水平层状介质孔中张量瞬变电磁响应的解析分析

在感应测井和瞬变电磁场理论基础上, 采用并矢格林函数计算三维感应测井中的9个谐变场分量, 应用余弦变换数值滤波法将磁张量转换到时间域, 得到孔中时间域张量电磁场理论公式, 提出了孔中时间域电磁张量测量方法.建立了竖井井旁层状介质模型和钻遇地层模型, 从理论上获得了井中不同深度处的时间域电磁张量响应, 初步验证了该方法在井旁远探和随钻前探中的可行性.研究结果表明: 孔中测点所处深度越深需要的采样时间窗口越宽; 与z相关的感应电位分量对目标层的识别能力更高, 可以作为井旁远探的首选参数; xx分量和zz分量异常响应较为明显, 可分别作为随钻前探的首选分量和辅助分量.多测道曲线和截距剖面能明显反映出目标层界面特征, 可以为资料处理与解释提供帮助.此外, zz分量可识别出高阻目标层, 显示出了方法的另一优势.研究成果可为电磁张量测井技术与装备研发提供理论参考.

Analytical analysis of tensor response for borehole transient electromagnetic method in a horizontal-layered medium

Based on induction logging and transient electromagnetic theory, a time domain electromagnetic tensor logging method was proposed. Dyadic Green's function was used to solve the 9 components of alternating magnetic field in 3D induction logging, and the magnetic tensors were converted into time domain with cosine transform of numerical filtering method, hence the theoretical formula was obtained for time domain electromagnetic tensor calculation. By building two straight well models of wellside layered medium and drilling formation, time domain electromagnetic tensor response at different depths was acquired, which initially verified the feasibility in wellside remote and well advance detection. The results indicate that, it needs wider sampling time window at deep observation points in borehole. The induced voltage components related to z component has a high sensitivity to target strata identification, which could be used as preferred parameters for wellside remote detection. Meanwhile, xx component and zz component, due to their strong anomaly responses, can be considered as a preferred parameter and an auxiliary parameter, respectively. Besides, the anomaly responses of wellside strata can be identified in multi-channel curves and intercept sections, the kinds of images are useful to data processing and interpretation. In addition, the method has an ability to distinguish the high resistivity layer. The study provides a theoretical preference to research and development of tensor logging technique with TEM.