Quantitative Evaluation of Multi-Annular Downhole Corrosion Using Time-Domain Electromagnetic Measurements

Abstract

Abstract A novel electromagnetic instrument is presented that uses transient or pulsed eddy current measurements to perform quantitative evaluation of downhole corrosion in four concentric tubulars individually, and to inspect a fifth tubular qualitatively. Case studies are presented that compare results from this instrument with industry-standard single-string evaluation tools such as multi-finger calipers and high-resolution magnetic flux leakage tools. The new instrument is based on transient or pulse eddy current technology and comprises three highly sensitive sensors that simultaneously achieve high-resolution of the inner barrier and high radial depth of investigation for up to five barriers. Each sensor induces coaxial rings of eddy currents in multiple concentric tubulars and measures a time-varying response from the outward-diffusing eddy currents. The full transient responses from multiple sensors are then interpreted to obtain individual tubular thickness profiles. Case studies are presented where the thickness profiles of outer barriers are obtained with the new instrument and are compared with high-resolution benchmark logs of multi-finger calipers and magnetic flux leakage tool. The benchmark logs were measured when the outer barrier was directly accessible because, either the inner barriers were not yet present, or the inner barriers were removed. These comparisons show that the new electromagnetic instrument is able to provide accurate individual tubular corrosion evaluation while logging through tubing. This ability is invaluable for proactive well integrity management because electrochemical corrosion, which is the primary corrosion mechanism in these wells, causes the outermost casing to fail first and then continues to penetrate inwards. Therefore, the new electromagnetic instrument enables early diagnosis of the outer tubulars to identify potential weak zones in the completion string while logging through tubing and eliminating the cost of pulling completions for this purpose. This paper describes the advantages and limitations of state-of-the-art multi-sensor pulsed eddy current measurements for individual barrier thicknesses of four or five strings. New case studies with high-resolution magnetic flux leakage tools and multi-finger calipers support these conclusions.