Analysis of the freezing circle and energy consumption for underground LNG storages in fractured rocks

Accurate prediction of the freezing process in underground liquefied natural gas (LNG) storages plays an important role in enhancing the sealing effect of surrounding rocks and the economic benefits of the storage system. This study established a freezing circle extension model of underground refrigerated storage based on the hydrothermal coupled method with nonlinear seepage effects in fracture networks. A novel method for analyzing energy consumption during the storage process is then presented. The effects of cavern parameters, seepage in the fracture network, and grouting on the freezing circle expansion process and energy consumption are further investigated. The results show that the proper expansion of the cavern is conducive to accelerating the growth of the freezing circle and reducing the energy consumption per unit volume of LNG storage. Setting the thickness of the polyurethane insulation layer up to 50 mm can effectively reduce energy consumption and ensure the emergence of freezing circles in a short time. The seepage within the fracture network inhibits the expansion of freezing circles and increases the energy consumption of the storage system. The increase in the groundwater pressure gradient and the connectivity of the fracture network will exacerbate the adverse effects of seepage on freezing circles. However, a grouting depth that matches the storage cycle allows the freezing circle expansion process and energy consumption to be virtually unaffected by fracture seepage.