Interface Decomposition and Stagewise Attenuation of Multilayer Ultrasonic Propagation in a Dual-String Wellbore

In dual-string wells with retained tubing, ultrasound must pass successively through the wellbore fluid, tubing, annular fluid, casing, cement sheath, and formation. Repeated fluid–solid transitions make signals from the outer layers difficult to separate directly from the final received waveform. This study combined 300 kHz ultrasonic through-transmission experiments with COMSOL simulations, progressing from individual media and fluid–solid combinations to an ideal intact dual-string wellbore. The principal wave packets in the experiments corresponded well to those in the respective models. For the water–steel combination, the characteristic peaks differed in time by 0.66 μs, and the local waveform correlation coefficient was 0.986. A two-dimensional axisymmetric model was then used to compare the envelope peaks of consistently identified early composite responses and cumulative outward work over 0–120 μs at successive radial sections. The responses near the tubing and casing did not decrease monotonically. Local amplitudes could increase even as the wave response continuing outward weakened, and the sections adjacent to the two steel strings exhibited distinct stagewise changes. Interface decomposition relates response changes along the complete propagation path to individual material layers, providing a basis for explaining the formation of weak outer-layer signals in dual-string wells.