Intrinsic Negative Chargeability of Soft Clays

Abstract

Lithologies with high clay contents can have negative chargeability. This response is usually attributed to geometric effects in induced polarization and electromagnetic surveys. Negative chargeability has also been noted in laboratory studies, but has remained unexplained. Traditional membrane polarisation models are of a series of membrane constrictions within a solid substrate, but this model produces only positive chargeability responses. Alternate soft clay polarisation model are considered, involving bulk sample ionic fluxes. These fluxes have been previously studied in electrokinetic dewatering and decontamination studies, but over longer time periods. Of particular note are the large pH gradients that form under an applied electric field. Relaxation of these gradients at the sample's surface produces a potential in the opposite sign to the applied potential, and so produces negative chargeability. Laboratory electrical impedance spectroscopy data is presented. The experimental data covers a variety of clay types at a variety of water contents. For pure clay samples, a relationship exists between negative chargeability and water content (as a percentage of liquid limit). This trend is not apparent in natural clays. Field measurements of negative chargeability are presented. Logging data through a soft clay lithology is shown to discriminate between clay and sand layers.