圆柱坐标系下三维瞬变电磁过套管响应特征研究

近年来，瞬变电磁过套管技术已成为电磁勘探领域的热点之一。由于井孔及地层非均匀介质的电导率和磁导率在小尺度（套管、泥浆等）上剧烈变化，为模拟并消除金属套管对电磁场的影响，有必要开展高效的过套管三维瞬变电磁正演模拟。正演计算中，为了有效减少网格数量进而降低方程组的求解规模，在圆柱坐标系下对求解区域进行离散，并基于有限体积法进行三维套管井瞬变电磁正演模拟。通过模拟并分析套管电导率、相对磁导率、厚度和内径等参数对电磁场的影响，为校正金属套管影响奠定了基础；对梯形波、半正弦波、三角波响应结果的对比分析表明发射波形采用梯形波的效果最好；分析梯形波各阶段持续时间对电磁响应结果的影响，表明较短的波形稳定时间对早期响应结果的影响明显，开启时间对响应结果的影响可以忽略，关断时间越短则电磁响应越强。研究成果为瞬变电磁过套管数据处理提供了技术指导。

Research on characteristics of 3D transient electromagnetic response through casing in cylindrical coordinate system

In recent years, transient electromagnetic through-casing technology has become one of the hotspots in electromagnetic prospecting. Since the conductivity and magnetic permeability of inhomogeneous media in wellbores and formation change drastically on small scales (casing, mud, etc.), efficient forward modeling for three-dimensional (3D) transient electromagnetic through casing is greatly important to simulate and eliminate the influence of metal casing on the electromagnetic field. In the calculations of forward modeling, the solution region is discretized in the cylindrical coordinate system to effectively reduce the number of grids and thus reduce the solution scale of the equation system, and the 3D cased-hole transient electromagnetic forward modeling is realized on the basis of the finite volume method. By simulating and analyzing the influence of parameters such as casing conductivity, relative magnetic permeability, thickness, and inner diameter on the electromagnetic field, a foundation is laid for correcting the influence of metal casing. The comparative analysis of the response results of trapezoidal waves, half-sine waves, and triangle waves de-monstrates that the transmitting waveform using trapezoidal waves performs the best. Moreover, the analysis on the influence of the duration of each stage of trapezoidal waves on electromagnetic response results reveals that the shorter waveform stabilization time has a significant impact on the early response results; the impact of the turn-on time on the response results can be ignored, and shorter turn-off time leads to stronger electromagnetic response. The research conclusions can provide technical support for research on transient electromagnetic through-casing data processing technology.