Coiled tubing (CT) is increasingly used in completion and workover applications, especially as a deployment method for acid and other corrosive materials. The occurrence of internal wall loss and external pitting has become more common, especially if proper tubing management is not followed. Not only is there internal wall loss associated from the use of corrosive materials, but also external pitting from severe downhole environments as well as surface environments. The recent industry down turn has reduced the utilization of coiled tubing strings. This sometime causes deterioration of the metal quality through external and/or internal corrosion, particularly when good maintenance procedures are not followed, and strings are not properly protected from the environment. This becomes costly, as strings are unable to be used to their full life potential or when a job failure occurs because of poor string management.
This paper presents case data demonstrating that the use of CT inspection and proper tubing management can avoid or reduce future job problems. The use of the described techniques and good pipe management will in turn reduce safety problems, and lost revenue due to pipe failures and shortened string life.
In the past, visual inspection would be made to the exterior of the CT, but the interior was never continuously inspected for wall loss or pitting until just recently. The internal wall loss in most cases was a function of CT models, where a value was entered for a percentage of wall loss due to acid or some other corrosive materials. This value was also applied if an abrasive material was used. If this value was too high, then the string may have been taken out of service and scrapped before the full life was reached. If the value was too low, underestimating internal metal loss, then this might have lead to a job failure. In either case, this would have been a negative economic outcome.
With proper tubing management (i.e., coiled tubing inspection and corrosion inhibition) the chances of onsite failure are reduced.
Most coiled tubing inspection units have a multi-function head that is placed over the OD of the coiled tubing, thus allowing for faster and more accurate detection of pipe flaws.
The inspection unit may use magnetic flux leakage, and eddy current as the detection of flaws. This inspection can also be referred to as a nondestructive inspection evaluation (NDIE).
The following information is a general overview of the methods by which a nondestructive inspection (NDI) is used to determine CT flaws. There is more information that details the use of NDI in refs. 1 through 3.
Magnetic Flux Leakage is utilized for the detection of wall thickness and transverse cracks. Detection of transverse cracks is accomplished by using rings of Hall probe sensors mounted in the inspection head around the coiled tubing surface. Disturbances in the magnetic field (e.g., corrosion pits, injector cuts, etc.) will cause flux leakage to occur which in turn produces an indication. A saturating magnetic field is applied to the CT. Wall variations are detected by measuring changes in the total magnetic flux density of the tubing. The strength of the field rises as the coiled tubing wall thickness decreases and falls as the CT wall thickness increases. The signals are averaged to produce a more reliable wall thickness signal. Wall thickness threshold is set on the unit to identify minor wall thickness variations.