瞬变电磁测井实验研究

摘要

This paper presents experimental verification of transient electromagnetic logging principles. We use a plexiglass tube with an out-side diameter of 113mm, wall thickness of 6mm, length 1m to simulates openhole borehole, and a plexiglass tube with an outside diameter of 350mm, wall thickness of 10mm thick to simulate formation, and use a one meter long of 5. 5 inches casing pipe to simulate cased well. Homemade transmitter and receiver coils are motivated by a spacing of 150ms square wave. While measuring the received waveform of vari-ous source , the transient electromagnetic response attenuation law with source distance is accessed. Changing brine resistivity and measuring again with the response subtract of different formation resistivities offseting the first response field stimulated by transient electromagnetic, and obtain the secondary field response waveform which is a direct reflection of formation resistivity. It was found that: secondary field re-sponse waveform characteristics consistent with theoretical calculations and experimental results that borehole secondary field in the transient excited sides ( on-off front and rear) both have extreme values, but opposite in sign;only one side of the cased well secondary field has extreme value. However, due to the decay constant of excitation circuit, the secondary field waveform magnitude is actually measured large in the current mutation side and small in exponentially decaying side. Borehole’s secondary field amplitude is much larger than the cased well’s secondary field amplitude. The original signal is small in measuring cased well formation resistivity.%文章给出瞬变电磁测井原理的实验验证。用外径为113 mm、壁厚6 mm、长1 m的有机玻璃管模拟裸眼井井眼、用外径350 mm、壁厚10 mm的粗有机玻璃管模拟地层，粗有机玻璃内放置盐水模拟地层的电阻率，用长1 m的5.5 in(1 in=25.4 mm)套管制作套管模型井，同样，用直径300 mm、壁厚10 mm的粗有机玻璃管模拟地层。自制发射和接收线圈，用间距150 ms的方波激发，测量不同源距的接收波形，获得了瞬变电磁响应随源距的衰减规律。改变盐水的电阻率重新测量，用不同地层电阻率的响应相减抵消瞬变电磁激发响应的一次场，获得了直接反映地层电阻率的二次场响应波形。实验发现：理论计算的二次场响应波形特征与实验结果一致，即裸眼井的二次场在瞬变激发的两侧(通断前后)均有极值，而且符号相反；套管井的二次场仅仅在一侧有极值。但是，由于激发电路衰减常数的影响，实际测量的二次场波形在电流突变一侧的幅度大，在指数衰减一侧的幅度小。裸眼井的二次场幅度远远大于套管井的二次场幅度，套管井地层电阻率测量的原始信号很小。