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Effect of Resistivity Anisotropy in Cased Hole Surface-Borehole EM Surveys
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 81st EAGE Conference and Exhibition 2019, Jun 2019, Volume 2019, p.1 - 5
Abstract
Summary
In surface-to-borehole and borehole-to-surface electromagnetic methods, a hybrid measurement system is implemented where a surface or downhole source is used to energize the subsurface, and the field distribution is measured along the wellbore or on the surface. Under ideal experimental conditions, these methods provide higher sensitivity to changes in formation resistivity than surface-based methods (e.g., land controlled-source electromagnetic). In practice, downhole electric field measurements can only be done in an open hole, underneath the casing shoe, while borehole antennas can be deployed inside or below the steel casing. In either case, current is channeled along the casing medium and, thereby, the formation is effectively energized by a distributed source. Interpreting the measurements requires understanding the distortion introduced by the casing and the interaction of the metallic medium with a generally anisotropic surrounding medium. We present a review of the effect of the steel casing in a representative 1D scenario of an electrical anisotropic formation in a surface-to-borehole configuration, and the analysis is expanded by deriving closed-form expressions of the fields due to a source embedded in an infinite homogeneous and anisotropic surrounding medium.
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