Effect of water inclusions on charge transport and polarization in porous media

Wideband dielectric spectroscopy measurements (100 Hz to 10 GHz) were carried out on different microporous systems (including sintered glass filters, sandstone and carbonate rocks) saturated with brine (electrolytic water solution) and a hydrocarbon oil at different relative fractions. Three main contributions were singled out in the dielectric spectra: low frequency dispersion (LFD) effects (<100 kHz) related to long range ionic transport and dependent on the connectivity of the water phase; Maxwell-Wagner-Sillars (MWS) polarization effects (100 kHz to 1 GHz), essentially controlled by the shape of the water inclusions; and high-frequency effects (>1 GHz) due to short-range ion transport and related to specific pore surface. For each analyzed system, oil-wet (o-W) and water-wet (w-w) samples were obtained by chemical treatment of the pore surfaces. Systematic differences were observed in the electrical response on varying wettability (and so the respective distribution of the fluids in the pore space). Parameters such as the loss tangent value, the strength and the exponent of the LFD power law, as well as the characteristic time and strength of the MWS relaxation were good indicators of wettability. Results were discussed by using some standard models that account for the pore wettability effects on dielectric spectra. A new method for wettability determination of reservoir rocks from both laboratory and borehole electric measurements was presented.