Experimental Study On The Characteristics Of Monopole Collar Wave In Acoustic Logging While Drilling

Acoustic logging while drilling(LWD)is one of the mainstream technology for the measurements of formation velocities and the evaluation of reservoir parameters.However,the collar wave in LWD has great influences on formation velocities in the field measurements,the scholars have conducted a lot of research works trying to get rid of the collar wave,but this problem is still not solved.In this paper,acoustic LWD experiments are carried out in the laboratory,the propagation and attenuation characteristics of direct collar waves and leaked collar waves are analyzed based on the experimental data.The author first introduce a set of high resolution data acquisition device and a high precision three-dimensional motion control system built in the lab,introduce the functions of various experimental instruments,present the rock samples and their physical parameters,discuss the noise interference in the laboratory.Then we verify the effectiveness of the test system by recording direct fluid acoustic signal between the source and the receivers.After that,five different experiments are designed for analyzing the propagation law of the collar waves excited by the monopole source in a water tank.In the first four experiments,the smooth drill collars are used to study the attenuation of leaked collar waves along the axis and radial directions,the effect of the separation of the source and the drill collar on the collar waves is discussed.We also compare the differences between the direct collar waves and the leaked collar waves from the recorded signals.The last experiment is conducted with a groove drill model,in which the direct collar wave and the leaked collar waves are measured as other experiments do,then the influences of the groove density on the drill collar wave propagation are discussed.The results show that the drill collar waves can still be excited with a strong energy even though the sound source is separated from the drill collar.However,both direct collar wave and the leaked collar wave disappear at the same time,when the drill collar is cut off,and these two collar waves transfer to fluid sound wave group to continue to propagate in the tank.By comparing the amplitudes of collar waves inside and outside the smooth collar,it shows that the amplitude of leaked collar wave is about a quarter of that of the direct drill collar wave.This result indicates that the amplitudes of direct and leaked collar waves are in the same order.Therefore,the influences of leaked collar wave on the detections of formation velocities can not be ignored in the field measurements.The results obtained in grooved model show that drill collar waves become weaker as the groove density increases.In addition,the attenuation curves of the collar waves along the axis and radial directions suggests a better method to weak the collar waves by putting the receivers outside of the collar,which is helpful for the LWD tool design.