Summary
When a seismic wave propagates in a fluid-saturated porous medium, a relative movement forms between the solid and fluid and induces an electric current due to the electronic double layer. As a result, two kinds of seismoelectric coupling responses are generated in this procedure, i.e. the localized electric/magnetic field and interfacial electromagnetic wave field. One important potential application of these two seismoelectric conversions is used for measuring formation P and S waves in well logging. Considering that the strong collar wave seriously affects the velocity measurements of formation P and S waves in current acoustic logging while drilling (LWD), the seismoelectric logging while drilling method, which combines seismoelectric conversion and acoustic LWD technique, was suggested to be a novel method in oil and gas exploration. Because the collar wave can't induce any seismoelectric signal on the metal collar, since there is no double layer formed on a metal surface. In this paper, acoustic and seismoelectric LWD measurements are conducted in the laboratory. We build a scaled multipole acoustic LWD tool to conduct acoustic measurements in a water tank and a sandstone borehole model. We also build a multipole seismoelectric LWD tool and record the seismoelectric signals induced with the same acoustic source. Then we compare the recorded acoustic and seismoelectric signals by using the experimental data. The result indicates that the apparent velocities of seismoelectric signals are equal to the formation P and S wave velocities and the collar waves do not induce any visible electric signal in the full waveforms. We further analyze the mechanism of seismoelectric LWD by a quantitative comparison of the amplitudes between the inner collar wave and outer collar wave. The results show that the amplitude of outer collar wave decreases significantly when it radiates out of the tool, so that the seismoelectric signals induced by collar waves are too weak to be distinguished in the full waveforms of seismoelectric LWD measurements. Thus, the formation P and S wave velocities are detected accurately from the recorded seismoelectric LWD data. These results verify the feasibility of seismoelectric LWD method for measuring acoustic velocities of the borehole formation.
