方位侧向电阻率成像随钻测井仪软聚焦仿真分析

Simulation Analysis of Soft Focusing of Azimuth Laterolog Resistivity Imaging LWD Tool

随钻方位侧向仪器工作时,由于钻铤长度较长,使表面电位衰减严重,钻铤流出的电流会产生轴向分量,不能严格垂直于钻铤表面流出,导致测量结果不准确。针对该问题,根据方位侧向电阻率成像随钻测井仪(RIT)的电极结构,设计5种软聚焦工作模式,并给出不同工作模式下的方位侧向电极电流和视电阻率的计算表达式;软聚焦工作模式可模拟不同源距的对称天线同时工作,以保持钻铤表面等电位,并通过线性叠加差值近似的方法获得更精确的轴向电流。通过三维有限元方法计算不同软聚焦工作模式在渐变薄互层的响应,并与原始测井响应进行对比,结果表明软聚焦方法能平滑单个发射天线界面处存在的尖峰现象及台阶现象,对薄层的识别及视电阻率计算更加准确,对仪器实际测井数据资料解释有一定的指导意义。

When logging while drilling directional lateral instruments work in formations,due to the long length of the drill collar,the surface potential attenuation is severe.The current flowing out of the drill collar will produce axial components,which cannot flow out strictly perpendicular to the surface of the drill collar,resulting in inaccurate measurement results.To address this issue,five soft focusing working modes are designed based on the electrode structure of the azimuth laterolog resistivity imaging logging tool(RIT),and expressions for calculating the azimuth lateral electrode current and apparent resistivity under different working modes are provided.The soft focusing working mode can simulate the simultaneous operation of symmetric antennas with different source distances to maintain the surface equipotential of the drill collar,and obtain more accurate axial current through linear superposition difference approximation.By using the three-dimensional finite element method to calculate the response of different soft focusing working modes in gradually changing thin interlayers,and comparing it with the original logging response.The results show that the soft focusing method can smooth out the peak and step phenomena at the interface of a single transmitting antenna,and is more accurate in identifying thin layers and calculating apparent resistivity.It has certain guiding significance for the interpretation of actual logging data of instruments.