Cryogenic strengthening effects on sandstone from 20 °C to −120 °C: Influence of initial water saturation and temperature

The underground storage of liquefied natural gas (LNG) is an emerging concept aimed at large-scale natural gas storage. A comprehensive understanding of rock mechanics at ultra-low temperatures is essential for the safe and efficient design of such storage facilities. This study conducted uniaxial compression tests on sandstones at temperatures ranging from 20 °C to −120 °C to assess the effects of temperature and initial water saturation prior to freezing on their mechanical properties. Results indicate that sandstone strength increases progressively as temperatures decline under varying water saturation conditions. At −40 °C and −80 °C, the sandstone initially weakens at low water saturation levels but strengthens as saturation rises further. In contrast, at −120 °C, strength correlates positively with higher initial water saturation, with fully saturated samples exhibiting a 425 % strength enhancement compared to water-saturated sandstone tested at 20 °C. Micro-CT imaging indicates minimal changes in pore structure at −120 °C, suggesting negligible frozen damage to the sandstone. The significant strengthening observed at ultra-low temperatures can be attributed to three main factors: (1) the enhanced mechanical properties of rock minerals at low temperatures, as confirmed by in-situ nanoindentation; (2) increased ice strength with decreasing temperature, allowing ice to bear compressive loads; and (3) the prestress provided by ice expansion to surrounding rock minerals, placing them in a multiaxial stress state that significantly enhances strength. These results suggest that rock strength, particularly under water-saturated conditions, improves substantially at ultra-low temperatures, which is advantageous for underground LNG storage applications.