瞬变电磁场穿过套管的过程与地层电导率测量

Process of the transient electromagnetic field passing through the casing and the measurement of the formation conductivity

利用瞬变电磁法穿过铁磁性套管实现套管外地层电导率测量对许多工程问题具有重要意义.实现该测量需要弄清瞬变电磁能量能否穿过套管,如何穿过套管,以及地层电导率信号如何耦合到井内液体的响应中.针对这些问题,本文用磁偶极子模拟激发线圈,利用实轴积分方法对径向分层的套管井瞬变电磁响应进行正演模拟,探索了瞬变电磁能量穿过套管的机理及其在地层中的分布规律.给出了不同时刻径向各层介质r-z截面中的电场强度分布图,展示了电磁信号穿过套管的过程、穿过套管后在水泥环和地层中的扩散过程,以及从套管外界面开始的耦合信号以类似于声波侧面波的形式再次穿越套管进入井内液体的过程.展示了套管的影响:(1)确定了瞬变电磁能量在发射线圈所在平面沿径向垂直穿过套管进入地层,解释了瞬变电磁能量如何穿过套管的问题;(2)给出瞬变电磁能量进入地层的位置和扩散方式,明确瞬变电磁测井的探测区域;(3)肯定了地层电导率信号随耦合信号进入到了井内液体,分布在每个时刻的接收波形幅度中.

The measurement of formation conductivity outside casing by Transient Electromagnetic(TEM)method through ferromagnetic casing is of great significance to many engineering problems.To realize the measurement,it is necessary to know whether TEM energy can pass through casing,how the signal passing through the casing,and how the formation conductivity signal is coupled to the response of the fluid in the well.In response to these problems,this paper uses a magnetic pole to simulate the excitation coil,using the real-axis integral method to simulate the TEM response of the cased well,explore the mechanism of TEM energy through the casing and the distribution law in the formation.The electric field intensity distribution in the r-z cross section of each layer in radial direction at different time is given,the process of the electromagnetic signal passing through the casing,the diffusion process in the cement sheath and the formation after passing through the casing,and the process of the coupling signal starting from the external surface of the casing passing through the casing again and entering the liquid in the well in a form similar to the side wave of sound wave are shown.It is confirmed that the TEM energy passes through the casing vertically along the radial direction at the plane where the transmitting coil is located,and how the TEM energy passes through the casing is explained.The location and diffusion mode of the TEM energy into the formation are given,and the detection area of the TEM logging is defined.It is confirmed that the formation conductivity signal enters into the well fluid with the coupling signal,and distributes in the receiving wave amplitude at each time.