| Noises generated during the drilling process have strong impact on the speed and accuracy of information transmission. In order to apply the acoustic transmission technology to the site, the noises’ interference to communication signals must be solved, which is also the key technique for the success of the acoustic LWD.In order to learn about the acoustic propagation characteristics in the drilling string, theoretical analysis and experimental study about the mechanism and features of acoustic wave propagation in the drilling string are firstly needed. Based on the theories of mutational cross-section pipe and waveguide acoustic waves, acoustics wave propagation in the drilling string was theoretically analyzed. Secondly, transfer matrix method was used to analyze the frequency spectrum of ideal drilling string. And the results show that the drilling string’s size, material, cross-sectional area and other relevant parameters have strong impact on the distribution and width of the drilling string’s frequency band. Based on the laboratory experiment of acoustic wave propagation in the drilling string, the propagation rules were studied. The results show that spreading under same frequency, signals with different wave forms have different distortion degrees, of which the distortion degree of sine wave is the lowest. In addition, drilling string channel change a little in different ways of supporting and it is also hardly affected by the amount of drilling strings. However, differences between single drilling strings shrink the communication band. The frequency of transmit from transducer can be affected by the joint materials. And the flow in annulus affects the width of transmission bands and accelerates the attenuation of acoustic wave energy.In order to study the characteristics of the drilling noise, live measures on the spot were taken to measure the drilling noise under different working conditions. The results show that with the change of working condition and parameters in the mines, the drilling noise changes. Downhole noise is also influenced by comb filter structure of drill string; through induction, it was found that in the drilling process major noise band distributes around 800Hz~1000Hz. Strength of composite drilling, the low frequency noise average drilling can be up to 50 dB, and stop drilling stop pump, low drilling noise intensity is only 20 dB.In order to study the influence of noise on acoustic signals, noise interference model was built. Through theoretical and experimental analysis, it was found that signal-to-noise ratio has nothing to do with the energy of both. It mainly affected by the average power. Transmission rate of communication signals is related to the selected bandwidth and signal-to-noise ratio. High frequency noise signals influence amplitude of signals, and further lead to waveform distortion of communication signals while it has less effects on signal spectral distribution. Noises that overlap with the bandwidth affect both the amplitude of acoustic signals and its spectral distribution. The impact of the real drilling noise on acoustic signals concentrates characteristics of all forms. Put forward solution to avoid noises according to their interference mode to acoustics transmission and choose the optimized communication frequency based on acoustics transmission rules within drilling strings Including sine pulse as a means of transmission, 600 Hz~700 Hz and 830 Hz~700 Hz as communication frequency band, the use of magneto stricture transducer for the transmitter, stop at stop drilling pump under the condition of transmission, maximum transmission rate can reach 30 BPS. |
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