随钻电磁波测井仪器结构影响的三维有限元模拟

利用三维有限元方法模拟钻铤、线圈衬底、天线槽、天线槽填充物及屏蔽罩等仪器结构对随钻电磁波测井响应的影响。结果表明:实际仪器结构中低阻的钻铤、线圈衬底和屏蔽罩导致电磁信号明显衰减,而高阻的天线槽填充物对仪器信号无明显影响。在兼顾仪器机械强度的条件下,通过减小天线槽深度、增大天线槽长度和占空比可以降低电磁信号衰减。仪器结构影响导致仪器响应随地层电阻率非线性变化,扣除仪器结构影响后,仪器响应恢复了随地层电阻率在较宽范围内线性变化的关系,可以满足仪器线性刻度需要。由于扣除仪器结构影响后线圈中磁通量减小,仪器响应略小于无仪器结构影响时的响应。

Structural effects analysis of an electromagnetic wave propagation resistivity LWD tool by 3D finite element method

The effects of metal mandrel, coil substrate, antenna slot, antenna slot filling material, and shield cover of an electromagnetic wave propagation resistivity logging while drilling(LWD) tool were simulated by a 3D finite element method. The results show that the low resistivity of metal mandrel, coil substrate, and shield cover can lead to significant electromagnetic signal reduction, whereas the effect of the slot filling material with high resistivity is nearly negligible. Under the consideration of mechanical strength, the attenuation of electromagnetic signal can be minimized via increasing the slot length, enlarging the space proportion of slot, and reducing the slot depth. The structural effects may result in nonlinear response changes with the formation resistivity; after eliminating the structural effects, the responses recover to change linearly with the formation resistivity in a broad range, satifying the requirement of linear calibration. However, due to the decrease of magnetic flux in the coils after eliminating the structural effects, the tool responses may become smaller than those without the structural effects.