Modern logging tools for the investigation of the condition of downhole tubulars provide multiple outputs derived from an array of sensors. These sensors provide simultaneous readings and are arranged so a single set of measured data is derived from a cross-section of the casing or production tubing.Data sets are rapidly updated as the tools are run in the well providing a detailed series of depth related cross-sections.
In addition to producing conventional logs in real-time, this data can be used to generate 3D images. Until recently the images produced from array tools were limited to internal dimensions. New thickness measuring tools, together with improved software, provide thickness data which can be displayed either as an image based solely on thickness or combined with internal measurements to provide 3D pictures that are analogues of the actual metal remaining within the well.These images can be used to assess both internal and external corrosion and/ or damage.
This paper describes in outline the tools used to produce the data. It employs examples from both the testing phase and initial operational deployment of the new tools to illustrate the benefits of the image approach to displaying data.
Failure of well casing or tubing due to corrosion, or to a lesser extent erosion, has long been recognised as causing production problems and even leading to well abandonment. While areas of damage that result in unwanted fluid movement can be identified using Production Logging or other non-dedicated logging methods, the results are qualitative. Such data can be useful, but it is only with quantitative measurements that precise remedial work can be designed.
Multi-fingered calipers designed to measure changes to tubulars within a well were introduced over fifty years ago as purely mechanical devices. Within the last ten years electronic versions of these tools have become available giving the ability to produce logs with greater resolution in real-time or via memory recording (Maxted 1995). The digital files available from these tools enable the rapid processing of the data to provide either statistical reports or 3D images of the interior of the well. However, accurate images are only possible when a sufficient array of depth related data is available.
There have also been corresponding developments with tools that provide similar data using different measurement techniques such as ultrasonic scanning tools.
However, while the images discussed above do provide extremely useful information, it is only part of the story. External defects are not detected by calipers unless they are of such a magnitude as to cause some change on the inner surface of the well. In other words it is quite possible that external corrosion or erosion that will lead to an integrity failure would go undetected and only be "seen" when an actual hole appears. Therefore, in order to produce an image that represents both the internal and external condition of a well it is necessary to integrate data from a second tool.
This second set of data must also be derived from an array which provides sufficient points around the tubular circumference that integration is achievable with confidence. It is essential that the two sets of data can be matched both with respect to depth and sensor orientation. It is not necessary for the two tool data sets to be obtained on the same run in the well, albeit this is often more convenient.