Research On Response Of Transient Electromagnetic Logging

This paper studies the response of transient electromagnetic logging.There are two kinds of open-hole wells and cased wells in actual engineering.Transient electromagnetic logging can be used for both casing and open-hole wells.In this paper,the feasibility of measuring the electrical conductivity of the formation outside the casing pipe is analyzed theoretically by using plane three-layer model and cylindrical three-layer model.A prototype of transient electromagnetic logging instrument with an one-lanuching and eight-receiving structure was designed.The response waveform of transient electromagnetic logging is obtained by experiments in open hole and cased hole models.Based on Maxwell's equations,the responses of the transient electromagnetic multilayer dielectric plane model and the logging model(two-layer medium,three-layer medium,and four-layer medium in the cylindrical interface)are derived respectively.According to the response characteristics of the three-layer dielectric logging models of plane and casing well,the influence of casing parameters on transient electromagnetic logging response is analyzed from the perspective of reflection and transmission coefficients,and it is proved that the transient electromagnetic response difference of different formation conductivity is proportional to the conductivity difference of the formation(less than 200Hz)when measured by transmitting and receiving coils in the cased hole,thus confirming transient electromagnetic,a non-contact measurement method,can effectively measure the formation conductivity through the casing.In addition,the propagation characteristics of magnetic field in formation during excitation in the cased hole are studied by using the response of four-layer logging model in casing well.In the frequency domain,the magnetic field response will continuously shrink toward the center of the well axis as the frequency increases.The radial distribution range is more concentrated around the casing.The magnetic field response in the time domain shrinks and expands at different times,which specifically shows the propagation process of the magnetic field in the casing well in the formation.In addition,in the frequency domain and time domain,the polarity of magnetic field changes during the propagation process.The study of the propagation characteristics of the magneti c field in the casing well in the formation is of great significance for determining the effective measurement radius and longitudinal range in the actual logging process.The transmitting system and the receiving system are studied.The design principle of the coil system is emphatically studied and two coil systems are designed and experimentally performed to obtain experimental waveforms.The open hole and casing well model experiments are carried out with the designed coil system,and the response waveform is obtained.The secondary field response waveform is obtained through the aluminum plate model experiment.Through the comparison between the open hole well and the casing well model experiment,it is found that the casing has a great influence on the shape of the response waveform,and response peaks at different source distances are at different time locations.