Modeling Of Seismoelectric Effects In A Borehole

Abstract

We present a method to simulate the propagation of seismic and converted electromagnetic waves generated by a mechanical borehole source embedded in a layered poroelastic medium. The electroseismic conversions occur at both the borehole wall and the layer boundaries. Most studies in electroseismic effects have been modelled and tested with seismic sources and detectors (geophones and antennas) at the surface. In this paper, we investigate the case of a seismic source in a borehole and receivers either at the surface or embedded in the medium. The method is formulated as a boundary element technique (where the poroelastic displacement and relative flow Green's functions are calculated by the discrete wavenumber method. The singular properties of the Green's functions are determined analytically using static Green's functions to regularize the integrals. This is necessary to calculate the element's self interaction. The borehole is cylindrical and its axis rulls normal to the interfaces. The coupled electroseismic effects in the layered media are included by using the global matrix technique. The developed method is an extension of the model of Biot-Rosenbaum, who applied the wavenumber integration technique to investigate the effect of formation permeability on Stoneley waves, using Biot's theory to model the wave propagation effects of a homogeneous permeable formation surrounding a borehole. We extend the Biot-Rosenbaum model by including the effect of a heterogeneous permeable formation surrounding the borehole. The effect of formation permeable zones (or fractured zones) on Stoneley waves can now be investigated. The other modification is the inclusion of conversions of mechanical into electromagnetic waves at mechanical and/or electrical contrasts in the poroelastic formation. The converted electromagnetic fields are sensitive to large permeability contrasts and fluid chemistry contrasts inside a reservoir. Using the electroseismic method downhole will provide more information about permeability/permeability contrasts in the formation, as well as additional lithological information (salinity of the fluids).