<p style="margin: 0cm 0cm 0pt; line-height: 150%">For cement bond evaluation, the conventional working method is to excite sound waves in the mud and radiate into the casing, and then induces the casing wave in the casing. But this method is not efficient in heavy mud or gas wells. This paper introduced the electromagnetic acoustic transducer (EMAT) into cased wells. EMAT can directly induce ultrasonic waves in the casing. The EMAT comprises the racetrack coil, periodic permanent magnet and the tested specimen (the casing). Periodic permanent magnet provide a bias magnetic field, the coil is used to produce a pulsed magnetic field and can induce pulsed eddy current at the inner wall of the casing. Lorentz force can be generated under the multi-field coupling between the static magnetic field, pulsed eddy current field and the mechanical filed. Based on Lorentz force mechanism finite element method, the simulation of EMAT is implemented. The simulation results show that the induced eddy current mainly exists within the inner wall of the casing due to the skin effect; Lorentz force occurs also within the inner wall of the casing; the amplitude values of the Lorentz force decay with increasing liftoff values of the coil. With optimized periodic permanent magnet and racetrack coil structure the quasi-SH0 or quasi-SH1 waves polarized axially and propagating circumferentially is excited in the casing. Refer to the simulation EMAT model, the EMAT is made in the Laboratory. The experiment results verify the simulation results. Both simulation and experiment results show that the dispersion of the quasi-SH0 mode is weak, and the waveform is almost unchanged when propagating circumferentially in the free casing; the quasi-SH1 mode shows strong dispersion, which makes it last for a long time in the time domain. The extracted dispersion curves from the measured array waveforms agrees with that calculated by analytic method.</p>

Finite Element Simulation and Experiment Study on Exciting Circumferential Quasi-Sh Waves  in a Casing

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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