The Seismoelectric Coupling in Shale

Seismic explorations are not sensitive to the mineral properties of shale reservoirs. The seismoelectric conversion induced based on the mechanism of electric double layer can reveal the mineral properties. We investigated 14 shale samples (Sichuan Basin, China) based on seismoelectric coupling for the first time. Combining the acoustic velocity and XRD method, we analyzed the relationship between minerals, shale anisotropy and seismoelectric coupling. This study shows that extremely low-permeability shale (0.00038-0.00495mD) can produce seismoelectric signals as strong as those of high-permeability sandstone (0.91412-58.74323mD). The effects of clay minerals and non-clay minerals on seismoelectric coupling indicate that clay minerals can enhance seismoelectric conversions in shale. The illite in clay minerals has the greatest influence on seismoelectric conversion. Seismoelectric coupling is also used to characterize the shale anisotropy caused by clay minerals. The anisotropy presented by seismoelectric conversion is in proportion to that characterized by acoustic velocities, it increases from approximately 0 to 0.25 with anisotropy parameters ranging from approximately 0.1 to 0.5 for ε and 0.15 to 0.35 for γ. This feature of seismoelectric effects, which can reflect both the clay minerals and the anisotropy, is helpful for intuitive understanding of the anisotropy caused by clay minerals in shale reservoirs.