| Acoustic logging while drilling(LWD)is characterized by simultaneous drilling and logging,accurate measurement results and fast measurement,which can guide drilling direction in real time.It can also be applied to highly deviated wells,horizontal wells and offshore drilling platforms.However,there are still some unsolved problems in acoustic field simulation and data interpretation.For example,the shear wave velocity of soft formation is not easy to be measured;There are differences between simulated waveform and experimental waveform;The signal types received by acoustic LWD are not clear;It is difficult to extract the inversion slowness dispersion curve of each individual wave;The aliased mode interferes with the true mode in the inversion slowness dispersion curve.To solve these problems,this paper analyses the characteristics of various acoustic LWD mode waves excited by monopole,dipole and quadrupole sources,simulates the acoustic field of LWD,and obtains the slowness and amplitude dispersion curves from waveform array.The main work and research progress are as follows:To explain the difference between simulated waveforms and experimental waveforms of monopole acoustic LWD,the waveform in the experiment is compared with the waveforms of acoustic pressure and radial displacement under the same parameters.It can be seen that the radial displacement signal can better reflect the signal characteristics received by the transducer in monopole acoustic LWD.In addition,to solve the problem of obtaining shear wave velocity in soft formation,we investigate the dispersion and sensitivity characteristics of Scholte wave and propose a method to obtain the shear wave velocity in soft formation by dipole Scholte wave.This method utilizes the one-to-one correspondence between Scholte wave velocity and shear wave velocity,and the highly sensitivity of Scholte wave velocity to shear wave velocity.Different from the previous finite element models which only simulate the piezoelectric effect of the transducer or the acoustic propagation process in logging,this paper establishes a finite element model which comprehensively considers the piezoelectric effect of the transducer and the propagation process of acoustic wave.We investigate the types of mechanical signals received by the transducer and calculate the mechanical responses and output voltage of the monopole acoustic LWD.Using the piezoelectric equation and the polarization relation of the transducer,we give an explicit expression of the voltage and mechanical quantities,and investigate the relationship between the output voltage recorded by the transducer and the radial displacement,pressure,radial strain and axial stress.The results show that the radial displacement,radial strain and axial stress contribute greatly to the voltage.At the same time,we study the effect of collar grooves on simulated waveform.The results show that the influence of grooves on the voltage waveform is closer to that of radial displacement waveform.To determine which dispersion mode the slowness values calculated at a given frequency belong to in the process of slowness dispersion curve inversion,we utilize density clustering method to classify frequency-slowness points belonging to an individual mode into a class,so as to extract the slowness of the individual wave.On this basis,combining with the intensity excitation curve,we elucidate the reasons for the inaccuracy of inversion slowness of some mode waves.Besides,the wrong choice of Riemannian sheet of inversion slowness leads to the interference of aliased mode to true mode.This paper explains that the reason for the aliased mode and proposes a method to remove the aliased mode.In this method,the true mode and the aliased mode are separated by density clustering of the amplitude spectrum.The aliased mode is moved to the correct Riemannian sheet according to the continuity of the dispersion curve to make it become the true mode,which fundamentally solves the interference of the aliased mode to the true mode. |
