Accounting for the effects of pore fluid chemistry on spectral induced polarization (SIP) measurements: the specific polarizability concept

Recent spectral induced polarization (SIP) research has advanced our understanding of the controls of the physical and hydraulic properties of porous media on both the polarization magnitude and relaxation length scales in porous media. A critical current challenge is to improve our understanding of how pore fluid chemistry modifies the interfacial polarization measured with the SIP technique. We report results from two laboratory-scale experiments designed to advance this understanding. In the first experiment, we analyzed the influence of electrolyte concentration and valence on the interfacial polarization of three sandstones with differing porosity and permeability. A Debye decomposition (DD) approach was used to determine normalized chargeability and average relaxation time from spectral data. We find that SIP measurements of the polarization magnitude (single frequency imaginary conductivity and normalized chargeability derived from the DD) of sandstone samples can be described by the product of the pore space related internal surface and a quantity that represents the polarizability of the mineral-fluid interface and depends on electrolyte concentration and valence. We introduce a new parameter, the specific polarizability, describing this dependence. In the second experiment, we investigated the effect of pH and hydroxyl ion concentration on the interfacial polarization of both silica gel and well-sorted sand. We find a strong dependence of the polarization on pH in the silica gel. Evidence for the same dependence exists for the sand, although the signal is only just above the noise threshold (~0.1 mrad) of the instrument. We relate the weaker signal observed in the sands to the much smaller pore space related internal surface relative to silica gel, a unique substance with surface area in excess of 500 m2/g. These observations suggest that the specific polarizability is also a function of pH, although the pH dependence is likely to be weak in SIP measurements of porous media characterized by an internal surface typical of natural unconsolidated rocks and soils. Our findings provide a framework for quantifying structural characteristics of porous media from SIP data in the presence of variable fluid chemistry.