Casing Effect in Borehole-surface (Surface-borehole) EM Fields

In this report the response of a steel casing in a borehole-surface (surface-borehole) source receiver configuration is analyzed using a 2D azimuthally symmetric code. It was demonstrated that the effect of the casing dominates the response at close distances from the pipe (<500 m), even when the source is placed well below the casing shoe (~ 100 m). The effect decreases with increasing radial offset, and it is found that for far enough distances (>500m), and deep enough sources (>100m below the casing) the response of the formation dominates over the distorting casing fields. It has been shown that the casing effect can be described by superimposing the field arising due to the vertical current induced in the pipe. This has been estimated as a function of depth, and its amplitude has been found to decrease exponentially along the length of the casing, away from the casing shoe. Its spatial rate of decay depends on the conductivity of the background formation, providing a leaking path for the current. The strength of the current induced on the pipe attenuates faster in a conductive medium than in a resistive one, due to the dependence on conductivity of the radial current density.