| Electromagnetic Measurement While Drilling（EM-MWD）technology uses electromagnetic wave signals as carriers and has advantages such as high speed for transmission rate,large data capacity,and no need for dedicated channels.However,due to the influence of different device parameters,geological structures,and well types on the transmission of electromagnetic signals,low-frequency electromagnetic signals will experience severe attenuation in the formation.Therefore,this paper carried out a study on the transmission characteristics of EM-MWD system borehole electromagnetic signals under different well conditions and analyzed the influence of various factors on the transmission of electromagnetic signals.（1）On the basis of thorough research on the research results of electromagnetic measurement while drilling systems both domestically and internationally,the working principle and excitation method of electromagnetic measurement while drilling systems were elaborated.A detailed analysis was conducted on the propagation theory of electromagnetic waves in the formation and the factors affecting the transmission of electromagnetic signals,and the electromagnetic excitation method of the asymmetric drill rod antenna was determined.（2）A theoretical model of uniform formation equivalent transmission line for electromagnetic signals while drilling was established for different wellbore structures.The theoretical formula for electromagnetic signal transmission was derived,and the attenuation law of electromagnetic signals in vertical and horizontal wells in uniform formations was analyzed.The simulation results based on COMSOL finite element software show that:The higher the emission frequency,the lower the ground receiving voltage,indicating the more severe attenuation of electromagnetic signals.Therefore,the frequency of electromagnetic signals while drilling is generally chosen to be less than 30Hz;When the length of the insulation short joint is greater than 0.3m in a wellbore with drilling fluid of 1S/m,it is beneficial for the transmission of electromagnetic signals;In a horizontal well,the azimuth angle of the receiving electrode is 0°,and the signal reception intensity is the highest.（3）A segmented equivalent transmission line theoretical model was established to address the non-uniform characteristics of the formation,and the attenuation law of wellbore electromagnetic signals in heterogeneous formations was analyzed.Using COMSOL finite element software for modeling and simulation,the influence of different stratigraphic intervals on the transmission of electromagnetic signals in the wellbore was analyzed.The results indicate that:When the conductivity of the intermediate interlayer is between 10-3 and 10-1 S/m,it is beneficial for the transmission of electromagnetic signals;When the conductivity of the drilled formation is less than 0.025S/m,it is not conducive to electromagnetic signal transmission;The conductivity and thickness of the lower strata have a relatively small impact on the transmission of electromagnetic signals.（4）A theoretical model for electromagnetic signal transmission was established for the influence of casing on the transmission of borehole electromagnetic signals.The theoretical formula for the excitation source inside and outside the casing was derived,and the distribution law of electromagnetic signals in the casing was analyzed.The influence of excitation source position,casing conductivity and length,and drill string attachment to the casing on electromagnetic signal transmission was analyzed using COMSOL.The results show that:When the excitation source is inside the casing,the casing has a shielding effect on the electromagnetic signals.When the excitation source is outside the casing at a depth of 1000m,the received signal strength increased by nearly 9 times;The longer the casing length and the higher the conductivity,the greater the received signal strength,indicating that the casing has a good guiding effect on electromagnetic signals;The contact between the drill string and the metal casing does not affect the transmission of borehole electromagnetic signals. |
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