Experimental and Theoretical Studies of Seismoelectric Effects in Boreholes

In a fluid-saturated porous formation, an impinging seismic wave induces fluid motion. The motion of fluid relative to the rock frame generates an electric streaming current. This current produces electric and magnetic fields, which are called seismoelectric and seismomagnetic fields, respectively. When there is a fracture or a discontinuity, a radiating electromagnetic wave is also generated, in addition to local fields. Seismoelectric and seismomagnetic fields depend on the amplitude, frequency, and mode of the seismic wave, as well as the formation porosity, permeability, pore size, and fluid conductivity. In this paper, we describe laboratory results of seismoelectric and seismomagnetic fields induced by an acoustic source in borehole models. We use a piezoelectric source for acoustic waves and a point electrode and a highsensitivity Hall-effect transducer for measuring the localized seismoelectric and seismomagnetic fields in fluid-saturated rocks. The dependence of seismoelectric conversions on porosity, permeability and fluid conductivity are investigated. Three components of the seismomagnetic field are measured by the Hall-effect transducer. At a horizontal fracture, the acoustic wave induces a radiating electromagnetic wave. Further, we conduct theoretical and numerical studies of electrokinetic conversions in fluid-filled boreholes. First, we derive the mathematical formulations for seismoelectric responses for an acoustic source in a borehole. Then we compute the electric field in boreholes penetrating formations with different rock compressibility, permeability, and porosity. We analyze the sensitivity of the converted electric fields to formation permeability and porosity. We find that the ratio of magnitude of electromagnetic waves to that of acoustic pressure increases with increasing porosity and permeability in fast and slow formations. Seismoelectric and seismomagnetic well logging might be a new means to determine formation properties in a borehole.