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 formation shear wave and determine formation anisotropy or fractures in the field of acoustic logging.In recent years,although there are a lot of studies on dipole acoustic logging,the propagation characteristics of dipole flexural waves in cased holes under different cement conditions are still not fully understood.Previous studies have found that the cut-off frequency of flexural wave dispersion curve will move to a very high frequency band in cased holes in soft formations,even far beyond the excitation frequency band of the conventional dipole source,which makes it difficult for the conventional dipole source to perform the flexural wave logging in cased hole embedded in soft formation.In order to solve the above problems,inspired by the application of low-density cement to low-pressure leaky formations,this paper studied the excitation and propagation characteristics of dipole flexural waves in cased hole under different bonding conditions.In this paper,the multipole acoustic field was studied theoretically,then,the numerical simulation analysis of the dipole flexural wave in cased hole under three kinds of bonded conditions were performed,mainly as follows: the effects of formation characteristic shear waves and cement parameters on dispersion curves of flexural waves 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,and the weighted spectral semblance is used to conduct the dispersion analysis for the full waves.The numerical results show that the cut-off frequency of flexural wave would exceed the frequency band of the current dipole logging tools in the well-bonded cased hole with conventional cement in soft formation.However,when ultralight cement is used,the cut-off frequency will shift towards low frequency,the ultralight cement bonding is conducive to measure flexural wave in soft formation.When the interface I is poorly bonded,the cut-off frequency would shift towards low frequency,the change is especially obvious in soft formation,and the cut-off frequency of the flexural wave is lower when the ultralight cement is cemented.If only the interface II is poorly bonded,whatever the cement,the cut-off frequency is always below 2 k Hz.The dipole full waves analysis in different bonding conditions further illustrates that the dispersion characteristics of flexural waves are helpful to determine cement bonding conditions of cased holes.