A Fundamental Research On The Application Of Flexural Lamb Waves In Primary Cement Evaluation

Acoustic technology is one of primary means of casing-cement cementation evaluation and the ultrasonic technology can achieve fine imaging near the borehole and do casing-evaluation integrally.To meet the need of reservoir protection and mud leak-proof,the low-density and lightweight cement(collectively known as low-impedance cement)have been used widely,however,the ultrasonic technology meets challenge to get a deeper investigation in such high acoustic impedance contrast behind casing and the flaw in a horizontally-layered system is insensitive to ultrasonic compressional wave with vertical incidence.The attenuation of the lowest order antisymmetric mode of leaky Lamb waves(hereinafter referred to as flexural Lamb wave)propagating along casing which is stimulated by transducers with inclined angle is sensitive to the acoustic impedance of media behind the casing as well as the bond condition between cement and casing(the first interface).The sensitivity of flexural Lamb wave to the bond condition between cement and formation(the second interface)is low but also matters.As mentioned above,flexural Lamb wave have been used for cement evaluation because it is sensitive to the acoustic parameters of layered media(especially the low-impedance cement)and cementation condition.This paper investigates the relationships among flexural mode dispersion and attenuation,cement acoustic properties,and bonding conditions between casing and cement using multi-layered media model,and then simulates the full waveforms launched and received by finite and directional emitting and receiving transducers as well as the time snapshots.The result shows that the flexural Lamb wave is mainly excited in casing,when the angle of incidence approximately equals 33 degree;the flexural mode dispersion curve is broken into two branches by a breakpoint when slow cement exists behind casing;the flexural mode attenuation maximizes when slow cement exists behind casing and it is sensitive to channel fluid thickness between casing and cement.Simulated time snapshots present that the flexural Lamb wave propagates inside the steel casing leaking bulk waves whose velocities are lower than the flexural mode's velocity in the upper water layer and the water or cement annulus below the casing at the same time.For the slow cement,whose compressional velocity and shear velocity are both lower than that of flexural mode,existing behind the casing,the attenuation is significantly large.By taking advantage of the characteristics of flexural Lamb wave mentioned above,it would be much easy and effective to distinguish the type of layered media behind casing(especially solid and fluid with similar impedance),bonding conditions between casing and cement and channel fluid thickness.The effect evaluation of low-impedance cement cementation could be improved correspondingly.This paper provide the theoretical basis for developing measuring instrument of flexural Lamb wave.