On the effect of the metal casing in surface-borehole electromagnetic methods

We analyze the electromagnetic fields arising in the vicinity of a vertical steel casing due to the excitation of a vertical electric field antenna placed in the borehole below the casing shoe. Numerical simulations of the fields were used to investigate the distorting effect of the casing on surface observations and in turn to define a distributed dipolar source to independently model the secondary fields arising due to the casing. The analysis indicates that the casing effect is mainly galvanic in nature, dominated by the conductivity of the metallic medium, with changes of the magnetic properties causing only small variations to the fields observed on the surface. Variations of the casing thickness also result in small perturbations to the fields; however, as the thickness decreases, the amplitude of the fields is first observed to increase and then to decrease. These observations are further investigated using approximate closed-form expressions derived for the surface impedance of the casing. The current induced on the casing was analyzed as a function of the resistivity distribution of the surrounding formation. The secondary field arising due to the presence of the casing was shown to be reproduced approximately by an array of dipole sources describing the continuous distribution of the current along the pipe. A method was tested in which the current was evaluated in a simplified configuration of the surrounding formation to reduce the computational burden. The approximate equivalent dipole array is then used to compute the open-hole response representative of the secondary casing response, which is in turn superimposed to that of the primary source located below the casing shoe. The resulting fields were found to describe the true response of the system very well, including the metallic casing.