Application of Ultrasonic Technology for Well Leak Detection

Abstract

Abstract: Identification of the source of pressure communication between well tubing-casing and casing-casing annuli presents an enormous challenge to petroleum engineers. Qatar Petroleum recently field-tested an acoustics based tool to successfully pinpoint the source of high pressure in the tubing casing annulus in a deep gas well in an onshore field. This paper presents a description of the technology and the findings from the field trial. An ultrasonic leak detection tool has been developed for down-hole applications to take advantage of the unique properties of ultrasound energy propagation through various media. This data acquisition equipment has been developed to allow continuous logging SRO (Surface Read Out) operation on conventional electric line or memory mode on slick line. The movement of fluids across a failed barrier creates turbulence which in turn creates ultrasonic frequency sound waves that are detected by the tool sensor. This new tool is different from the conventional noise logs as its' highly customized multiple sensitivities enable the detection of leaks as small as 0.02 Litres per Minute (LPM) with an accuracy of inches in the production tubing, casing and other completion equipment. It is also practically immune to disturbances from distant noise sources. The tool was run in a deep gas well with a history of high pressure in the tubing-casing annulus. Diagnostic work prior to the trial had established that the wellhead seals were not the source of the pressure. A leak rate of 3.5 LPM was estimated based on the pressure build-up trend in the annulus. The paper describes the procedure for running the survey and discusses the interpretation of the results that confirmed a casing leak at a failed recirculation valve (DV). The field trial confirmed that ultrasonic based well leak detector can accurately and efficiently detect very small tubing and casing leaks. Introduction: A growing concern in the life of an oil or gas well is the pressure buildup in its annuli. The first challenge for a petroleum engineer is to identify the source of the leak to enable design of an effective remedial activity. There are many methods to identify the source of leaks in a well. The simplest, cheapest and most common is the "Pony Tail" method. If the leak is big enough (>100 LPM), flowing from the tubing into the A-annulus then a highly experienced slickline operator running a "Pony Tail" assembly may be able to detect the leak location. This method will, obviously, not work with very small leaks. More advanced technologies have been used over the years to identify such leaks more accurately. Over time, a philosophical approach has been used to decide what technological gadget to use. Temperature logs, spinner logs, downhole cameras, and noise logs, etc. or a combination thereof can help identify rather small leaks (with leak rates varying between 10 LPM and 100 LPM) between the tubing and A-Annulus. The optimal tool string depends in part on the magnitude of the leak. At the lower range of leak rates, the noise log is often the most sensitive. The temperature log works over a broader range of leak rates, but is usually not as sensitive as the noise log. A spinner can be used for higher leak rates.2 The problem becomes more complicated when the leaks are caused by packer (seal element) failure or holes in the casing causing fluid movement from the B-Annulus to A-Annulus, or fluid movement from the formation through the annuli and reaching to the A-Annulus. (i.e. problems behind the tubing wall). Matters are further complicated if the leak rate at the time of investigation is lower than the sensitivity of the tool being used for identification. Based on extensive R&D in the ultrasound technology domain, it was discovered that an active leak at rates ranging from 0.02 LPM to 150 LPM would create turbulence with a high frequency wave that would generate an ultrasonic signature.