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Borehole Electroseismic Phenomena: Field Measurements and Theory

Oleg Mikhailov

Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on July 27, 1998 in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Abstract

A Stoneley wave propagating in a borehole generates a flow of pore fluid in permeable zones intersected by the borehole. In turn, this flow of pore fluid induces a streaming electrical field. This thesis is an experimental and theoretical investigation of the electrical fields induced by Stoneley waves. The main emphasis of this thesis is to understand the electroseismic phenomena that are observed in the field.

In the first experiment described in this thesis, we measured Stoneley wave-induced electrical fields in a borehole drilled through fractured igneous rocks. Analysis of field data confirms that the electrical fields that we measured were induced by fluid flow in fractures. The normalized amplitude of these electrical fields correlated with the fracture density log.

In the second experiment, we measured Stoneley-wave-induced electrical fields in several boreholes in vuggy dolomite. In dolomite, the normalized amplitude of the Stoneley-wave-induced electrical field correlates with the porosity of the formation around the borehole. Further, the Stoneley-wave-induced electrical fields have anomalously high amplitudes at an isolated fracture that intersected two boreholes.

To explain the experimental results, we developed a theoretical model for the Stoneley-wave-induced electrical fields. According to the model, the normalized amplitude of the Stoneley-wave-induced electrical field is proportional to the porosity and inversely proportional to the pore space tortuosity of a formation around a borehole. Moreover, the amplitude-versus-frequency behavior of this electrical field depends on the permeability of the formation.

To further test the theory's prediction, we measured electrical potentials induced by the borehole Stoneley wave in the frequency range from 100Hz to 4kHz. The normalized amplitudes of the Stoneley-wave-induced electrical potentials measured in the field were consistent with the amplitudes predicted by the theory. Also, the amplitude-versus-frequency dependence of the electroseismic signals recorded at the depth of the large fracture roughly followed the trend predicted by the theory. However, the general amplitude-versus-frequency dependence of the electroseismic signals recorded in the field is more complicated than that predicted by the theory.

The main contributions of this thesis are:

1. This thesis develops a borehole electroseismic measurement technique and demonstrates that it works in the field. This technique can be used to make electroseismic logging measurements.
2. This thesis investigates an electrical field induced by a borehole Stoneley wave. This electroseismic phenomenon is explained, measured in the field and modeled theoretically.
3. This thesis derives from field data a parameter that describes local electroseismic coupling in a formation around a borehole. This parameter, the normalized amplitude of theStoneley-wave-induced electrical field, is defined as the ratio of an electrical field amplitude to a pressure amplitude in the Stoneley wave at a certain depth. This thesis demonstrates that the normalized amplitude of the Stoneley-wave-induced electrical field can be used to identify permeable fractures in situ.
4. This thesis uses field electroseismic measurements to quantitatively characterize rock formations around a borehole. Using the theoretical model developed in this thesis, a porosity log for fractured granite is derived from electroseismic field data.