瞬变电磁过套管电阻率测井响应模拟及分析

瞬变电磁过套管电阻率测井采用线圈电流的导通和关断方式进行大功率激发,测量套管井内不同源距的瞬态响应波形。现场测井得的瞬态波形有2个响应:一个是脉冲,位于激发时刻;一个是单峰波形——上升沿变化快、下降沿变化慢、位于激发时刻之后。用实轴积分法模拟套管井的瞬变电磁响应,得到的2个响应与所测量的波形特征一致,它们分别对应于位移电流(电磁波)和传导电流(电磁感应)的响应。传导电流密度与地层电导率成正比,过套管电阻率测井主要利用第2个瞬态响应获得地层的电导率。由于瞬变激发的连续谱以低频为主,测井波形中直接耦合的无用信号幅度很大。用同一源距不同深度测量的波形相减可以去掉响应中的无用信号(直接耦合响应和井内液体、套管、水泥环的二次场响应),得到2个深度点所测量地层的二次场差,该二次场差与所测量的不同区域地层电导率成正比,从二次场差波形中任取一点的幅度均可得到1条曲线,对其反褶积并刻度可以得到地层电导率随深度的变化曲线,加上初值便得到地层的电导率,实现过套管地层电阻率的连续测量。

Simulation and Analysis of Transient Electromagnetic Resistivity Logging Through Casing

In the process of cased-hole TEM logging, transient electromagnetic wave(TEM)is excited by turning on/off transmitter current, and the responses from different points are recorded. The TEM responses obtained by field logging has two response peaks, one is a pulse located at the exciting time, and the other is a single peak appearing after the exciting time, which changes rapidly along the rising edge and slowly along the descending edge. In this study, a real axis integral method was used to simulate the TEM responses in a cased-hole. The stimulated results are consistent with measured waveforms. The two response peaks correspond to displacement current (electromagnetic wave)and conduction current(electromagnetic induction). Because conductive current density is directly proportional to formation conductivity, the formation conductivity outside the casing is obtained by the second response peak. However, the amplitude of the useless signal directly coupled in TEM logging response is very large because the transient continuous spectra are of low frequency. To remove the useless signals(i.e., directly coupled signals and secondary field responses of fluid, casing and cement sheath in a well), the waveform measured at one depth is subtracted by that measured at another depth from the same source, to obtain the difference of secondary field responses at two depths. The difference is proportional to the conductivity of the detected interval. Then, a curve can be obtained from the amplitude of any point in the waveform of the second field difference. After deconvoluting and calibrating the curve, the conductivity curve of formation varying with depth can be obtained. Finally, by adding the known initial value, the formation conductivity can be obtained, and the continuous measurement of formation resistivity through casing can be realized.