| Oil and gas resources are the lifeblood of a country's economic development,and my country also attaches great importance to it.Measurement while drilling(MWD)technology plays an important role in improving oil and gas drilling efficiency and reducing drilling costs.The currently commonly used wireless measurement-whiledrilling method has complex structure and slow information transmission rate,which greatly limits the further development of drilling technology.Electromagnetic measurement-while-drilling(EM-MWD)is a new measurement while drilling method.It uses electromagnetic signals as carrier waves to transmit information between downhole and surface.It has the advantages of fast transmission rate,simple operation and high reliability.more attention.However,due to the complex underground environment,there is still no clear theory at home and abroad for the attenuation characteristics of electromagnetic signal transmission in the underground.This paper firstly analyzes the transmission channel of electromagnetic signal while drilling and the way of downhole electromagnetic excitation,and discusses the factors affecting the transmission of electromagnetic signal while drilling in detail.According to the classical electromagnetic theory combined with the principle of "equivalent transmission line",the theoretical research on the signal transmission process is carried out.Then,an electromagnetic measurement-while-drilling(EMMWD)finite element model was established based on ANSYS software.From the perspectives of signal transmission power and frequency,four aspects of drill string,casing,formation conduction inhomogeneity and formation temperature change were analyzed.Influence law on electromagnetic signal transmission while drilling.Finally,based on the ground simulation experiment designed by ourselves,the simulation analysis results are verified from three aspects: the ohmic loss generated by the electromagnetic signal along the drill string,the drill string of different specifications,and the non-uniformity of formation conduction.The research results show that the ohmic loss generated by the transmission of the electromagnetic signal while drilling along the drill string is proportional to the cross-sectional area of the drill string;the specification of the drill string has a certain influence on the transmission of the electromagnetic signal while drilling.When the ratio of the outer diameter is about 0.08-0.20,the signal transmission effect is better;the excitation and the sleeve will have a serious shielding effect on the transmission of electromagnetic signals,especially if the frequency exceeds 50 Hz,the ground receiving voltage will be seriously attenuated,but when the length of the sleeve is less than 40 m,the sleeve The influence of the pipe is small;the signal transmission effect is worse than that of the uniform formation in the formation with uneven distribution of electrical conductivity,and the higher the signal transmission frequency,the more serious the attenuation;the formation temperature causes the wellbore temperature to change,which makes the electromagnetic signal transmitted along the drill string attenuated while drilling increase,thus affecting the signal transmission effect.Finally,when the signal frequency is 20-60 Hz,the overall transmission performance of the EM-MWD system is relatively high.Based on the above conclusions,suggestions are provided for the design and improvement of the on-site EM-MWD system and related instruments,and a reference for evaluating the application scope and environment of the EM-MWD system. |
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