Laboratory determination of chloride diffusion coefficient in an intact shale

Abstract

An experimental investigation of diffusive transport of a nonreactive solute (chloride) in saturated, intact Queenston Shale is described. Laboratory tests were performed by placing distilled water in contact with samples of shale having a high initial concentration of chloride in their pore water. Chloride was then permitted to diffuse out of the shale and into the distilled water reservoir for a period of up to 65 days. At the end of each test, the shale sample was sectioned to determine the variation in chloride pore-water concentration with depth through the sample. Fickian diffusion theory was then used to deduce the diffusion coefficient (D). The experimental diffusion coefficient for chloride at a temperature of 22 ± 1 °C ranged from 1.4 × 10−6 to 1.6 × 10−6 cm2/s, which corresponds to a tortuosity (τ) ranging from 0.095 to 0.108. Based on pore size measurements and consideration of the ionic diameter of hydrated chloride, the "effective porosity" available for chloride diffusion is estimated to be greater than 75% of the total porosity calculated from the moisture content of the shale. Key words: diffusion, chloride, rock matrix, Queenston Shale, laboratory study.