套管井弯曲型Lamb波衰减测量的数值仿真和试验

Numerical simulation and experimental research on flexural Lamb-wave attenuation measurement in a cased well

利用数值仿真和试验测量相结合,分析换能器指向性以及弯曲型Lamb波在套管中传播时的扩散特征,给出换能器在距离套管内壁不同位置时测量的衰减值的变化规律。结果表明:随着换能器距离套管内壁越近(仪器直径变大),测量的衰减值越大,但此衰减值的变化与水泥胶结质量无关;在测井仪器偏心时,声系在朝向井周不同方位上测量时其换能器与套管内壁距离也发生改变,且辐射声束传播到套管内壁时入射角也会发生变化,测量的衰减值随方位近似呈现余弦曲线式的变化规律;在声系和套管厚度均保持不变,随着套管外径在一定范围内逐渐增大时,测量的衰减值先稍有增大后又逐渐降低。

By analyzing the radiation spreading of Lamb wave and radiation-receiver directivity of the transducers,the trend of flexural Lamb-wave attenuation with instrument diameter increasing is studied.Both numerical simulations and experimental measurements show that the closer the ultrasonic pitch-catch system is to the inner wall of the casing,the greater the measured attenuation value is.The reason is that the flexural Lamb wave radiation spreads as it propagates in the arcuate casing and the covered sound-field sector becomes wider.Therefore,the correction for radiation spreading should be done with the measured attenuation applied to evaluate the cementing quality.When the logging instrument is eccentric,both the distance from the transducer to the inner wall of the casing and the incident angle vary.The measured attenuation values show a trend of cosine-like curve with the tool rotating around casing circumference direction.When the logging tool diameter and casing thickness remain unchanged,the measured attenuation value increases slightly at the beginning and then decreases gradually with gradually increasing casing outer diameter.