基于黏弹滑移界面理论揭示套管中模式波的传播特征

目前固井质量评价主要是利用套管中模式波的幅度或衰减变化判断套管与水泥间的胶结状况.在超声测量频段（70~500 kHz），现有声波测井仪器工作时在套管中激发的模式波类似薄板中的对称兰姆波（拉伸波）或反对称兰姆波（弯曲型兰姆波）.本文将黏弹滑移界面理论应用到套管和水泥的耦合界面，探讨了套管中的准兰姆波（包括拉伸波和弯曲型兰姆波）与套管后物质的耦合方式，拉伸波与套后物质主要以剪切刚度耦合，即便是0.01 mm的微环也会使得拉伸波的衰减明显降低，接近套后是流体时的响应特征；而弯曲型兰姆波与套管后物质主要以法向刚度耦合，这使得在套后胶结轻质水泥时弯曲型兰姆波对微环不敏感，在套后耦合常规水泥时微环的存在又会使得弯曲型兰姆波的衰减明显增强，衰减的增加说明其与套后物质的声耦合更紧密，也表明微环与常规水泥的等效声阻抗适当降低，更有利于弯曲型兰姆波传播时与套后介质实现法向刚度的匹配.拉伸波和弯曲型兰姆波对同一胶结状况的不同响应特征也为固井质量综合评价时区分不同的水泥胶结状况提供了可能.

Study on the propagation characteristics of the casing waves in cased wells by viscoelastic slip theory

The amplitude or attenuation of casing waves is used to evaluate cement bonding quality in cased wells. At the 70~500 kHz frequency range, the casing waves generated by the acoustic tools are similar to the fundamental symmetric Lamb waves or anti-symmetric Lamb waves in acoustic terminology. The fundamental symmetric Lamb waves are called extensional waves and the fundamental anti-symmetric Lamb waves are called flexural Lamb waves. The two kinds of quasi-Lamb models show different propagation characteristics. The viscoelastic slip theory is applied to reveal the motion characteristics of extensional waves and flexural Lamb waves. The extensional wave propagating in the casing couples with the medium behind casing by shear stiffness. The existence of 0.01 mm micro annulus can cause sharp decrease of the extensional wave attenuation. By contrary, flexural Lamb-wave attenuation is not sensitive to micro annulus with lightweight cement behind the casing. The response characteristic of the flexural Lamb-wave attenuation with heavy cement behind casing is different from that with lightweight cement behind casing. The attenuation of the flexural waves shows evident increase with micro annulus existence. So the flexural Lamb-waves couple with the medium behind casing mainly by normal stiffness. With fluid micro annulus between casing and cement, the equivalent impedance of the media behind casing is less than that of heavy cement. The lower impedance can be prone to normal deformation and acoustic coupling between casing and the medium behind casing. The different response characteristics of extensional waves and flexural Lamb waves in cased wells provide possibility to determine the cementing situation in the comprehensive evaluation of cement bonding quality.