Method for Improving the Evaluation Quality of the Second Cementing Interface Using Acoustic Time Difference Curve

Acoustic variable density logging uses the formation wave amplitude received by the 5 ft probe to evaluate the cement bonding quality at the second cementing interface, which is one of the few logging methods that can effectively evaluate the second cementing interface. However, due to factors such as lithology, pore structure and compaction of the formation, the arrival time and frequency of formation waves are uncertain, making it difficult for evaluation software to effectively identify and lock formation waves in both time and frequency domains. At the same time, the bonding condition of the first cementing interface directly affects the energy of sound waves entering the formation, which also affects the evaluation of the second cementing interface. Based on the acoustic time difference curve of open hole, a movable detection time window which can change with formation wave velocity in time domain is established to accurately lock formation wave. The first interface cementation correction coefficient is set to correct the amplitude of formation wave detected by the movable time window, and the influence of the first interface cementation on the amplitude of formation wave is eliminated. The practical application shows that this method can effectively improve the evaluation accuracy of the second cementing interface in most cases. However, when the time difference curve of acoustic wave is straight in some formations, regardless of the cementation of the first cementing surface, the formation wave spectrum characteristics should be used to determine whether it is a formation wave, then the evaluation of the second cementing interface should be carried out. This method enhances the application potential of acoustic variable density logging technology.