# Experimental Study on the Influence of Low Temperature on the Gas Permeability of Granite
## 3 Results and Discussion
### NMR Test Results
#### 3.4.2 Changes in Pore Size Distribution  
Based on the data presented in Figure 13 and Equation (7), the pore size distribution of coarse and fine granite before and after freeze-thaw cycles can be derived. In this study, granite rock pores are classified into three categories: micropores (r < 0.1 \(\upmu\)m), mesopores (0.1 \(\upmu\)m \(\leq\) r \(\leq\) 1 \(\upmu\)m), and macropores (r > 1 \(\upmu\)m) as per previous studies [45]. The area under the curves in Figure 14 directly reflects the trend of changes in various pore sizes within the granite specimens. Figure 14: Pore size distribution of granite before and after freeze–thaw cycles. Figure 13: Variation rule of pore volume with temperature in granite. The pore volume of the granite specimens correlates positively with the area of the pore size distribution curve. By integrating the area of the three pore size intervals shown in Figure 14, the volume occupied by each pore type can be calculated. The variation trends in pore volume and the proportion of each pore type with temperature for the two granites are shown in Figures 15-17. The volume proportion of macropores in coarse-grained granite is the largest, followed by mesopores, while micropores constitute the smallest proportion. From the pore distribution changes in Figures 15 and 17, it can be seen that when the temperature is higher than \(-20\)\({}^{\circ}\)C, new micropores form within coarse-grained granite, while mesopores expand and transition into macropores. This results in an increase in both the volume and proportion of micropores and macropores, while the volume and proportion of mesopores decrease. As the freezing temperature decreases further, micropores continue to form and gradually develop into mesopores, resulting in the stabilization of micropore volume, although its proportion decreases. Simultaneously, the volume of macropores increases, but their volume proportion decreases, indicating that mesopores continue to expand into macropores, although the rate of increase in macropore volume is slower than that of mesopores. Therefore, at lower freezing temperatures, the freeze-thaw damage to coarse-grained granite is primarily characterized by the continuous formation of micropores, which subsequently develop into mesopores. Fine-grained granites do not have macropores within them and contain only micropores and mesopores. The change in pore volume inside fine-grained granite takes \(-60\)\({}^{\circ}\)C as the cut-off point; when the freezing temperature is higher than \(-60\)\({}^{\circ}\)C, the volume of micropores inside fine-grained granite increases, and there are no mesopores. When the freezing temperature dropped below \(-60\)\({}^{\circ}\)C, the volume of micropores began to decrease, while mesopores began to appear and gradually increased in volume, and finally the proportion of mesopore volume exceeded that of micropores at \(-120\)\({}^{\circ}\)C. This suggests that new micropores are constantly produced inside the fine-grained granite when the freezing temperature is high. With the further decrease in temperature, these micropores gradually developed and expanded into mesopores, resulting in a continuous increase in mesopore volume. The decrease in the volume of micropores further suggests that the freeze-thaw damage of fine-grained granite at lower freezing temperatures is mainly reflected in the transformation and expansion of micropores into mesopores. When considering the effects of confining pressure and temperature on the gas permeability of granite after freeze-thaw cycles, it is observed that at higher freezing temperatures,  
Figure 16: Variation in pore volumes of fine-grained granite with temperature. Figure 17: Pore volume percentage of granite before and after freeze–thaw cycles. only the water in the smaller pores of both granite types freezes into ice. As a result, freeze-thaw damage is minimal, leading to the formation of only tiny pores in both types of granite.