Summary
In order to ensure that waste remains isolated from the biosphere reliably over the long term when a subterranean dumpsite is closed down, the waste is encased by a barrier system. Optimising the barriers' hydraulic properties guarantees secure pollutant insulation. However, ingress of water or saline solutions may produce gas, in turn generating impermissibly high pressures in the dump chambers. Therefore, pressure relief must be possible via the barrier seals. Selecting suitable clay-mineral mixtures allows the permeability for water/solutions and (in the case of a two-phase flow system) for gas/fluids to be adjusted specifically. However, which material is best-suited is presently unclear.
The project therefore performed laboratory research on clay-mineral mixtures under two-phase flow conditions using gas, water and brine in order to characterise the barrier materials and to assess their sealing efficacy. Further, the effect of larger-grained components on hydraulic properties as well as connection of the sealing material to host rock were examined.
Results:
- In terms of relative permeability, two-phase flow behaviour is unaffected by the proportion of clay in the clay-sand mixture.
- The samples saturated with magnesium-chloride brine showed a stagnation of the gas flow after gas penetration. Increasing the pressure results in an increased gas flow rate with subsequent stagnation. This finding can be assumed indicative of gypsum precipitation in the pore chambers.
- Hydraulic behaviour here is primarily determined by the sealing material installed. Uncontrolled outflow of fluids via the aqueous rock can be largely precluded.
- Comparison of gas permeabilities shows that there is no substantial difference between sealing material filled by bulk and seals installed with pre-compacted samples and annulus plugging.
