A Study of Seismoelectric Signals in Measurement While Drilling By Xin Zhan

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