Earth Resources Laboratory :: Abstracts

A Study of Seismoelectric Signals in Measurement While Drilling

Xin Zhan

Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on September 9, 2005 in partial fulfillment of the requirements for the degree of Master of Science

Abstract

An LWD acoustic wave can move the excess charge in the electric double layer along the borehole wall to generate a streaming electric field. This thesis is an experimental and theoretical investigation of the electric field induced by the multipole LWD acoustic wave. The main goal of this thesis is to understand the mechanism in the seismoelectric conversion under the LWD geometry and prove the absence of the tool mode in the LWD-acoustic-wave induced electric signals.

In this experimental study, we measured the seismoelectric signals excited by an acoustic multipole source in the scaled logging-while-drilling model. We put the scaled tool in a sandstone borehole to perform LWD seismoelectric and acoustic measurements. Monopole and dipole acoustic and the induced electric signals were recorded separately under exactly the same settings. The recorded acoustic and seismoelectric signals were analyzed in both time and frequency domains using a semblance method.

We found no tool mode components in the electric signals by examining both the waveforms and the time and frequency domain semblances. The underlying mechanism is the electric double layer (EDL) at the steel water interface is much weaker than the one at the formation water interface. When the tool is electrically grounded, there can be no excess charge at the tool surface which is always a zero potential plane. Thus, in the LWD seismoelectric signal, there should be no component with an apparent velocity of tool mode. Since only formation acoustic modes have their corresponding components in the electric signal, we calculated the coherence of the two kinds of signals in the frequency domain. By applying the coherence curve to filter the acoustic signals, we can eliminate the tool modes and pick out the formation acoustic modes.