A Study of Seismoelectric Signals in Measurement While Drilling (M.S. Thesis)
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zhan_msthesis06.pdf
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Author(s)
Zhan, Xin
Date Issued
September 2005
Publisher
Massachusetts Institute of Technology. Earth Resources Laboratory
Series/Report no.
Earth Resources Laboratory Industry Consortia Annual Report;2006-15
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. 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.
In the theoretical study, we developed a Pride-theory-based model for the
LWD-acoustic-wave induced electric field. The electric field strength is calculated at the electrode positions along the borehole wall, analogous to what was done in the experiment.
The electric boundary conditions, which are the continuity of the electric field at the
borehole wall and disappearance at the LWD tool surface, reveal the underlying mechanism
in the LWD sismoelectric conversion which is also the basis of our lab experiment. The
absence of the tool modes in the synthetic waveforms of the electric field coincides with
what we have observed in the experimental study.
Both the experiment and the theoretical results confirm that measuring the seismoelectric
signal generated by an acoustic multipole source during the LWD process can be an
effective way to eliminate the tool wave contamination on the LWD acoustic measurements.
This thesis research shows that seismoelectric logging-while-drilling may be a potential
new method in formation property evaluation.
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