Novel Through-Tubing Casing Measurement With Azimuthal Sensitivity for Game-Changing Proactive Multi-Casing Corrosion Measurement

Corrosion monitoring of multi-casing systems is an integral part of well integrity management because it can provide timely information to operators for well intervention and workovers. Conventional multi-casing corrosion measurements investigate more than three concentric tubulars but only yield average non-azimuthal wall thickness measurements of each tubular. These lead to non-unique corrosion interpretation whereby a catastrophic failure in a small azimuthal region of the tubular may be indistinguishable from a minor metal loss that is circumferentially spread out. Hence, conventional multi-casing corrosion measurements can sometimes be limited to being used as a precursor to pulling the tubing and evaluating the casing again with single barrier evaluation services. A new transient electromagnetic sensor is presented that can provide azimuthal measurements of the casing through-tubing and can thereby help provide a better definition of conventional multi-casing corrosion measurements. The new sensor follows the pulsed-eddy current diffusion principle by inducing time-decaying eddy currents in multiple concentric tubulars and measuring corresponding time-decaying voltages generated by outward-diffusing eddy currents. The sensor design evolved through multiple iterations involving modeling, simulation, rapid prototyping, and laboratory testing. In some instances, laboratory testing results were used to validate modeling results. In other instances, modeling was used as a tool to understand and explain interesting or unexpected behaviors and results observed in laboratory testing. Many tests were also performed to decouple two or more factors affecting the measurement. Basic interpretation workflows were developed as a means of quality control of test data, as well as to visualize and evaluate casing features measured through tubing. The sensor comprises multiple coils oriented and operated differently from conventional multi-casing instruments. Simulation studies and experimental results are presented to delineate the sensor performance in terms of sensitivity to channel flaws, vertical resolution, depth of investigation, and azimuthal resolution. These measurements can also complement through tubing evaluation of cement behind casing in some cases by providing an independent assessment of metal features, while other measurements might provide sensitivity to both cement features and casing features. The novel sensor design has the potential to plug a critical gap in conventional downhole well integrity measurements during intervention or plug and abandonment, which in the present day only provide non-azimuthal multi-casing corrosion, thickness, and metal loss measurements.