| Acoustic logging while drilling plays an important role in geological guidance while drilling,formation evaluation,etc.,taking up less time of wellsite drilling rig.,and its characteristics of "measuring while drilling" can meet the logging operation of various complex formation conditions,and it has obvious advantages in improving efficiency and reducing cost.Isolator is one of the key components of acoustic logging tools.It can attenuate collar waves to obtain formation wave information,which is especially important in acoustic logging tools while drilling.In this paper,firstly,the acoustic wave equation in the form of velocity-stress in twodimensional cylindrical coordinate system is derived,and then the finite difference scheme of staggered grid wave equation is obtained,the problems of grid division,source loading,medium interface,well axis singularity,negative r-axis processing and the application of non-split perfect matching layer are solved,and a program is compiled to calculate the sound field generated by monopole sound source in borehole in soft and hard formation,and the comparison shows that it is consistent with the analytical solution of real axis integral method,which verifies the accuracy of the program.On this basis,the attenuation performance of different grooving ways and variable diameter and their combination is studied by numerical simulation,and the full-wave numerical simulation while drilling is carried out.Numerical simulation of uniform grooving of thick and thin collars in different ways and gradual grooving of thick collars are carried out,and spectrum analysis of collar waves is carried out.It is found that when the number of grooves reaches a certain amount,the attenuation of collar wave will not increase obviously if the number of grooves is increased,for example,for a thin drill collar with a radius of 3.5/6.5 cm,when the groove depth is 1.5 cm,there is little difference in sound insulation performance between 11 grooves and 16 grooves.and the maximum attenuation will be obtained when the groove width and the groove spacing are equal,and the grooving depth is the main parameter for attenuation of collar waves,and thick collars need to be grooved deeper,otherwise low frequency collar waves may be enhanced.The closer the sound source is to the groove structure,the more favorable it is to extract the information of gliding P-wave.It is found that the gradual groove has greater attenuation to the low frequency drill collar than the uniform groove,and can obtain a wider stopband and a larger relative collar wave attenuation.The optimized isolator is used to calculate the time-domain waveform while drilling,and good sliding P-wave is obtained.According to the characteristics of timedomain waveform while drilling,STC is simply and effectively improved to extract the velocity of sliding P-wave successfully.The waveforms while drilling with different spacing between collar and formation are calculated,it is found that the spacing between collar and formation highly affects the effective extraction of gliding P-wave velocity.The numerical study on the isolator of variable diameter collar shows that the variable diameter length of only one wavelength can’t completely add up the inherent stop bands,and a longer length is needed.The performance of isolator combined with variable diameter and groove is calculated,and it is found that on the basis of shorter variable diameter length,the attenuation effect of shallower groove depth is better than best that of only variable diameter,and the attenuation of high-frequency collar wave by external groove is stronger than that by internal grooving.The groove depth is an important parameter,which shows that the attenuation of thin collar is more difficult,and the calculation shows that the combination of variable diameter and shallow groove has great attenuation potential.This paper has certain reference significance for finite difference numerical simulation and isolator design of acoustic logging while drilling. |
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