Real-axis Integral Calculation Method Of Cased-hole TEM Response And Theoretical And Experimental Study On TEM Logging

Increasing domestic oil and gas production is an important measure to alleviate China’s increasingly severe energy situation.Because oil and gas reservoirs with different oil(gas)content exhibit different formation electrical conductivity,people use electrical conductivity,one of the physical parameters of the formation,as an important basis for the evaluation of oil and gas reservoir properties and the identification of remaining oil and gas,and its detection is helpful to achieve stable and increased production of oil and gas.However,the detection of formation conductivity is very difficult due to the steel casing in cased-holes.The existing method uses an electrode system pushed against the inner wall of the casing to measure the nanovolt DC voltage drop on the casing to obtain the formation conductivity.This method can only detect at a single point,with a long time,low efficiency,and high requirements for wellbore conditions.In order to improve the detection efficiency,transient electromagnetic(TEM)logging is used to detect the formation conductivity of the cased hole.This method uses non-contact detection to obtain a continuous formation conductivity curve,and the signal frequency components used are rich,so it has unique advantages.However,the logging theory it relies on is not yet perfect and needs to be studied in depth.It is difficult to accurately and continuously detect the formation conductivity of the oil and gas cased-hole and to study the propagation law of the TEM energy through the casing.By studying the real-axis integral algorithm of the TEM responses in casedhole model,this paper explores the distribution law of the formation conductivity information in cased-hole response,the mechanism of TEM energy passing through casing and its distribution law in formation,which provides a method for theoretical research of TEM logging in cased-hole.The main achievements are as follows:1.In terms of theoretical research:(1)the real-axis integration algorithm for casedhole TEM response is deduced,implemented and verified,and the full-wave response waveform inside and outside the hole and the TEM field distribution at different times are obtained,which lays the foundation for the theoretical research of this paper(2)this paper studies the characteristics of the responses in the cased-hole,establishes the relationship between the in-hole response,the source-receiver offset and the out-ofcasing formation conductivity,and determines the linear relationship between the response difference calculated with different formation conductivities and the formation conductivity difference,so as to provide the basis for the data processing of the formation conductivity detection through the casing;(3)this paper studies the spatial distribution of the responses in the casing and formation with time,determines the propagation and distribution characteristics of electromagnetic energy in the casing and formation,makes clear how the TEM energy passes through the casing,and gives the detection area of the TEM logging.2.In terms of model experiment:(1)a high-precision TEM signal acquisition system is developed,and the TEM response waveforms consistent with the theoretical calculation is measured in the cased-hole model,which verifies the theoretical calculation results;(2)the variation rules of the in-hole response and the response difference calculated with different formation conductivities with the source-receiver offset,out-of-casing liquid level and out-of-casing liquid conductivity are studied,which confirm that the TEM response difference in-hole can reflect the change of the out-of-casing liquid level and liquid conductivity,proving that it is feasible to detect the formation conductivity outside the casing by the in-hole TEM responses.3.In terms of verification test:(1)the method of obtaining apparent conductivity curve from the in-hole TEM response of the open-hole is given,which lays the foundation for data processing in this paper;(2)the radial geometric factors of the openhole and cased-hole are calculated and compared to determine the radial detection depth of TEM logging;(3)the TEM response inside the actual cased-hole is measured,and the continuous apparent conductivity curve of the cased-hole formation is obtained.The consistency comparison between the curve and the induction logging curve of the openhole verifies the theoretical research results of this paper,achieving the purpose of distinguishing different formation conductivities preliminarily.