Study on mechanical properties of granite under ultra-low temperature and stability of underground LNG storage facility

Granite is characterized as wide-distribution and high strength, which is an ideal medium for underground storage of low-temperature liquefied natural gas (LNG). However, LNG formed extreme low temperature (-162℃) causes changes in the properties of granite, which affects the storage safety. In order to explore the mechanical properties of granite at ultra-low temperature, uniaxial compression test, thermal expansion test and microscopic observations were conducted on dry and saturated granite under -90°C to -165°C. The experimental results show that when the temperature decreases from -90°C to -165°C, 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%, and its compressive strength shows no significant change. The main reason for the increase in mechanical strength is that with the decrease of temperature, the dry granite experiences a low-temperature shrinkage of minerals, leading to enhanced internal particle bonding. Due to the presence of pore water, saturated granite freezes into ice at low temperatures, causing the rock voids and cracks to adhere more tightly. Hence, internal structure of granite becomes denser. A negative correlation empirical formula between rock linear expansion coefficient and temperature was established through experimental data, and embedded into the thermal mechanical coupling model in COMSOL to achieve long-term stability analysis of low-temperature LNG storage. With the increase of operating years, the surrounding rock of underground storage will experience frost heave effect with the decrease of temperature, resulting in compression deformation between rock layers and the risk of surface uplift.