Pulsed eddy current screening of corrosions in the in-service tubing based on gradient magnetic field sensing

As the inner most pipe of the oil-and-gas well shaft, the tubing is used to directly extract either oil or gas. The in-service tubing is subject to the sulfurous substances which lead to the corrosion occurring on its internal surface. This undermines the integrity of the well shaft. In this paper a pulsed eddy current probe was proposed for high-sensitivity imaging of internal-surface corrosion of the in-service tubing, based on gradient magnetic field sensing alongside the differential mechanism of dual magnetic sensors. Following the determination of the probe structure, two signal features, i.e., the gradient of the logarithm curve of the testing signal and peak value of normalized differential signal were intensively investigated through experiments. The correlation of each signal feature with the corrosion sizing parameters was established. And the corrosion imaging method was explored based on each signal feature. A signal-feature fusion approach was proposed. Based on the fusion of the two signal features, the visualization method for tubing corrosion was exploited in conjunction with the fused feature. The results revealed that compared with the traditional pulsed eddy current testing, the fused feature-based imaging method was advantageous in terms of higher signal-to-noise ratio (SNR) of produced corrosion images. This further indicated that the proposed probe together with the signal-feature fusion approach was beneficial to the visualization of internal-surface corrosion in the in-service tubing with high sensitivity and SNR.