Research on Wall Thickness Inversion Method for Multilayer String Based on Pulsed Eddy Current

When conducting integrity assessments of metal well-bore casings in oil and gas wells using pulsed eddy current testing (PECT), the quantitative inversion of multi-layer wall thickness becomes challenging due to the complex effects of electromagnetic field propagation and eddy current coupling. We investigated the time-domain analytical solution of multi-layered sting for PECT and propose a time-window-integral (TWI) inversion method based on Tikhonov regular optimization. The correlation between numerical integration of time window and wall thickness reduction is analyzed according to eddy current response characteristics in time domain. Furthermore, the mathematical equation is established and corrected by regularization method. Finally, the simulation model of three-layer string is tested. The experimental results show that the average wall thickness error of each layer obtained by TWI inversion is 0.032mm, 0.049mm and 0.253mm respectively, and the average relative error is less than 5.5%, which can quickly obtain accurate wall thickness information.