Applying squeezing technique to clayrocks: lessons learned from experiments at Mont Terri Rock Laboratory

Abstract

Knowledge of the pore water chemistry in clayrock formations plays an important role in determining radionuclide

migration in the context of nuclear waste disposal. Among the different in situ and ex-situ techniques for pore water

sampling in clay sediments and soils, squeezing technique dates back 115 years. Although different studies have been

performed about the reliability and representativeness of squeezed pore waters, more of them were achieved on highporosity,

high water content and unconsolidated clay sediments. A very few of them tackled the analysis of squeezed

pore water from low-porosity, low water content and highly consolidated clayrocks.

In this work, a specially designed and fabricated one-dimensional compression cell two directional fluid flow was

used to extract and analyse the pore water composition of Opalinus Clay core samples from Mont Terri (Switzerland).

The reproducibility of the technique is good and no ionic ultrafiltration, chemical fractionation or anion exclusion was

found in the range of pressures analysed: 70-200 MPa. Pore waters extracted in this range of pressures do not decrease

in concentration, which would indicate a dilution of water by mixing of the free pore water and the outer layers of

double layer water (Donnan water). A threshold (safety) squeezing pressure of 175 MPa was established for avoiding

membrane effects (ion filtering, anion exclusion, etc.) from clay particles induced by increasing pressures. Besides, the

pore waters extracted at these pressures are representative of the Opalinus Clay formation from a direct comparison

against in situ collected borehole waters.