Seismoelectric Wave Measurements in Borehole Models

Abstract:

An experimental system was built in the laboratory based on the electrokinetic theory, which contains a small scaled seismoelectric detector and a high resolution digitizer ( 1 MS/s, 22 bits ). The electrokinetic measurements are carried out with seismoelectric well logging technique in borehole models at high frequency (90 kHz), and the localized electrokinetic fields that accompany compressional wave, shear wave and Stoneley wave are clearly observed with monopole source in two sandstone models that are saturated by tap water. The magnitudes of these seismoelectric waves are in the range of 1-100 microvolt, which is useful for designing the seismoelectric logging instruments. The experimental results also show that the seismoelectric well logging signals are related to the permeability of borehole formations. Their amplitudes become larger in the high permeability model, which can be used to measure the permeability of rock formation although no such relationship has ever been provided in existing theories. We also made seismoelectric measurements in a lucite borehole model, but no observable seismoelectric signals were recorded by the electrode. This is not out of our expectation because the lucite formation is not porous and no electrokinetic conversion occurs in such material. However, the electric signal recorded in the Lucite borehole represents the background noise of our measurement system, which is less than 0.5 microvolt. This study verifies the feasibility of seismoelectric well logging, and also presents the range of seismoelectric signals in borehole saturated by tap water that is much closer to the condition of actual formation.