High cement bond quality is required to keep an oil well from hydraulic communication between zones.In the cement bond evaluation,the ultrasonic echo method is widely used for its capability of channeling azimuth detection.Full waveforms reflected from the cased hole are simulated for different bonding conditions by the generalized transfer matrix method.Because of the high acoustic impedance of casing,the amplitudes of the reflected waves from the cement and the formation are small and cannot easily be used to evaluate the cementing condition.The wave that can propagate into the cement and the formation through the casing concentrates closely on the casing resonant frequency.To reduce the amplitude of the reflected wave from the casing inner surface and highlight the part of the reflected wave which carries the cementing quality information,the reflected full wave is filtered according to the casing resonant frequency.There are several wave packets in the filtered waveform.When the amplitude of the second wave packet is low,the cement bonds well with the casing,otherwise poorly.A low amplitude third wave packet is an indication of a good bond of the cement with the formation,otherwise poor.To reveal the sensitivity of the reflection wave amplitudes to the incident angle,reflected full waveform is modeled when an acoustic beam with finite width is incident on the casing.It is shown that the bond evaluation method based on filtered wave packets is valid for incident angle less than 5 degrees.
