套管井瞬变电磁响应波形中的地层电导率信息

Formation conductivity information revealed by the transient electromagnetic response waveform of a cased well

瞬变电磁测井是测量套管外地层电导率的一种有效方法。采用实轴积分法试算了套管井模型的瞬变电磁响应,不同源距响应波形的形状差异明显。对同一源距,将两种不同地层电导率的响应波形相减得到响应差波形,不同源距的响应差波形形状不变。各个时刻的响应差与地层电导率差呈线性关系。首先将实际测量波形减去同一深度的测量波形,取某个时刻的波形差,然后刻度平移得到瞬变电磁视电导率曲线,该曲线形状与裸眼井阵列感应测井曲线形状一致,最后仿照Doll的地层电流环,使用导电薄板对地层中涡流再次激发的二次场响应进行分析,发现其传播特征为单向,得到的传播速度为常数,这解释了响应差波形形状不随源距改变的计算结果,确定了瞬变电磁测井波形中的地层电导率信号由地层涡流激发。上述研究成果为利用二次场响应的传播速度计算过套管地层电导率提供了理论依据。

Transient electromagnetic(TEM)logging is an effective method for measuring the conductivity of the layer surrounding a casing.In this study,the real-axis integral method was used to calculate the TEM response waveform(RW)in a cased-well model.For a given source-receiver offset,the RW difference was obtained by subtracting the RW of two formations with different conductivities.The shape of the RW at different distances from the source remained unchanged for different source-receiver offsets.A linear relationship was observed between the RW difference at a given time and the difference in formation conductivity.By subtracting the calculated RW at a given depth from the actual measured RW and taking the RW difference at a given time,the TEM apparent conductivity curve could be obtained.The shape of this curve was consistent with that obtained in an open-well configuration.Finally,the secondary field response re-excited by eddy current in the formation was analyzed using a conductive thin plate,which was modeled based on Doll's formation current loop.The one-way propagation characteristics of the secondary field were evaluated,and it was found that the velocity of propagation was constant.This can explain that,as shown by the calculation,the RW difference is independent of the source-receiver offset.It was also confirmed that the formation conductivity signal in the transient electromagnetic logging waveform was excited by the formation eddy current.