Joel Johns, TecWel
When operators are faced with leaks in wellbore tubulars, a number of methods exist to detect them. When leaks preclude the use of conventional logging tools, however, it is often difficult, if not impossible, to detect their source.
Methods such as spinners and temperature logs are useful, but small leaks (<1 gpm) often do not generate enough temperature or velocity change to register on these tools. Downhole cameras are useful in some cases, but they are limited to optically clear fluid. Conventional noise logs can detect fluid and gas movement, but must be used in a stationary mode and are affected by sounds other than leaks. Most of these techniques are not useful in analyzing leaks behind strings of pipe.
The properties of ultrasonic energy have been used for a number of years to detect leaks in surface equipment. Experience has shown that a leak generates an ultrasonic frequency or audible frequency, depending upon its rate, pressure drop and geometry. In addition, the medium in which the leak occurs plays a role in the frequency spectrum it generates.
Ultrasonic energy passes through steel, gas, and other media, but normally travels very short distances, about 3-4.6 m (10-15 ft). A tool that can detect these attributes would be useful in detecting very small leaks and leaks behind production tubulars.
TecWel and a major operator initiated a joint venture project to develop a leak detection tool based on ultrasonic principles.
The resulting tool can:
The tool uses an ultrasonic sensor in conjunction with digital signal processing (DSP). As the tool passes near a leak, the ultrasonic sensor detects frequencies generated by the turbulent flow at the leak point. The signal is amplified and sent through the DSP module in the tool.
The DSP unit is equipped with a large amount of flash RAM running a series of modular signal processing programs. This process filters unwanted background energies caused by mechanical noise or other interference. The result is a digitized signal of the leak signature that is transmitted uphole via wireline telemetry to the surface read-out system. The tool can also be run in memory mode on slickline or coiled tubing.
Tubing leak detection
Following successful field trials, the well leak detector (WLD) was used to detect a wide variety of leak scenarios in wells all over the world. One case involved a 0.1-gpm leak in a tubing connection just above a mud line hanger.
The graphical representation of the tool response shows three frequency windows of investigation. The three traces used are the total energy level, a medium-high frequency range, and a very high frequency range.
The traces are unitless measurements of signal strength. A casing collar locator (CCL) was also present for correlation purposes. This signature was detected while logging on e-line at 10 m/min (32.8 ft/min.).
An operator in Norway encountered a pressure buildup in a well in the “B” annulus when gas lift operations were under way. Production operations were suspended.
The WLD was used to determine the source of the pressure buildup to diagnose the leak. Both the “A” annulus and tubing were pressurized using lift gas to activate the leak. The leak measured 0.005 cubic feet per minute during logging .
The leak signature appeared to be near a tubing joint on the CCL, but because the tubing and “A” annulus were operated at the same pressure, the leak was confirmed in the “B” annulus casing. This leak likely would have been undetectable by other means.
The WLD tool has found leaks in over 200 wells world wide. The data generated by this tool also have been used in planning, saving operators millions of dollars by allowing them to select methods of leak repair that do not require a workover.
Refinements to the tool and ultrasonic logging process are ongoing, and the limits of this method of leak detection continue to be extended, with the smallest leak detected as of this writing of 0.005 gpm.
