Study on the dispersion and attenuation characteristics of quasi-flexural Lamb wave propagating circumferentially along the casing

To evaluate cementing quality in the absence of a liquid medium inside the casing, Electromagnetic Acoustic Transducers (EMATs) can be employed to generate quasi-Lamb waves that propagate circumferentially along the casing. The attenuation characteristics of these waves can be analyzed to assess the bonding quality at the casing-cement interface. This paper investigates the dispersion and attenuation characteristics of quasi-flexural Lamb waves propagating circumferentially through both theoretical and numerical simulation methods, comparing the results with those of flexural Lamb waves propagating axially. The findings indicate that when the casing is coupled with fluid, the phase velocity of the quasi-flexural Lamb wave propagating circumferentially is slightly lower than that of the flexural Lamb wave propagating axially, but its attenuation is significantly higher. In terms of wave structure, the primary displacement during propagation along the casing is radial in both cases. While the displacement distribution between the inner and outer walls is uniform during axial propagation, the displacement amplitude of the outer wall is higher than that of the inner wall during circumferential propagation, resulting in energy leaking outward. When the casing is coupled with lightweight cement, the dispersion curve of the flexural Lamb waves propagating axially exhibits discontinuities, whereas the dispersion curve of the quasi-flexural Lamb wave propagating circumferentially remains continuous. Additionally, as the cement acoustic impedance increases, the attenuation of both axially and circumferentially propagating flexural Lamb waves initially rises and then declines. However, the attenuation of the quasi-flexural Lamb wave propagating circumferentially is significantly higher, and the cement impedance corresponding to the maximum attenuation is notably higher for the circumferentially propagating quasi-flexural Lamb wave compared to the axially propagating flexural Lamb waves. Using finite element simulation software, the well logging responses of the quasi-flexural Lamb wave propagating circumferentially and axially along the casing were simulated. The extracted attenuation values are consistent with the theoretical calculations. The study lays a theoretical foundation for further research on cementing quality evaluation using quasi-flexural Lamb wave that propagates circumferentially along the casing.