Seismo-electric conversion in shale: experiment and analytical modelling

The development of seismo-electric exploration techniques relies critically upon the strength of the seismo-electric conversion. However, there have been very few seismo-electric measurements or modelling on shales, despite shales accounting for the majority of unconventional reservoirs. We have carried out seismo-electric measurements on Sichuan Basin shales (permeability 0.00147–0.107 mD), together with some comparative measurements on sandstones (permeability 0.2–60 mD). Experimental results show that the amplitudes of the seismo-electric coupling coefficient in shales are comparable to that exhibited by sandstones, and are approximately independent of frequency in the seismic frequency range (<1 kHz). Numerical modelling has also been used to examine the effects of varying (i) dimensionless number, (ii) porosity, (iii) permeability, (iv) tortuosity and (v) zeta potential on seismo-electric conversion in porous media. It was found that while changes in dimensionless number and permeability seem to have little effect, seismo-electric coupling coefficient is highly sensitive to changes in porosity, tortuosity and zeta potential. Numerical modelling suggests that the origin of the seismo-electric conversion in shales is enhanced zeta potentials caused by clay minerals, which are highly frequency dependent. This is supported by a comparison of our numerical modelling with our experimental data, together with an analysis of seismo-electric conversion as a function of clay mineral composition from XRD measurements. The sensitivity of seismo-electric coupling to the clay minerals suggests that seismo-electric exploration may have potential for the characterization of clay minerals in shale gas and shale oil reservoirs.