Quantitative Multi-Barrier Evaluation Through Dual Completion Using Time-Domain Electromagnetic Measurements

Abstract

An innovative electromagnetic tool is presented which utilizes transient or pulsed eddy current measurements to provide individual quantitative evaluation of downhole corrosion in up to four concentric pipes. Case studies from dual string completions are presented that compare the results from passes logged through short string (SS) and long string (LS) tubing. The novel instrument utilizes transient electromagnetics or pulse eddy current technology and contains three sensors that achieve a trade-off between high-resolution measurements of the inner barrier and high radial depth of investigation for up to five barriers. Each sensor induces coaxial outward-diffusing rings of eddy currents in multiple concentric tubulars and measures the consequent time-varying response. 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 tool. Logging was carried out in two passes; one pass was logged through SS tubing and the second pass was recorded through LS tubing. The comparisons show that the new electromagnetic instrument provides accurate corrosion evaluation while logging through tubing, even in the presence of a second tubing string. This feature of the electromagnetic tool is important for proactive well integrity management because electrochemical external 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 tool allows early identification of the outer tubulars with potential weak zones in the completion string without the need or 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 evaluation of up to five concentric strings even in the presence of dual tubing strings. The presented case studies in dual completions support these conclusions, with additional evidence from other diagnostic tools.