Electromagnetic wave transmission characteristics in narrow casing-drill pipe annulus
The downhole information transmission technology based on electromagnetic wave can effectively provide real-time transmission of downhole drilling parameters in the process of gas drilling.At present,however,the influence laws of environmental factors on electromagnetic wave transmission in narrow annulus are less researched.To further study the electromagnetic wave transmission characteristics in the narrow casing-drill pipe annulus and clarify the key factors influencing the attenuation rate of electromagnetic wave intensity,this paper establishes a model of narrow casing-drill pipe annulus using the finite element numerical simulation method,according to the electromagnetic field and electromagnetic wave transmission theory.Then,with the electromagnetic wave intensity as an evaluation index,the distribution of electromagnetic wave on the section of the annulus and its attenuation rate with the change of transmission distance are investigated,and the influences of drill pipe eccentricity,pipe surface roughness and electromagnetic response parameters,and annulus filling medium properties on the attenuation rate of electromagnetic wave intensity are analyzed.Finally,the ways to effectively increase electromagnetic wave transmission distance are discussed.And the following research results are obtained.First,the electromagnetic energy shows approximately linear attenuation with the increase of distance,but it may turn into exponential attenuation under the influence of a variety of factors.Second,the drill pipe eccentricity has a direct effect on the energy distribution and attenuation rate of electromagnetic wave.Third,the surface roughness,electrical conductivity and relative magnetic permeability of pipe and the electrical conductivity of air are the key factors affecting the attenuation of energy transmitted by electromagnetic waves.In conclusion,the research results clarify the attenuation laws of electromagnetic wave when it is transmitted in the wellbore annulus,and provide a theoretical basis for broadening electromagnetic wave transmission channels and developing downhole device and equipment based on electromagnetic wave measurement and control.
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