带钢套管的可控源电磁三维响应计算与分析

近年来，井中电磁法勘探在油气工业中的应用备受关注，但完井时钢套管的带入使激发的电磁场信号产生强烈畸变，常规的可控源电磁正反演处理及解释技术是不适用的。目前，关于钢套管效应的研究是电磁法研究的热点问题。该项目就带钢套管的可控源电磁三维响应正演进行研究，可以从理论上分析钢套管对电磁响应的影响规律。具体的研究内容包括常规可控源电磁三维正演、三维模型下钢套管上电流密度分布的计算及带钢套管的可控源电磁三维正演。利用TetGen工具生成非结构化四面体网格，结合矢量有限元对可控源电磁双旋度方程进行离散求解，实现常规可控源电磁三维响应的计算；采用矩量法，将钢套管剖分为一系列单元，结合欧姆定律及基尔霍夫电流定律求解钢套管上电流密度的分布；进一步利用互惠原理可计算钢套管产生的二次电磁响应。该项目的研究可为井中电磁观测设计提供理论依据，为井中电磁数据资料解释打下基础，将进一步促进电磁法技术在油气工业中的应用。

In recent years, extensive concerns are attracted by the application of the borehole electromagnetic (EM) methods in the oil and gas industry. Nevertheless, the existing of steel casing after borehole completion will distort the EM signal significantly, making it inappropriate for the numerical modeling, processing and interpretation techniques of the regular controlled-source EM methods. Presently, it is one of the research highlights in EM regarding the casing effects. The project researches on the calculation of the 3D controlled-source EM responses with a steel-cased borehole. By doing this, the casing effects in 3D space can be analyzed. Detailed research aspects contains the 3D forward modeling of the controlled-source EM method, the calculation of the distribution of the current densities along the casing within a 3D model, and the 3D forward modeling with a steel casing. The TetGen tool is used to generate unstructured tetrahedral mesh. The vector finite element is utilized to solve the 3D curl-curl equation. Thus, the 3D controlled source EM response is obtained in this way. The steel casing is separated into a series of elements, and its current density distribution is solved by the joint application of Method of Moments, Ohm’s Law and Kirchhoff’ s Current Law. Furthermore, the secondary EM response generated by the steel casing is calculated by the application of reciprocity. The project could guide the design of borehole EM measurements, act as the basis for the geophysical interpretation of borehole EM data, and could accelerate the application of EM methods in oil and gas industry.