Numerical study of dispersion characteristics of dipole flexural waves in a cased hole with different cement conditions.

Crossed-dipole acoustic logging technology is mainly used to measure shear wave and determine the anisotropy or fractures of formation in the field of acoustic logging. Based on the propagation characteristics of dipole flexural waves in cased holes, and the wavefield of fundamental flexural mode at three bonding conditions: good cement bonding, poorly bonded interface I, and poorly bonded interface II. The dispersion curves of flexural waves are calculated, and the effects of cement parameters (elastic parameters and geometric dimensions) on dispersion curves are investigated. The RAI method is used to calculate the dipole full waveforms in a cased hole with conventional or ultralight cement surrounded by a soft formation. Then, the weighted spectral semblance is used to conduct the dispersion analysis of the full waves. The numerical results show that the cutoff frequency of flexural waves exceeds the frequency band of the current dipole logging tools in the well-bonded cased hole with conventional cement in soft formations. However, when ultralight cement is used, the cutoff frequency shifts toward low values, and the ultralight cement bonding is conducive to measuring flexural waves in soft formations. When interface I is poorly bonded, the cutoff frequency shifts toward low values, and the change is evident in soft formations. The cutoff frequency in the case of ultralight cement bonding is more evident than that of conventional cement bonding. If only interface II is poorly bonded, regardless of the cement used, the cutoff frequencies are all below 2 kHz. The dipole full-wave analysis of different bonding conditions further illustrates that the dispersion characteristics of flexural waves are sensitive to the cement bonding quality of cased holes. [ABSTRACT FROM AUTHOR]