The Magnetic Imaging Defectoscope (MID) is a tool designed to record the time response to high-energy electromagnetic pulses. Metal presence around the tool is evaluated by matching measured and numerically modelled magnetisation decays. The air response decays faster than if the tool is placed inside a metal pipe. If multiple pipes are present around the tool, the near pipe responds at earlier times and the more distant ones at later times. This phenomenon is key to detecting metal presence in each individual pipe. The MID tool features two sensors, short and long, to ensure reliable detection of responses from a wide range of multi-barrier completions and differentiate between defects in each barrier independently.
Despite this simple concept, the response detected by the tool has a complex time behaviour and cannot be modelled in a simple analytical way, which put time-domain technologies on hold for many years. The recent advances in computer speed and multicore parallel computing enable accurate numerical modelling of complex responses and determining metal presence in two metal barriers separately. In addition to thickness profiles for each barrier, the MID tool generates differential (DELTA) data panels, based on the difference between numerical models and actual responses, that visualise electromagnetic signatures of metal structures and can be used to locate and recognise various completion components, such as packers or SSD.
The paper describes the basics of the time-domain magnetic-pulse technology, specifications of the tool itself and laboratory test data. This technology has been tested in a dual-string producer of the Raudhatain field operated by the Kuwait Oil Company. The long and short strings were suspected to be communicating due to identical water cut trends. The MID tool was run once in memory mode on slickline through the long string and detected a 1.5-inch corrosion hole between packers that created communication with the short string. This communication flow left a footprint in a shut-in temperature log right across the corrosion hole. Another corrosion hole was located in the casing below the tubing shoe, immediately above the perforations. During a subsequent workover, the tubing was retrieved and a 1.5-inch corrosion hole was located in the exact place where it was identified by the MID tool.