Seismoelectric measurements in rock samples and borehole models

Abstract

A seismic wave propagating in a fluid-saturated porous rock generates relative movement between the rock matrix and the fluid. Because of the electric double layer between fluid and rock, the moving charge produces an electric field. The magnitude of the induced electric field (i. e. seismoelectric conversion) depends on rock and fluid properties, pore geometry, and seismic wave frequency. To study the seismoelectric coupling coefficients, we conducted a series of laboratory experiments in several kinds of rock samples using transient seismic waves and measuring the electric field. Experimental results show that borehole acoustic waves generate seismoelectric fields in fluid-saturated formations. The seismoelectric fields can be detected by an electrode in the borehole. The amplitude of the seismoelectric field is related not only to the seismic wave, but also to the properties of formation such as permeability, conductivity, etc. For example, the seismoelectric conversion increases as the porosity and permeability of the rock samples increase. Seismoelectric and seismomagnetic fields generated by seismic waves in fluid-saturated fractured borehole models are experimentally investigated with an electrode and a Hall-effect sensor. In a borehole with a horizontal fracture, the Stoneley waves induce seismoelectric and seismomagnetic fields in the borehole and an electromagnetic wave propagates at light speed along the borehole.