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Simulation of characteristic waves in acoustic full waveform relating to cement bond

Cement channeling is usually generated on the first and/or secondary interfaces which lie on between the cement and casing, and the cement and formation, respectively. It is an inherently complicated problem to evaluate the cement bond of the secondary interface (SICB) in a cased borehole. So we need to find a useful wave for evaluating SICB and provide its propagation features. To achieve this, we simulated the acoustic full waveforms and dispersion curves in cased boreholes using the real axis integration method, and a propagator matrix. The results indicated that “casing-cement” wave can be utilized to evaluate the SICB, as it is most sensitive to water channeling on the secondary interface. The velocity of this wave is in between that of the casing wave and the formation wave, so that it is easy to extract it in time. The dispersion curves showed that the casing-cement wave is a dispersive wave too, but not a pseudo-Rayleigh wave. Simulation results further indicated that the amplitude of casing-cement wave is decreasing with increasing cement sheath thickness, but not water channeling thickness, and its travel time is influenced by casing and cement together. Moreover, we investigated factors influencing the casing-cement wave and found that casing diameter and peak frequency are major factors influencing the amplitude and the travel time. But none of these factors greatly influences the bond index which is computed from the amplitude. So the bond index has an advantage over the amplitude during evaluating SICB. Experimental data in calibration pits and field data of detection of channeling supported the simulation results.