Studies with gel-filled sandstone samples with implications for the origin of induced polarization
A. Hördt and S. Milde
Issue: Vol 10, No 6, December 2012 pp. 469 - 478
Special Topic: Induced Polarization for Near-surface Investigations
Info: Article, PDF ( 2.48Mb )
The origin of induced polarization (IP) in the pore space of sediments is not completely understood. Existing theories may be separated into two groups. The electrochemical polarization mechanism requires only an electrical double layer around the sediment grains, with the grain size determining the spatial scale. The membrane polarization mechanism is based on a coupling between pores of different sizes and ion mobilities to produce a frequency-dependent conductivity. 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 conductivity spectra of the gel-filled sandstone samples are distinctly different from those of water-filled samples. The phase shifts decrease, the spectrum is flattened and the maximum moves towards higher frequencies. In terms of Cole-Cole parameters, the gel decreases the chargeabilities and decay times. The effect can be clearly separated from that of increasing water salinity. We conclude that ion mobility in large pores is an important factor for the generation of the IP-effect. Whereas the decrease in phase shift by the gel can be explained with both electrochemical and membrane polarization, the observed decrease in relaxation times seems to be inconsistent with either type of theories.