# Experimental Study on the Influence of Low Temperature on the Gas Permeability of Granite
###### Abstract  
Granite is widely regarded as an ideal material for the construction of underground liquefied natural gas (LNG) storage reservoirs due to its high mechanical strength and broad geological availability. However, the ultra-low storage temperature of LNG (\(-162\)\({}^{\circ}\)C) poses potential risks in altering the permeability of granite, which may compromise the long-term safety and integrity of the reservoir. To investigate the permeability characteristics and microstructural degradation of granite under low-temperature conditions, both coarse-grained and fine-grained granite samples were subjected to a series of experiments, including one-dimensional (1D) gas permeability tests (conducted before and after freeze-thaw cycles ranging from \(-20\)\({}^{\circ}\)C to \(-120\)\({}^{\circ}\)C), nuclear magnetic resonance (NMR) tests, and two-dimensional (2D) gas permeability tests performed under real-time low-temperature conditions. Experimental results indicated that the gas permeability of granite under real-time low-temperature conditions exhibited a linear increase as the temperature decreased. In contrast, the gas permeability after freeze-thaw cycling followed a nonlinear trend: it increased initially, plateaued, and then increased again as the freezing temperature continued to drop. A further analysis of pore structure evolution and permeability changes revealed distinct degradation mechanisms depending on grain size. In coarse-grained granite, freeze-thaw damage was primarily characterized by the initiation and propagation of new microcracks, which originated as micropores and expanded into mesopores. In fine-grained granite, the damage primarily resulted from the progressive widening of existing fissures, with micropores gradually evolving into mesopores over successive cycles. The study's findings provide a useful theoretical foundation for the secure subterranean storage of LNG. fontsize=, frames=single-, frames  
The operating temperature of liquefied natural gas (LNG) storage tanks is approximately \(-162\)\({}^{\circ}\)C, making it essential to ensure the thermal stability and structural integrity of foundation materials and surrounding geological formations under cryogenic conditions when designing and operating underground LNG storage systems. In recent years, extensive experimental research has been conducted to investigate the physical, mechanical, and microstructural properties of rocks subjected to low temperatures and freeze-thaw cycles. These studies have primarily focused on various lithologies, including granite [5; 6; 7], marble [8; 9], shale [10] and sandstone [11; 12], and have explored the influence of factors such as rock type [13], water content [14], freezing temperature [15; 16; 17], and the number of freeze-thaw cycles [18] on rock deterioration and strength degradation under cryogenic conditions. While these investigations have provided valuable insights into macroscopic strength changes, such analyses alone are insufficient to fully elucidate the mechanisms of low-temperature damage in rocks. A more comprehensive understanding requires precise identification and quantification of internal pore damage, which forms the foundation for evaluating cryogenic degradation and developing predictive models of rock behavior in low-temperature environments. At present, a range of advanced techniques are employed to investigate changes in the internal pore structure of rocks under low-temperature conditions, including acoustic emission (AE) [6], nuclear magnetic resonance (NMR) [19; 20], scanning electron microscopy (SEM) [21] and computed tomography (CT) [22]. These methods provide multi-scale insights into pore evolution and damage mechanisms. For example, using CT and SEM imaging, [PERSON] et al. [23] observed grain disaggregation, crack initiation and propagation, and increased porosity in igneous rocks resulting from internal water volume expansion during freeze-thaw cycling. [PERSON] et al.