well integrity issues as the field matures. Non-rig tubing repairs have become a viable alternative to RWO's, which can easily exceed 1 million dollars. Repair methods include tubing straddles and coiled tubing packer repairs. The advantage over a conventional RWO is that there is no need to pull tubing, resulting in the well being returned to service faster. However, the main limitation for non-rig candidate selection has been in identifying leaks which are below the resolution of conventional leak detection methods. Often a well with annular communication had to be worked over because the leak point could not be determined. The ultrasonic leak detection tool has provided a step change in leak identification. Prior to its introduction, it was virtually impossible to detect leaks smaller than 1 gpm. Often the velocity and temperature changes associated with these leaks are below the resolution of conventional logging tools, including spinners, temperature logs, down-hole cameras, and noise logs. These tools are even more limited when trying to detect leaks that occur behind tubing. The ultrasonic leak detection tool can identify leaks so small as to be almost unbelievable. Tool Principles and Operation. SPE paper 1028151 details the tool physics and development history of the ultrasonic leak detection log. Tool principles are briefly summarized here. The frequency spectrum a leak produces is a function of differential pressure, leak magnitude, and leak geometry. These properties determine whether the frequency is audible, ultrasonic, or both. The ultrasonic logging tool (Figure 1) utilizes a sensor that detects a frequency spectrum, including those typically produced by leaks. The signal is processed by a series of band-pass algorithms that focus on frequencies in the ultrasonic range. Virtually all audible noise associated with tool movement is filtered out, allowing continuous logging. Typical logging speed is 30 feet per minute (fpm) and leaks can be identified while logging in either an up or down direction. Greater accuracy is achievable due to the characteristics of ultrasound, which attenuates, or dies away, quickly in fluids. Ultrasound typically travels only 3–10 ft in a wellbore before attenuating. This attenuation results in a very sharp leak character, typically identifying the leak within 1 to 2 feet.

Acoustic signal processing for well integrity monitoring systems

“…It is found that the estimation performance of the dROSA follows closely with the CRB of the differential sensory array [99] when the source is located radially at approximately 6 cm, the typical range of the most common leaks (i.e., those in the tubing region). It is also found that the localization performance of the dROSA remains relatively constant for the moving sensor with a velocity of 15.24 cm/s along the sensor axis, the typical operating range of single-leak detection sensors [27]. The performance of the dROSA starts to degrade when the source is located radially further from the sensor axis.…”

“…Typically, the sensors used in ultrasonic leak-detection can reach a velocity of 15.24 cm/s [27]. The third simulation studies the RMSE performance of the dROSA at varying velocities.…”

“…It can be seen that the RMSE performance is relatively constant within this varying range of moving-sensor velocity. Thus, the proposed approach at the typical moving velocity [27] performs similarly to a static array using the differential received-signal strength approach of [99].…”

“…In the previous chapters, the problem of bridging the research gap between the noiselogging approach [27], [102] and the acoustic-array approach proposed by Walters et al [20] was discussed in terms of extending these approaches' operating frequency in arrays (Chapter 3 and Chapter 4) and achieving localization using a single sensor (Chapter 5). Aside from localization, additional knowledge about leak phases can further enhance an operator's ability to resolve any well integrity problems at an early stage.…”

“…A signal propagating through space and time is often corrupted by noise and interference [25]. A great deal of research has been conducted on this problem and focuses on detecting the presence of the desired signal and reducing the effect of noise when using a single sensor [17], [26], [27]. However, Johnson and Dudgeon [25] acknowledged that it is often advantageous to use an array of sensors for better performance.…”