Lightweight cements is widely used in oil and gas fields cementing,which results in traditional logging methods can not evaluate cement bond quality effectively.This paper is based on the characteristics of flexural Lamb waves which are sensitive to cement bond quality evaluation.Firstly,a suitable layered medium model is established for casing well,the plane wave reflection coefficient of layered medium is calculated,and the conditions of the flexural Lamb wave excitation in the casing well are discussed in terms of reflection coefficient.Then calculate the waveform signal of leaky flexural Lamb according to the conditions combined with angular spectrum theory when the steady-state finite width pulse beam incident in casing well,also analyse the relationship between waveform signal characteristics and the cement bond quality.The results show that,the incident acoustic wave can excite the flexural Lamb waves in the casing under certain conditions,and it's attenuation rate correlated both with the cement bond quality and acoustic parameters.When it comes to common or lightweight cement,the attenuation rate decreases as the increase of the water layer thickness between casing and cement,when it comes to fast cement,the attenuation rate increases as the decrease of the water layer thickness,while attenuation rate shifts to decrease as the water layer disappears which means the casing bonded with fast cement well.%轻质水泥在油气田固井中的广泛应用使得传统声阻抗测井方式难以准确地对固井水泥胶结质量作出评价.本文基于弯曲型Lamb波传播特征对层状介质胶结质量较为敏感的特点,首先对套管井建立合适的层状介质模型,计算了层状介质的平面波反射系数,从反射系数的角度探讨了在套管井内激发弯曲型Lamb波的条件.然后依据该条件并结合声波角谱理论计算了稳态有限宽脉冲束入射时在套管井内产生的弯曲型Lamb波泄漏波的时间波列信号.通过对泄漏波波列信号的分析,得到了弯曲型Lamb波与套管井水泥胶结质量间的关系.研究表明,入射声波在满足一定条件下可以在套管内激发弯曲型Lamb波,其传播时的衰减率与套管水泥胶结质量以及水泥的声学参数均相关,当水泥为轻质水泥或者普通水泥时,其衰减率随着套管水泥间水层厚度的增加而减小,当水泥为快水泥时,其衰减率随着水层厚度的减小而增加,但当水层厚度减为零即套管水泥胶结良好情况下,其衰减率转而变小.
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