Simulation Analysis of Influencing Factors on Ultrasonic Lamb Wave Scanning Imaging Logging

To address the limitations of traditional cementing quality detection and evaluation methods, such as acoustic cement bond log/variable density logging (CBL/VDL) and segmented cement bonding tool (SBT) logging, which have low sensitivity to low-density cement, suffer from significant influence by micro annulus, lack of azimuthal identification capability and casing damage detect capability. By solving the reflection coefficients of multi-layer media and the excitation acoustic field, the excitation conditions of ultrasonic Lamb waves were analyzed. The basic structure and key parameters of the ultrasonic Lamb wave scanning imaging logging instrument were designed. The influence of factors such as bond cement, wellbore fluid, and casing specifications on the flexural Lamb waves, as well as the influence of post-casing medium and micro annulus on the resonance waves, were simulated and analyzed. The analysis reveals that in slow-setting cement, flexural Lamb waves exhibit greater attenuation, and different micro annulus thicknesses have a significant impact on their amplitude values. Furthermore, the attenuation coefficient of flexural Lamb waves increases with the density of the drilling fluid, and as the casing gets thinner, the attenuation coefficient of the flexural Lamb waves gets higher. The acoustic impedance of the post-casing medium plays a crucial role in the amplitude of resonance waves and resonance efficiency, although the identification capability of resonance waves for micro annulus is relatively low. The research results provide theoretical guidance for the development and data processing of the flexural ultrasonic Lamb wave scanning imaging logging instrument and help improve the evaluation accuracy of cementing quality by low-density cement.