| Borehole time domain electromagnetic detection is a new transient induction logging method,which has obvious advantages of short source distance and long detection.In this paper,the transient induction logging model is constructed,the parameters of the tool are optimized,and the detection characteristics of the tool in the borehole environment are studied by numerical calculation.The graduate results provide theoretical basis for the development and engineering application of the open hole time domain induction logging tool.The main research contents and achievements are divided into the following four parts.The first part is the theory of borehole time domain electromagnetic detection and the numerical simulation method based on COMSOL software.Firstly,the basic principle of time domain electromagnetic logging is studied,and the physical meaning of transient induction logging is revealed by formula decomposition and graphic display analytical solution,and the transient response characteristics of uniform formation are analyzed.Based on COMSOL finite element software,a time domain transient induction logging model is constructed,and the rationality of numerical calculation and mesh generation is verified by comparing with analytical solution.At the same time,it is found that when the formation conductivity is greater than 1S/m,only the numerical solution can reflect the transient characteristics at the existing initial time.Finally,the "smoke ring" effect of transient induction logging is explained by numerical calculation.The second part is the parameter design of borehole time domain electromagnetic detection tool.The effects of rod conductivity,source distance,coil turns,core thickness and relative permeability,pulse signal shape and amplitude on time domain electromagnetic detection signal are studied by numerical calculation.The results show that the coil of borehole time domain electromagnetic detection can not be wound on the metal drill pipe;In engineering,the steeper the lower step function is,the better;Tool spacing affects the variation characteristics of neararea signals,but has little influence on far-area signals,which provides a theoretical basis for remote detection with short source spacing.The third part studies the response characteristics of time domain electromagnetic detector.The influence of mud conductivity,well diameter and eccentricity on logging response is studied by numerical calculation The radial detection depth of high-resistivity intrusion and lowresistivity intrusion and the longitudinal resolution of high-resistivity surrounding rock and lowresistivity surrounding rock are studied.The results show that when the mud conductivity is greater than 1 S/m,the logging response affects the conductivity along the mud,and the increase of well diameter and eccentricity becomes serious,so it is necessary to establish a response library for borehole correction.The radial detection depth of high resistivity invasion is greater than that of low resistivity invasion.The resolution of high resistivity surrounding rock is greater than that of low resistivity surrounding rock.The radial detection depth is contradictory to the longitudinal resolution.Therefore,it is necessary to design multiple receiving coils with different source distances in order to detect the formation characteristics near the tool.In the fourth part,the characteristics and depth of remote detection of transient induction logging are studied,and a three-dimensional three-layer formation model is established in COMSOL for calculation.The results show that the time-domain electromagnetic method for remote detection represents the far-field characteristics of the instrument,independent of the instrument source distance.When the conductivity of the surrounding rock is 1 S/m and the conductivity of the formation is 0.02 S/m,the farthest detection distance is 50 m. |
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