A Theoretical And Experimental Study Of Casing Flexural Lamb Waves

Acoustic technology is one of primary means of casing-cement evaluation and the ultrasonic technology can achieve fine imaging for 360 degree near the borehole and do casing-evaluation. To meet the need of reservoir protection and mud leak-proof, the low-density and lightweight cement have been used widely, however, the ultrasonic technology meets challenge to get a deeper investigation in such high acoustic impedance contrast behind casing. Since the flaw in a horizontally-layered system is insensitive to ultrasonic compressional wave with vertical incidence, the attenuation of flexural Lamb wave propagating along casing is sensitive to the bond condition among cement and casing and the acoustic impedance behind casing.. Based on the above theory and knowledge, in this paper the propagation characterization of leaky-Lamb wave has been analyzed through the theoretical and experimental means which lays the foundation for low impedance cement evaluation and the annular channelling identification in cased-borehole.In this dissertation, firstly, according to the ultrasonic propagation characteristics in the cased well, we have simplified the cylindrically-layered casing system as a horizontally-layered system. The dispersion and attenuation of the wave modes existed in the thin plate on the logging frequency, and the propagation characteristics comparison between the flexural Lamb wave(A0) and extensional Lamb wave(S0) under different cement bond conditions have been studied. The simulation result show that the flexural Lamb wave has more advantages on lightweight cement bond evaluation. Furtherly, We studied the attenuation characteristics of the flexural Lamb wave under different cement bond condition and different cement acoustic impedance. Directional radiation of the source and directional reception of the receiver have been simulated by the Gaussian-beam theory and the simula-tion results indicate the full waveform is the result of specular reflection phase-matching with the leaky Lamb wave. The impact of several factors on the waveform, such as the transducer inclination angle, the standoff between the transducer and the inner wall of the casing, the acoustic impedance of the cement, the thickness of the casing and cementing condition have been studied and the proper spacing to receive flexural Lamb wave has been found which provide the theoretical basis for developing measuring instrument of flexural Lamb wave.A laboratory experiment has been conducted along with the theoretical modeling and verifies the excitation of Lamb wave modes. We have investigated the influence of specular reflection on the waveform by conducting the flap experiment. The characteristic of waveform amplitude as spacing varying and the characteristic of flexural Lamb wave attenuation as acoustic impedance behind casing varying are in good agreement with modeling results. The research in this paper will lay the theoretical and experimental foundation for acoustic instrument design based on flexural Lamb wave for lightweight cement evaluation.