# Experimental Study on the Influence of Low Temperature on the Gas Permeability of Granite
## 3 Results and Discussion
### NMR Test Results  
#### 3.4.1 Analysis of T\({}_{2}\) Distribution Curve  
Nuclear magnetic resonance (NMR) tests were conducted on water-saturated granite specimens before and after freeze-thaw cycles, resulting in T\({}_{2}\) distribution curves for granite samples with different particle sizes. As shown in Figure 12, the T\({}_{2}\) values for coarse-grained granite ranged from 0.01 to 1000 ms, while those for fine-grained granite ranged from 0.01 to 100 ms. This indicates that, compared to fine-grained granite, coarse-grained granite exhibits a broader pore size distribution and a larger maximum pore radius. The T\({}_{2}\) distribution curves of fine-grained granite before and after the freeze-thaw cycles were primarily characterized by two peaks, with the first peak showing a significantly higher signal intensity than the second. This suggests that the internal pore structure of fine-grained granite is dominated by small-sized pores. As the freezing temperature decreased, the position of the first peak shifted slightly to the right, while its amplitude remained largely unchanged. In contrast, the second peak not only shifted to the right but also increased in intensity. These observations imply that the pore size corresponding to the first peak increased after the freeze-thaw cycle, though the quantity of these pores remained stable, whereas both the size and number of pores associated with the second peak increased. Figure 11: Two-Dimensional gas permeability of granite plotted against temperature. For coarse-grained granite, the T\({}_{2}\) distribution curve before the freeze-thaw cycle also displayed two distinct peaks. However, when the freezing temperature was reduced to between \(-20\)\({}^{\circ}\)C and \(-90\)\({}^{\circ}\)C, the curve exhibited three peaks, indicating the emergence of a new class of pores. When the temperature was further reduced to \(-120\)\({}^{\circ}\)C, the number of peaks reverted to two. This suggests that the freeze-thaw process led to the development of a new range of pore sizes in coarse-grained granite, resulting in significant alterations to its pore structure. Notably, at \(-120\)\({}^{\circ}\)C, the pore structure of the coarse-grained granite appeared to be more fully developed, with enhanced connectivity between pores. The microstructural changes observed in both granite types after freeze-thaw treatment align with the trends in intrinsic gas permeability under varying confining pressures, thereby validating the consistency between microscopic pore evolution and macroscopic permeability behavior. The total area under the T\({}_{2}\) distribution curve can be used to represent the porosity of the rock, serving as a critical indicator of changes in pore volume. The variation in the total area of the T\({}_{2}\) distribution curves with freezing temperature is shown in Figure 13. As the freezing temperature gradually decreases, pore initiation and propagation occur within both granite types due to the combined effects of the water-ice phase transition and the contraction and deformation of mineral particles. This leads to an upward trend in the total area of the T\({}_{2}\) distribution curve, indicating a significant increase in pore volume as a result of freeze-thaw-induced damage. Under identical freeze-thaw conditions, coarse-grained granite consistently exhibits a larger total T\({}_{2}\) area compared to fine-grained granite, suggesting that the latter has a lower porosity and a denser pore structure. As freezing temperature decreases, the total T\({}_{2}\) area for both granite types generally follows a pattern of increase-decrease-increase. Compared to the range of \(-20\)\({}^{\circ}\)C to \(-60\)\({}^{\circ}\)C, a more substantial increase in the total T\({}_{2}\) area is observed when temperatures drop to \(-90\)\({}^{\circ}\)C and \(-120\)\({}^{\circ}\)C, and a declining trend is seen in the \(-40\)\({}^{\circ}\)C to \(-60\)\({}^{\circ}\)C range. It coincides with the law that the rate of change of 1D gas permeability of granite changes with temperature.