Mechanical properties of granite under ultra-low temperature and stability of underground LNG storage facilities

The granite is widely distributed and has high strength, making it an ideal medium for underground storage of low-temperature liquefied natural gas (LNG). The LNG exhibits extremely low temperatures (-162℃), which causes changes in the properties of the granite and affects storage safety. This study aims to investigate the mechanical properties of dry and saturated granite at ultra-low temperatures (-90℃ to -165℃) through uniaxial compression, thermal expansion, and microscopic tests. The results show that when the temperature decreases from -90℃ to -165℃, the compressive strength and elastic modulus of saturated granite increase by 31.1% and 24%, respectively, while the elastic modulus of dry granite increases by 12.8% without a significant change in the compressive strength. This is attributed to the decrease in temperature, resulting in shrinkage of minerals in the dry granite, which enhances the bonding of internal particles. The presence of pore water in the saturated granite causes it to freeze into ice at low temperatures, resulting in the tighter adhesion of the rock voids and cracks. Consequently, the internal structure of granite becomes denser. Using the test data an empirical formula showing a negative correlation between the rock linear expansion coefficient and the temperature was established. This formula is integrated into the thermal-mechanical coupling model in COMSOL for conducting long-term stability analysis of low-temperature LNG storage. As operating years increase, the surrounding rock of the underground storage will undergo frost heave effects due to temperatures decreases. This can lead to compression deformation between rock layers and risk of surface uplift, while this research is of great significance for the development of underground space engineering.