斜井层状介质孔中时间域电磁张量响应特征

在石油勘探、金属矿找矿以及地下空间探测领域,孔旁远探和随钻前探是迫切性技术需求。结合三分量感应测井和瞬变电磁场理论,提出了一种孔中时间域电磁张量测井方法。基于谐变场三维感应测井9个磁场分量求解方法,采用传输线法实现并矢格林函数的快速计算,通过将谐变场的磁张量转换到时间域,获得孔中时间域电磁场张量计算公式; 然后,建立一个斜井三层介质模型,分析该模型下感应电位张量各分量的时间衰减规律,得到目标层电磁张量响应特征; 最后,在地-井瞬变电磁实测资料分析基础上,比较孔中张量测井方法的效果。结果表明:感应电位张量xz、yz、zx以及zy分量衰减曲线符合垂直磁偶源激励场特征,其他分量满足水平磁偶源激励场特征; 与z相关的分量对地层电性界面的反映最为明显,可以作为张量解释的首选分量,其他分量可以作为辅助分量; 低阻目标层在多测道曲线和感应电位张量时间剖面中得到清晰识别,为孔中张量电磁资料处理与解释提供了手段。

Tensor Response Characteristics of Time-domain Electromagnetic Field in a Layered Medium with a Deviated Well

It urgently demands wellside remote and well advance detection technique in petroleum exploration and metallic minerals prospecting, as well as underground space detection. Combining the theory of three-component induction logging and transient electromagnetic field, a time domain electromagnetic tensor logging method was proposed. Based on the solution of the 9 components of alternating magnetic field in 3D induction logging, a transmission line algorithm was adopted to fastly solve Dyadic Green’s function. Hence, the theoretical formula was obtained for time domain electromagnetic tensor field by converting magnetic tensors into time domain. The attenuation versus time for all of the induced voltage components was analyzed by making use of a three-layered medium with a deviated well, and the tensor electromagnetic response characteristics of the target layer was obtained. Eventually, the effect of time domain electromagnetic tensor measurement was testified on the basis of downhole transient electromagnetic data. The results show that the attenuation curves for xz, yz, zx and zy components of induced voltage tensor have an electromagnetic feature excited by a vertical magnetic dipole, while the rest of components meet with the feature excited by a horizontal magnetic dipole; the voltage components related to z component can be used as preferred parameters for their significant resolution to the electrical interface of strata, while the others can be considered as auxiliary parameters; the low resistivity target layer can be clearly identified in the multi-channel curves and induced voltage tensor time sections, which provide an effective tool for the measured data processing and interpretation.