金属套管对多分量电磁感应测井信号的影响

研究金属套管对低频电磁场各分量透射信号和反射信号的影响规律对采用过套管电磁感应测井技术监测生产过程中的流体移动具有重要价值.采用柱状成层各向异性介质中的并矢Green函数模拟当发射源在套管内、接收器分别在套管内和套管外时多分量电磁感应的响应，并系统分析了金属套管对透射磁场和反射磁场各主分量的影响规律.计算结果表明，无论是透射场还是反射场，由于所采用的频率较低，金属套管对每个分量信号的影响可以与地层对该分量信号的影响分开.在低频段，磁性金属套管对磁场各分量仍存在稳定的反射，但不同分量被反射的程度不同，受套管磁屏蔽作用大的分量（如ρρ分量）被反射的程度高于受套管磁屏蔽作用小的分量（如zz分量）.随着频率的升高，反射磁场的强度逐渐增大并最终达到饱和，信号被全反射.磁性金属套管的存在导致透射磁场各分量出现不同程度的幅度衰减和相位延迟，受套管磁屏蔽作用大的分量的幅度衰减程度大.在固定套管参数和频率的情况下，磁性套管对透射磁场不同分量的影响程度虽然不同但是恒定，其所导致的各分量的幅度衰减和相位延迟与线圈系位置和地层电参数无关.在低频段，套管外透射场的各分量处于准静态，其幅度和相位均不随频率而改变，受套管磁屏蔽作用大的分量的幅度随套管磁导率和厚度的不同而存在明显变化.当频率超过某一临界值后，套管的影响逐渐从以静态屏蔽为主转到以趋肤效应为主，导致透射场幅度的衰减和相位的延迟，套管的厚度越大、套管的电导率和相对磁导率值越大，所对应的频率临界值越小.

Effect of metal casing on multi-component electromagnetic induction logging signals

Study the effect of metal casing on each component of transmitted and reflected low-frequency electromagnetic fields is of great value for monitoring fluid movement during production by using through-the-casing electromagnetic induction logging technology. The dyadic Green's functions in cylindrically stratified anisotropic media were applied to simulate the multi-component electromagnetic induction responses when the transmitter is inside the casing and the receiver is inside or outside the casing, and the law of effect of metal casing on the main components of transmitted and reflected magnetic fields were analyzed systematically. Numerical results showed that the casing effects on each component of both the transmitted and the reflected magnetic fields can be separated from the formation effects since the frequency is low. There still exists a stable reflection for each component of magnetic fields by the magnetic metal casing at low frequencies, but the degree of reflection is different for different component. The component which is more affected by the magnetic shielding effect of the casing (e.g. ρρ-component) will be more reflected than the component which is less affected by the magnetic shielding effect of the casing (e.g. zz-component). The intensity of the reflected magnetic fields will become bigger with the increasing frequency, and finally it will reach a saturation value and the signal is thus totally reflected. The existence of magnetic metal casing will lead to amplitude attenuation and phase delay for all the components of transmitted magnetic fields, but the degree of amplitude attenuation and phase delay is different for different component, and the amplitude attenuation of the component which is more affected by the magnetic shielding effect of the casing will be more obvious. Under the condition of fixed casing parameters and frequency, the degree of magnetic metal casing effect is different for different component of the transmitted magnetic fields but keeps constant, and the amplitude attenuation and phase delay caused by the casing are irrelevant to the coil's position and the formation conductivity. At low frequencies, each component of the transmitted magnetic fields outside the casing is in quasi-static state, its amplitude and phase do not change with the increasing frequency, and the amplitude of the component which is more affected by the magnetic shielding effect will have a more obvious change when the casing's permeability or thickness is changing. The fields will be gradually dominated by the skin effect than by the static shielding effect of the magnetic metal casing after the frequency exceeds a critical value, and the transmitted magnetic fields will undergo an extra amplitude attenuation and phase delay. The larger the casing thickness, conductivity and magnetic permeability, the lower the critical value of the frequency.