Insights on electromagnetic scattering by steel casings in surface-to-borehole and borehole-to-surface methods

Abstract

The development of surface-hole electromagnetic technologies has gained renewed interest in recent years. This is motivated by the method's capability to provide the sensitivity of the resistivity distribution in a hydrocarbon reservoir and its variation over time associated with fluid displacement. However, it has been long recognized that data acquired in a realistic experimental setting will carry a distorting component to the measured fields, superimposed to those expected in an idealized open-hole setup. Investigating the processes driving the casing effect can lead to properly identifying distortions of the measurements and ultimately defining adequate strategies to deal with the inversion of casing-contaminated data sets. In this paper, I discuss the main aspects of the electromagnetic physics behind the conceptual model and describe current channeling and radial flow arising when the formation is excited by sources embedded inside the pipe. I revisit these concepts to explain measurement distortion of receivers placed in the vicinity of the casing, describe measurement sensitivity of resistive targets in surveys deploying casing antennas, and review suitable strategies for the inversion of casing-distorted data. I also briefly outline active topics of research intended to provide further insight to better understand the casing effect.