Gas migration through water-saturated bentonite-sand mixtures, COx argillite, and their interfaces

France's deep-seated nuclear waste repository consists of a natural barrier located at a depth of 500 m in a Callovo-Oxfordian clayey formation. This repository has artificial barriers that include plugs of swelling clay (MX80 bentonite - sand mixtures) for sealing purposes. This paper focuses on the gas migration properties of water-saturated bentonite-sand mixtures and their interfaces with COx argillite. The main contribution of our study is the identification of a preferential gas migration pathway by measuring the downstream gas breakthrough pressures and gas flow rates. The water permeabilities of the bentonite-sand mixtures and their interfaces with COx argillite or COx argillite itself are the same order of magnitude (10^-20-10^-21 m²). Thus, water tightness can be obtained for these materials when they become completely saturated. The results obtained from the gas breakthrough tests suggest that both the COx argillite and its interface with the bentonite-sand mixture can act as preferential pathways for gas migration. The transport of the gas through the COx argillite or through its interface with the bentonite-sand mixture depends on the initial state of the COx argillite. (author)