Experimental Study On Collar Wave Characteristics Of Monopole Source And Measurement Of P-wave Velocity In Logging While Drilling

Since acoustic logging while drilling(LWD)technology was born,it has quickly gained a place in the logging industry with it's unique advantages,and has received great attention and development.Different from traditional wireline logging,acoustic LWD can realize the measurement while drilling,which improves the logging efficiency and greatly saves the cost.It is also used in horizontal wells and high-angle wells.However,the acoustic LWD technology also has its shortcomings.During the logging process,there is a collar wave propagating along the metal drill collar and the its amplitude is very large,which can interfere or even cover the formation wave signal.The predecessors conducted a series of theoretical simulations and experimental studies on this issue,and obtained many valuable research results.However,the problem that the collar wave interfers with the formation-wave measurement hasn't been solved so far.Based on previous studies,this paper further conducts experimental studies on the propagation characteristics of the collar wave.We analyze the collar-wave attenuation characteristics in different excitation frequencies and source distances.Meanwhile,We highly proposes a method for extracting P-wave velocity in LWD,and the effectiveness of the method is verified through experiments.Firstly,we introduce the instruments and devices required for the experiment,detailedly explain functions of each experimental instrument and the parameters used in the experiment,and give the specific flow of the experiment.Next,we list the physical indicators and parameters of the model well used in the experiment.We also propose the method of reducing noise interference and experimental data processing methods in the laboratory.Secondly,the paper conducts experimental studies on the collar-wave propagation characteristics.We conduct monopole source acoustic LWD experiments in an infinitely large fluid well and study the response characteristics of the internal and external collar-wave amplitudes in various acoustic-source excitation frequency.Then we give the change curves of the internal and external collar waves and analyze them in detail.Besides,we record the waveform curves of the internal collar wave in different source distances,and qualitatively discuss the internal collar-wave attenuation characteristics.The experimental results indicate that the amplitudes of the internal and external collar waves show a trend of increasing first and then decreasing as the sound source excitation frequency increases.The external collar wave response frequency band is much wider than that of the internal collar wave,and the external collar-wave amplitude increase rate is slower than that of the internal collar-wave.Moreover,the amplitude of the internal collar wave gradually weakens as the source distance increases,and it conforms to the exponential decay law.Finally,by analyzing the coupling relationship between the internal/external collar waves and the formation waves in the acoustic LWD,we propose a method to extract P wave by removing the collar wave from the acoustic LWD signals.In addition,we conduct the acoustic LWD experiments in the sandstone model well to verify the effectiveness of the method.The experimental results indicate that even if the drill pipe is not grooved,we can greatly attenuate the collar-wave amplitude in the acoustic LWD waveform with our method.Thus we can obtain the P-wave velocity accurately.Besides,the effectiveness and repeatability of this method are verfied by repeating the experiments in sandstone borehole model.