Experimental study on the permeability characteristics of reconsolidated salt: Effects of gas and confining pressure

Återkonsoliderad salt från krossat halitmineral uppvisar extremt låg gaspermeabilitet — ner till 10⁻¹⁹ m² — vilket gör den attraktiv som tätningsmaterial i djupa geologiska förvar för högaktivt radioaktivt avfall. Forskare från Luleå tekniska universitet och kinesiska institutioner testade gaspermeabiliteten under varierande tryck med kvävegas och kärnmagnetisk resonans (NMR). Resultaten visar att gaspermeabiliteten minskar när både gasförtryck och inneslutande tryck ökar, där gastrycket har starkast effekt. Efter Klinkenberg-korrigering — som förklarar gasglirning längs porväggarna — framkom att den absoluta permeabiliteten är betydligt lägre än den uppmätta. Permeabiliteten följer en potenslagsfunktion mot porositet och uppvisar logaritmisk beroende av inneslutande tryck. Dessa värden överträffar eller motsvarar bentonits tätningskapacitet. Resultaten är kritiska för långsiktiga säkerhetsbedömningar av slutförvar och underlättar leverantörsval av tätningsmaterial i kommande kärnkraftsprojekt.

<p>Reconsolidated salt, formed from crushed halite under compaction, is a promising buffer and sealing material for deep geological repositories of high-level radioactive waste (HLW) because of its low permeability and self-healing properties. This study investigated the gas permeability behavior of reconsolidated salt with varying porosities under different confining pressures and inlet gas pressures using nitrogen gas. Based on nuclear magnetic resonance (NMR) technology, the pore structure of reconsolidated salt specimens with different porosities was tested and imaged. The experimental results demonstrate that gas permeability decreases with increasing gas and confining pressures, with gas pressure having a more pronounced effect. The observed permeability‒pressure relationship is attributed primarily to the Klinkenberg effect, with gas slippage along pore walls enhancing the measured permeability under low-pressure conditions. Using the Klinkenberg correction, the absolute permeability values of reconsolidated salt were derived, reaching as low as 10−19 m2 in low-porosity samples. These values are significantly lower than the apparent gas permeability, indicating excellent sealing performance comparable to or superior to that of bentonite. A logarithmic relationship between the absolute permeability and confining pressure was established, providing a quantitative basis for permeability prediction under repository stress conditions. NMR imaging results indicate that with decreasing porosity, the connectivity between pores also gradually diminishes. Additionally, the slip factor was found to increase with increasing confining pressure, underscoring the evolving influence of pore geometry on gas transport mechanisms. Permeability of reconsolidated granular salt decreases with porosity following a power-law relationship, and the healing supports its sealing effectiveness. This study provides essential data and theoretical insights for evaluating the long-term sealing performance of reconsolidated salt in salt-based HLW repositories.</p>