Effect of well casings on surface electrical surveys

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Abstract
Surface electrical surveys are being used as alternative oil exploration tools in both developed and frontier areas. We have examined the resistivity‐IP response of steel oil well casings, which are common in developed oil fields. Two numerical models were constructed: a solution of Laplace’s equation in the spatial wavenumber domain, and an integral equation approach using the Galerkin weighted residual technique, which simulate hollow, infinite‐length cylinders and finite‐length cylindrical shells, respectively, embedded in a uniform half‐space. Induced‐polarization effects can be simulated in these models by including a complex, frequency‐dependent surface impedance on the outer surface of the cylinder. Significant anomalies are generated by pipes having physically realistic dimensions and conductivities. Important parameters governing the response are the longitudinal conductance, length and radius of the pipe, and the number of pipes and their distances from the survey line. Comparison of field data with model results suggests that a significant part of the observed response may be due to well casings. However, more complicated models including electrical connections at the surface and earth layering are required before accurate modeling of many situations will be possible.

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