| In recent years,with the increase of deepwater and deep drilling and the increase of horizontal and highly deviated Wells,LWD(Logging-while-drilling)technology is urgently needed.Compared with conventional wireline logging technology,LWD has natural advantages such as low logging cost and geosteering.However,in multipole acoustic logging while drilling,the introduction of drill collars will make the sound field complicated.The collar wave propagating along the instrument will greatly interfere with the formation wave signal,thus attenuating the formation signal acquisition.For the practical problem of severe coupling between drill collar wave and formation wave,this thesis from the point view of the elastic wave propagation theory carries out in-depth and meticulous research on the well for isotropic formation model,the anisotropic formation model(VTI)and casing well(isotropic formation)model with the method of partial wave calculation.Then the propagation rules of guided waves and body waves in the sound field while drilling are clarified.This thesis is divided into the following three parts:In the first part,the theoretical analysis model of multipole acoustic logging while drilling in isotropic formation is established.The Riemannian leaf selection of the integrand in the complex wavenumber plane is determined based on the radiation condition of the sound field.After the singularization of the sound field function is completed,the poles of the integrand are determined by the two-dimensional search method,and the contributions of the branch points and the poles to the sound field are obtained by the vertical secant integral and the residue theorem respectively.Compared with the wireline logging results,it is found that the dispersion of pseudo-Rayleigh wave and second order stratified quadrupole wave in monopole and quadrupole while-drilling are greatly reduced,which is beneficial to the measurement of shear wave velocity of the formation.Further study find that the energy of collar wave is not uniformly distributed in the drill collar.With the increase of the dominant frequency(5kHz～30kHz),the radial position corresponding to the excitation peak of collar wave moves from the inner wall of drill collar to the outer wall of drill collar.This feature contributes to optimize the design of grooves in the drill collars,in order to achieve sound insulation by blocking the propagation of the collar wave at the location of their peak energy.In the second part,the theoretical analysis model of multipole acoustic logging while drilling in transversely isotropic formation(VTI)is established.The propagation law of multipole acoustic waves while drilling in transversely isotropic formation is studied in detail by the method of partial wave calculation.It is worth pointing out that the single-value definition of borehole acoustic field response function changes with the anisotropic parameters for anisotropic formation.In the formation with anisotropic parametersδ>ε+c44/2c33,the wave velocities at the cut-off frequencies of pseudo Rayleigh wave,flexural wave and quadrupole wave are all less than the shear wave velocities of the formation in the vertical direction,and tend to the corresponding velocities vh at the branch point kh.By measuring the velocity and combining with anisotropic parameters,the real vertical shear wave velocity can be obtained.In the third part,Aiming at the hot issues of evaluating cementing quality with acoustic logging while drilling,the sonic logging model of cementing while drilling characterized by sliding interface theory is established.The dispersion characteristics of casing wave and collar wave in cased borehole models with different cemented conditions are analyzed.Combined with partial wave calculation,it is found that the influence of residual drill collar wave on the casing wave amplitude increases with the interface bonding strength. |
