Investigation of Percolation-Driven Fluid Transport in Rock Salt under Repository-Relevant Conditions (PeTroS)
Authors: Lüdeling Christoph , Naumann Dirk , Minkley Wolfgang 
Published: November 2021

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