Studies on the Origin of the IP-effect with Gel-filled Sandstone Samples

The origin of the induced polarization in the pore space of sediments is not completely understood. There are two groups of theories, which are either based on the grains, or on the structure of the pore space. We have carried out complex electrical conductivity measurements on gel-filled sandstone samples with different gel concentrations and fluid salinities. The idea is to reduce the ion mobility in the pore space, allowing to test hypotheses resulting from the different theories. The electrical conductivity of pure gel increases with gel concentration, indicating an increase in ion density. Thus, adding gel to a saline fluid reduces ion mobility, but does not completely destroy it. The conductivity spectra from the gel-filled sandstone samples are distinctly different from water-filled samples. The phase shifts decrease, the spectrum is flattened, and the maximum moves towards higher frequencies. We conclude that the geometry of the pore space, and the mobilities in the large pores, are important factors for the generation of the IP-effect. The reduction of the IP-effect by a reduction of mobilities is consistent with a quantitative theory which is based on the pore space geometry.