Investigation of Percolation-Driven Fluid Transport in Rock Salt under Repository-Relevant Conditions (PeTroS)

Abstract. According to the state of art in mining and repository research, undisturbed rock salt is impermeable fluids. Hence, formations are considered as host for nuclear waste repositories. Viscous, polycrystalline with low humidity contains no connected pore spaces. Two mechanisms known fluid transport: (a) damage due large deviatoric tensile stresses generates dilatancy, hence permeability. (b) Fluid pressure exceeding minor principal stress can open pathways (pressure-driven percolation, Minkley et al., 2013). To assess barrier integrity barriers, dilatancy minimal criteria have been derived. Recently (Ghanbarzadeh 2015; Lewis Holness, 1996), high permeabilities postulated under certain conditions. In particular, at temperatures, including possible conditions, claimed develop a connected, thus permeable, space. PeTroS project (Minkley 2020), we investigated transport supposedly permeable region. Five points pressure-temperature space were defined – pressures 18 36 MPa, temperatures 140, 160, 180 ∘C. At each point, experiments both nitrogen saturated NaCl solution (brine) performed. Samples prepared from natural German Zechstein formations, bedded domal salt. Sample material was generally relatively pure impurities. Cylindrical samples (diameter 100 mm, length 200 mm) loaded triaxial (Kármán) cell. applied central chamber; any transmitted collected extracted secondary side. The entire cell heated specified temperature. Experiments comprised an isotropic phase (several stages almost up confining stress) breakthrough (lowering axial by strain-controlled extension). After test, coloured tracer injected visualise discharge points. breakthroughs above observed all five Some showed approximately Darcian flow below stress, order 10−22 m2, regularly small size initial sample preparation (Popp 2007). Tests consistently gradual decrease rate, i.e. reduction damage. A stable permeability over longer times, would be expected formation network, not experiments. Intriguingly, brine even though wetting should plausibly favour network. Predictions static scale theory 2015) could confirmed. Regarding repositories heat-generating waste, it concluded that geomechanical point view, relevant higher