Transient electromagnetic (TEM) or pulsed eddy current (PEC) tools offer salient advantages over conventional continuous-wave remote field eddy current (RFEC) tools for the inspection of multi-barrier well systems. In this paper, we describe a state of the art transient electromagnetic tool that is capable of inspecting up to five concentric barriers. We use this tool to illustrate the working physics of this technology and delineate its features and advantages.
The measurements result from a diffusion process, and therefore consist of time-decay curves. These curves exhibit space-time mapping, meaning that the signal at early time is sensitive to proximal barriers while the signal at late time is sensitive to proximal as well as distal barriers. This behavior enables the measurements to independently yield the wall thicknesses and/or metal loss/gain of multiple barriers. This technology yields extremely rich measurements because it uses wide-band excitation. Through diverse modeled examples and field examples, we demonstrate techniques to maximize the information gleaned from these measurements. These techniques can complement automated inversion of measured data and are amenable to the use of machine learning and artificial intelligence methods.
Examples and features discussed include pipe collars, discrimination of internal (inner tubular wall) anomalies vs. external (outer tubular wall), tubular eccentricity and buckling or ovality. Key peculiarities often observed in multi-barrier transient electromagnetic measurements are also discussed.
Well integrity problems place a huge economic burden on oil and gas producers. Corrosion of pipes is a major cause of well integrity problems. There are multiple mechanisms for corrosion, such as electrochemical, chemical, and mechanical. There are several techniques available to mitigate each corrosion mechanism, but it has been impossible to eliminate corrosion altogether. Monitoring of casings and early detection of corrosion and other flaws allow operators to perform timely and economical intervention and/or workover. There is a large spectrum of casing inspection tools that follow diverse measurement principles. Multi-finger calipers can provide a profile of the inner surface of the tubing at good resolution while ultrasonic tools can evaluate both the inner and outer surfaces of the tubing. Magnetic flux leakage tools evaluate the inner and outer surfaces of the first barrier at a very high resolution. To date, only electromagnetic tools are able to evaluate multiple barriers. Being able to independently inspect multiple barriers enables operators to assess the condition of outer barriers without having to cut and pull tubing and/or other inner barriers. This translates to cost savings over the life of the well. Being able to do this offline without a rig translates to cost savings during the plugging and abandonment stage. In this paper, we briefly describe a state of the art transient electromagnetic (TEM) tool that provides quantitative evaluation of up to four barriers, and qualitative assessment of a fifth barrier. The tool comprises three sensors, called the short sensor (SS), medium sensor (MS) and long sensor (LS), respectively. The short sensor provides higher resolution data from the inner barrier, whereas the medium and long sensors provide large depth of investigation to assess the outer barriers. The bulk of this paper focuses on the analysis and interpretation of measurements provided by such an instrument.