Research On Transient Electromagnetic Logging Theory And Experiment

Transient electromagnetic logging is an effective method for measuring the resistivity of casing wells in recent years.Like induction logging,it measures the secondary eddy current field signal excited by conductive medium according to the principle of electromagnetic induction.However,the casing is closest to the coil,its electrical conductivity and magnetic permeability are both high,the response amplitude is large,and constitutes the main component of the primary and secondary field response.Removing the unwanted signal in the response is a top priority for casing resistivity logging.The transient electromagnetic response is a transient waveform,and the response shapes of different source distances are very different.It is not possible to use the array-induced three-coil method.In this paper,the characteristic data of the secondary field is analyzed by the inductive logging Doll geometry factor,and the natural potential experiment is carried out to eliminate the influence of the natural potential on the received signal in the logging.The response subtraction method of the adjacent depth point measurement is used to remove the unwanted signal and the set.The secondary field signal of the tube and cement ring is only the difference of the conductivity of the formation measured at two depth points,which is the directly measured conductivity information of the formation.The geometry factor of the casing well is studied using the difference between the measured waveforms of the same source distance and the adjacent depth at the casing hoop.At the casing hoop,the waveform of the response difference with depth is taken out,and the geometrical factor law of the casing well is found by comparing the difference between the two Doll geometry factors of the cosine distance.The geometrical factor of the casing well is similar to the shape of the Doll geometry factor,but The casing stretches the transient electromagnetic field in the longitudinal direction and compresses in the radial direction,so that the geometric factor of the casing well formation changes accordingly.The response depth of the different source distances is still different.The casing cuff is short and the thickness is small.It can be approximated as the impact function in space.The difference in the response of the casing cuff reflects the geometric factor difference of the casing wellstratum.After normalization and integration,it can be approximated as the geometric factor of cased hole transient electromagnetic logging.This geometric factor is twice as large in the longitudinal direction as the open-hole Doll geometry factor,and is reduced by two times in the radial direction.Based on this,the transient electromagnetic logging response is analyzed and the processing methods of different resolution logging curves are established.