Osmotic flow through a Cretaceous clay in southern Saskatchewan, Canada

Abstract

Clays and shales can act as semipermeable membranes, causing osmotic flow and (or) osmotically induced fluid pressures. Despite laboratory proof of the osmotic behaviour of clay-rich geologic material, the influence of this osmotic behaviour on in situ groundwater flow and solute transport is inconclusive. This is due in part to the lack of laboratory experimental work involving undisturbed core samples, and to the lack of field-scale research. The ability of undisturbed clay samples to conduct flow hydraulically and osmotically was investigated using dilute salt solutions. Undisturbed Cretaceous clay samples from southern Saskatchewan were used in a laboratory experimental program. The experiments included constant-head hydraulic conductivity (Kh) tests and osmotic flow tests conducted over a range of pore-fluid concentrations of 0.054–1.12 equiv./L. The clay samples exhibited semipermeable membrane behaviour by conducting flow osmotically. The hydraulic conductivity of the clay increased by a factor of two as the pore-fluid concentration increased from 0.070 to 0.56 equiv./L. Osmotic efficiencies ranged from 0.0028 to 0.42 for concentrations from 0.84 to 0.096 equiv./L. Both the osmotic compressibility and osmotic efficiency decreased with increasing pore-fluid concentration. The behaviour of the clay was consistent with diffuse double-layer theory.Key words: osmotic flow, Cretaceous clay, salt, hydraulic conductivity.