1D subsurface electromagnetic fields excited by energized steel casing

Abstract

We use numerical simulations to investigate the possibility of enabling steel-cased wells as galvanic sources to detect and quantify spatial variations of electrical conductivity in the subsurface. The study assumes a vertical steel-cased well that penetrates electrically anisotropic horizontal layers. Simulations include a steel-cased vertical well with a finite-length thin wire of piecewise-constant electric conductivity and magnetic permeability. The steel-cased well is energized at the surface or within the borehole at an arbitrary depth with an electrode connected to a current source of variable frequency. Electromagnetic (EM) fields excited by the energized steel-cased well are simulated with an integral-equation approach. Results confirm the accuracy of the simulations when benchmarked against the whole-space solution of EM fields excited by a vertical electric dipole. Additional simulations consider a wide range of frequencies and subsurface conductivity values for several transmitter-receiver configurations, including borehole-to-surface and crosswell. The distribution of electric current along the steel-cased well is sensitive to vertical variations of electric conductivity in the host rock. In addition, numerical simulations indicate that crosswell and borehole-to-surface receiver configurations could reliably estimate vertical variations of electric conductivity within radial distances of up to [Formula: see text] for frequencies below [Formula: see text] and for average host rock electric conductivities below [Formula: see text].