Pipe corrosion is a natural phenomenon in the harsh environment of oil and gas production wells, costing billions of dollars each year. Monitoring such corrosion allows for timely mitigation to prevent leaks, environmental damage, or catastrophic failures.
Corrosion of pipes is inferred from measurement of internal diameter and wall thickness. Mechanical finger calipers provide a high-resolution image of the internal pipe surface. Ultrasonic tools measure both the internal diameter and thickness of a single pipe, providing the pipe is filled with a fluid. Production tubing is a barrier to caliper, ultrasonic, and flux-leakage electromagnetic tools, preventing the assessment of outer casings. Remote-Field Eddy-Current (RFEC) tools respond to the total metal thickness of all the combined casings. The measurement is valid in single and multiple casing strings and is largely insensitive to the kind of material filling the pipe.
This paper presents a new slim electromagnetic (EM) imaging tool enabling casing assessment without removing the production tubing. The tool excites eddy-currents in the tubulars using a powerful low-frequency solenoidal transmitter. Distribution and strength of the eddy-currents depend on the geometric properties of the casings, their EM properties, and the presence of defects. The tool operates in the RFEC regime using large transmitter-receiver spacings, in which the measurements directly determine the total metal thickness. Mandrel-mounted coils provide the average total metal thickness, and eighteen small pad sensors around the tool provide an image of local pipe wall metal loss.
The combination of low- and high-frequency images, together with the average total metal thickness, provides a quantitative, high-resolution assessment of pipe condition. Log examples show detection of casing joints, additional casings strings, and presence of corrosion damage in the low- and high-frequency images.
Corrosion is a relentless process with electrochemical, chemical or mechanical origins that is difficult to slow down: it is a 'silent killer' (Acuña 2010). In the oil and gas industry corrosion monitoring is used to assess the well condition as a basis for planning intervention and mitigation strategies. Among all the high-technology systems and equipment that are installed in oil and gas production wells, the corrosion of well casings causes the most problems. Apparently stable casing can suddenly develop a leak that may be the first evidence of a severely corroded section that left untreated may force abandonment of the well in the near future.
