瞬变电磁井间勘探方法研究

Research on transient electromagnetic interwell exploration method

依据瞬变电磁全空间响应的传播特征,本文提出了一种井间勘探方法.在井内用线圈激发,在本井和邻近井接收瞬态响应波形.基于Maxwell方程分析了位移电流和传导电流所引起的两种传播特征,明确了在邻井接收到的波形中包含界面的电导率信息.详细研究了线圈在无限大均匀介质中激发的瞬变电磁响应波形以及电场在不同时刻的空间分布.瞬变电磁近源距响应波形幅度随源距的增大衰减很快,响应持续时间短;远源距对应的响应波形上升速度快,达到峰值之后缓慢下降.随着源距的增加,响应波形上升和下降的速度均减慢,峰值向后移动,能够看到明显的传播特征(与位移电流引起的电磁波形态不变差异很大).与此对应,瞬变电场的空间分布也随着波形幅度的上升迅速扩大,随着幅度的减小快速收缩.在电磁能量向外传播的过程中,遇到界面时会发生透射和反射,使得测量得到的波形形状会在无界面条件下波形基础上发生改变.界面信息隐含在这种波形形状改变中,需要通过瞬变电磁反射和透射模型进行提取,实现井间勘探.

According to the propagation characteristics of transient electromagnetic response in full space, an interwell exploration method is proposed in this paper. A coil in the well is used to generate transient electromagnetic field, and coils in the same well and adjacent well are used to receive transient electromagnetic response. The two propagation characteristics caused by displacement current and conduction current are analyzed based on Maxwell equations, and it is known clearly that the waveform received from the adjacent well include the electrical conductivity information of the interface. The transient electromagnetic response waveform of a coil excited in an infinite homogeneous medium,the spatial distribution of electromagnetic field at different time are carefully studied. The amplitude of transient electromagnetic near-source response waveform decreases rapidly with the increase of source distance and the response duration is short. The response waveform corresponding to the far-source distance rises quickly and then slowly declines after reaching the peak value. With the increase of the source distance, the speed of rise and fall of the response waveform slows down, and the peak value moves backward, obvious propagation characteristics can be seen(quite different from the shape of electromagnetic wave caused by displacement current). Correspondingly, the spatial distribution of transient electromagnetic field also expands rapidly with the rise of waveform amplitude and shrinks rapidly with the decrease of amplitude. In the process of outward propagation of electromagnetic energy, transmission and reflection occur when it encounters an interface, making the received waveform shape changing based on the waveform shape without interface. The interface information is hidden in the shape change of waveform, which needs to be extracted through transient electromagnetic reflection and transmission models to realize the interwell exploration.