The Research On Response Characteristics Of Multipole Array Acoustic Logging In Cased Borehole

Cementing is an important part of petroleum exploration and development,and the cementing quality seriously affects the safe production and overall benefits of oil and gas exploitation.Accurate evaluation of cementing quality has important guidance and practical significance for adjusting and reforming the existing oilfields.At present,acoustic logging technology is still the main method for evaluating the cementing quality,among which the existing problems and application potential are mainly shown in the following three points.Firstly,the method of CBL/VDL can not effectively evaluate the cementing condition of the second interface when the first interface of cement ring is poorly cemented.Secondly,with the increasing application of multipole array acoustic logging in cased boreholes,it is worth studying to evaluate the bond quality of each interface of cement ring by using the velocity of casing wave or the dispersion characteristics of borehole mode wave.Finally,with the extensive application of special cement systems in cementing in recent years,the logging response of special cement systems is not fully understood.To solve above problems,based on the basic theory of acoustic field excited by multipole acoustic source in cased well,the acoustic field characteristics of monopole and dipole source in cased well with different cementing conditions are studied by analytical method.The characteristics of dispersion,attenuation and excitation intensity of borehole mode wave are also analyzed.And then,this paper focuses on the variation of borehole mode wave with fluid ring thickness,and further discusses the response characteristics of multipole acoustic logging in cased well with special cement cementing.The theoretical calculation results show that,for monopole logging,the velocity of casing wave is significantly slower when the second interface is poorly cemented,compared with the poor cementation at the first interface of cement ring,indicating that the cementing condition of the second interface can be evaluated according to the velocity of casing wave.For dipole logging,the casing flexural wave will be measured in the casing well with poor cementation,and the waveform and dispersion characteristics of formation and casing flexural waves are related to the cementing condition of the two interfaces of the cement ring,especially the phase velocity of casing flexural wave is very sensitive to the thickness of the fluid ring at the cement interface.The thickness of the fluid ring at the cement interface can be estimated by comparing the dispersion data extracted from the dipole waveform measured in the field with the dispersion curves calculated by the theoretical model.In addition,when low-density cement is used for cementing,it is more conducive to extract formation wave information from dipole acoustic logging data in cased wells,which is particularly important in the soft formation.The results of this paper,therefore,lay a theoretical foundation for the evaluation of cementing quality by using the borehole mode wave of multipole acoustic logging in cased wells,and open up a new idea for evaluating cementing quality.