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.