Study On Propagation Characteristics Of Acoustic Waves Along Drill String

Acoustic transmission technology is one kind of the MWD information transmission technologies.Compared with the transmission technologies using mud-pulse,electromagnetic wave,intelligent drill string and optical fiber,the acoustic transmission technology has unique advantages,such as the stable characteristics of drill string,independent of the circulation of mud and it can be widely used in all present drilling modes.Therefore,research on acoustic transmission along drill string is beneficial to provide theoretical basis for down-hole acoustic transmission technology and research and development of LWD,and has some practical value in the promotion of intelligent drilling technology.Based on the acoustic theory,the mathematical model of acoustic wave transmission along the one-dimensional drill pipe was established,and studying the frequency response characteristics and transmission rules of acoustic wave in the ideal drill pipe;according to the impedance mismatch of steady structure when the cross section changes suddenly,the frequency dispersion characteristics of drill pipe with joint had been analyzed,which provides a theoretical basis for the design of sound insulation structure;the frequency response characteristics of periodic and non-periodic drill string was studied using the finite difference method and transfer matrix method respectively,the results indicate that the pass-band and stop-band appears alternately,namely,the periodic drill string has the property of comb filter,and the drill string structure is more complicated,the width of pass-band is more narrow;the effect of the tapered part at the ends of joint on frequency response characteristics of drill string was considered in this thesis,the tapered part increases the width of pass-band,it is more conducive to the acoustic transmission.Combined with the actual work environment in down hole,considering the viscous damping of fluid and stratum boundary,the attenuation rules of acoustic wave along the drill string was studied and the maximum transmission distance was computed,the results indicate that repeater is necessary in down hole acoustic communication;moreover,due to the reflection of stratum boundary,the carrier signal is distorted,so the suggestion about determining the installation site of sound generator was given.According on the analysis of the acoustic properties of the drill string system,a set of system which can realize the acoustic data transmission is designed,using combined sound-insulation structure,double acoustic receiving device and determining reasonable position of acoustic generator and receiver to reduce the bottom-hole,ground and channel echo noise,the three methods were able to gain good effect of noise reduction through simulation with computer,for other noise in signal,the design method and circuit diagram of 4-order band-pass filter was given(the digital filter is used in the simulation);comparative analysis of amplitude shift keying(ASK),frequency shift keying(FSK)and phase shift keying(PSK),the binary phase shift keying(BPSK)was selected to modulate the sine wave using 100 binary signal generated randomly,then the modulated signal was applied to drill string,filtering and demodulating the received signal,the error rate is 8 percentage,the simulation had a good effect.