ABSTRACT
Gas transport properties are key issues in the long term assessment of storage of high level radioactive or toxic waste in salt formations. Whereas extensive knowledge exists regarding the initial and dilatant rock salt properties, little is known about consequences due to the long term gas generation in a radioactive waste repository. Because rock salt is attributed to be impermeable for gases and fluids gas pressures will be built up with time until a level that may exceed the fracturing pressure of the rock (generally discussed as gasfrac-scenario). For an assessment of the provable impact of increasing gas pressures on the integrity of rock salt we present preliminary results from a long-term field test with progressive gas injection in a gas-tight sealed borehole. To detect micro-cracking a highly sensitive micro-seismic network was installed. Remarkably, in the multi-stage injection tests the gas-breakthrough was obtained at a gas pressure of 140 bar slightly above the primary stress state inducing a pressure build-up in two neighbored control bore holes. Due to the associated permeability increase of 3 orders (up to 10−20 m2) transient pressure decay occurs coevally in the pressurized injection-borehole. Reaching equilibrium at around 100 bar the primary gas-integrity is partly restored in the order of 10−22 m2. Most important, no pressure induced micro-seismic activity was observed during the gas-breakthrough which clearly contradicts the gas-frac-scenario. For comparison, we performed additional laboratory investigations highlighting the impact of increasing pore pressures on permeability whereby the effect of the gas-breakthrough could be attributed to pressure induced opening of grain boundaries. In addition, special account is taken to the effect of anhydrite bearing intercalations which may canalize the spatial gas migration in salt.
