影响随钻电磁波电阻率测井仪器响应的因素主要来自两方面:一方面是自身结构因素,包含线圈、天线凹槽、盖板、钻铤等,这些因素通过影响接收线圈感应电动势的幅度,进而对电阻率测量结果产生影响;另一方面因素来自外界环境,包括井眼泥浆、围岩、侵入、介电常数以及地层各向异性等,测井过程中的仪器响应,是对井下多种不同介质的综合响应,为了达到探测原状地层电阻率的目的,需要扣除其它因素对仪器响应的影响。因而研究影响随钻电磁波电阻率测井响应的因素,对测井资料的解释与应用是十分必要的。本文以随钻电磁波电阻率仪器WPR(Wave Propagation Resistivity)为算例,建立真实的3维仪器结构,采用3维有限元3 D FEM(Three-Dimensional Finite Element Model)方法,针对真实仪器自身结构以及外界环境因素对仪器响应造成的影响进行了数值模拟,并利用解析解对本文的数值解进行了验证。感应电动势绝对值随着线圈半径、线圈凹槽宽度、天线槽长度、盖板占空比增大而明显增大;随着凹槽深度增加而减小。其中,浅探测模式受以上因素影响较深探测模式更大。此外,感应电动势绝对值受金属钻铤的电导率影响较小。在外界环境因素方面,井眼与泥浆存在使视幅度比电阻率偏小,视相位差电阻率偏大;泥浆电阻率越大,对仪器响应造成的影响越小;各向异性使视幅度比电阻率与视相位差电阻率均偏大;泥浆侵入、围岩、介电常数以及各向异性因素使视幅度比电阻率与视相位差电阻率在数值上差异逐渐增大;泥浆侵入使视相位差电阻率与视幅度比电阻率差异先增大再减小;介电常数影响在高阻地层中更加明显,其中视相位差电阻率受影响更大。
A new three-dimensional laterolog array sonde（3D-LS） is presented. The 3DLS is based on existing high-resolution laterolog array and azimuthal resistivity imaging sondes with radial, longitudinal, and circumferential detection abilities. Six investigation modes are designed using the 3D finite-element method and different investigation depths are simulated based on the pseudo-geometrical factor of the six modes. The invasion profile is described using multi-array radial logs. From the analysis of the pseudo-geometrical factor, the investigation depth of the 3D-LS is about 1.5 m for conductive invasion, which is close to that of the dual laterolog tool but greater than that of the highly integrated azimuthal laterolog sonde. The vertical and azimuthal resolution is also analyzed with the same method. The 3DLS can detect low-resistivity anomalies of 0.5 m thickness and 15？ around the borehole for infinitely thick formations. This study lays the foundation for more work on 3D laterolog array sonde for evaluating low-resistivity anomalies.
