套管中弯曲型Lamb波数值仿真和物理模拟

Numerical and physical simulations of flexural Lamb wave in cased wells

低密度水泥在低压易漏复杂井固井中的广泛应用使得声阻抗类测井方法难以准确、有效地评价固井质量。基于套管中传播的弯曲型Lamb波对套后介质的声学参数和胶结状况的敏感性,该文通过理论计算和实验测量研究了不同频率下弯曲型Lamb波衰减与水泥声学性质的关系,较低的工作频率会使得不同水泥阻抗下的衰减动态变化范围降低,且对水泥环第I界面的窜槽厚度也有很高的灵敏度;利用套管外胶结流体和固体时衰减的差异,还可较好地区分套管外声阻抗接近的流体和固体;但弯曲型Lamb波的同一个衰减值可与两个水泥声阻抗值相对应,因此需通过建立弯曲型Lamb波的衰减与水泥声阻抗的解释图版以提高低密度水泥固井质量评价的可信度。该文的计算分析结果对进一步应用弯曲型Lamb评价低密度水泥具有指导意义。

The wide use of lightweight cement in the oil and gas wells makes it difficult to accurately and effectively evaluate cement bond quality using the acoustic-impedance-based logging method(e.g.,pulse-echo ultrasonic technique).Lamb waves have been used for cement evaluation because the wave propagation in casing is sensitive to layered media acoustic parameters and cementation condition.This paper investigates the relationships among flexural mode attenuation,cement acoustic properties,and bonding conditions between casing and cement using theoretical modeling and analyses.The results show that the flexural mode attenuation is sensitive to the condition of cementation between casing and lightweight cement and,in the presence of free pipe condition,to channel fluid thickness between casing and cement.Using the attenuation difference between bonding fluid and bonding solid outside of casing,it is possible to distinguish fluid and solid even when the two situations have similar acoustic impedance values.However,using flexural mode attenuation alone is not sufficient because one attenuation value may correspond to two different cement acoustic impedance values.For this situation it is helpful to use the attenuation-acoustic impedance cross plot to improve reliability of the lightweight cement evaluation.The analysis results can be used to provide a guideline for lightweight cement bond quality evaluation.