| In recent years,logging while drilling technology has developed rapidly and is widely used in oil and gas exploration in deep sea and land.However,acoustic logging while drilling faces special difficulties.One of these is that unipolar sound sources will excite the drill waves that propagate along the drill collar.Because the arriving time of drilling wave and formation refracted longitudinal wave are similar,and the amplitude of the drilling wave is much larger than the amplitude of the formation refraction longitudinal wave,this results in the drilling wave obscuring the formation refracted longitudinal wave and unable to obtain the longitudinal wave information of the formation.In order to weaken the influence of drilling wave on stratum information measurement,at present,most of the logging-while-drilling tools adopt the method of notching on the outer surface of the drill to reduce the interference of drilling waves.In this paper,the finite element software Abaqus is used to calculate the sound field of the acoustic logging while drilling,and the influence of the surface groove of the drill collar on the full wave waveform of the acoustic wave logging while drilling is studied.Firstly,the real-axis integration method and the finite element method were used for theoretical simulation and numerical simulation respectively,the full-wave waveform of the sound field of cable sonic logging under the sound source at the expansion point is calculated.Through the comparison of the two results,the correctness of finite element software in the calculation of wellhole sound field problems was verified.Afterwards,using the un-grooved drill collar placed in the borehole as the object,the finite element software modeling was used to calculate the response waveform of the acoustic wave logging while drilling on the surface of the drill collar,and slowness-time correlation method was used to process the full-wave array.The detailed analysis of the wave groups in the full wave while drilling.The results show that the large amplitude and long delay of the drilling wave obscures the longitudinal wave signal of the formation and must try to weaken the drilling wave.The focus of the paper is to use finite element software to analyze the sonic logging sound field after drilling groove surface.Firstly,the logging while drilling sound field under the condition of external groove was calculated.The grooves on the outer surface of the drill does not separate the stratigraphic refracted longitudinal wave group from the drill collar wave group,but also generates an additional Stoneley wave with a large amplitude,so that the formation refracted shear wave group becomes insignificant.Then,the logging while drilling sound field under the condition of internal groove was calculated.The method of groove cutting on the inner surface of the drill has obvious attenuation effect on the drilling wave,and it has little effect on Stoneley's propagation.In the STC processing chart,it can be clearly seen that the drill wave group and the stratum refraction longitudinal wave group are separated,and the stratum can refract the longitudinal wave group velocity accurately.Therefore,it is recommended to design the grooved groove structure by selecting the inner groove.Finally,based on the internal groove,the effect of groove depth and groove length on the attenuation of the drilling wave was analyzed.The increase of the groove depth is beneficial to the attenuation of the boring wave,but the groove depth is too deep to make the formation refracted shear wave information difficult to obtain;the change of the groove length has little effect on the weakening effect of the drill boring wave.Therefore,when designing the grooved structure of the drill boring,the appropriate groove depth should be selected to meet the acoustic attenuation requirements. |
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