# Experimental Study on the Influence of Low Temperature on the Gas Permeability of Granite
## 3 Results and Discussion
### One-Dimensional Gas Permeability
#### 3.2.3 Effect of Confining Pressure on Intrinsic Gas Permeability  
Based on the results of intrinsic gas permeability measurements before and after freeze-thaw cycles, the variation curves of granite's intrinsic gas permeability with confining pressure at different freezing temperatures are plotted in Figure 10. Overall, both coarse- and fine-grained granite exhibit a similar trend: intrinsic gas permeability decreases with increasing confining pressure, and the rate of this decrease gradually diminishes as the pressure continues to rise. Among the fine-grained granite specimens, the reduction in intrinsic gas permeability in Group B is more pronounced than that in Group C.  
Before freeze-thaw cycling, the intrinsic gas permeability of the three granite types decreased by 55.59%, 79.32%, and 76.74%, respectively, as confining pressure increased from 5 MPa to 15 MPa. After freeze-thaw cycling, the average reduction in intrinsic gas permeability for coarse-grained granite at freezing temperatures of \(-20\)\({}^{\circ}\)C, \(-40\)\({}^{\circ}\)C, \(-60\)\({}^{\circ}\)C, \(-90\)\({}^{\circ}\)C, and \(-120\)\({}^{\circ}\)C was 63.80%, 59.72%, 59.09%, 58.50%, and 59.57%, respectively. For fine-grained granite, Group B exhibited reductions of 81.66%, 78.61%, 76.38%, 84.55%, and 80.21%, while Group C showed corresponding reductions of 75.36%, 64.18%, 86.74%, 78.25%, and 78.93%.  
The primary determinant of freeze-thaw damage to rocks is their lithology [44]. Due to differences in mineral composition and particle size between coarse- and fine-grained granite, the damage mechanisms under freeze-thaw cycles also differ. After freeze-thaw cycling, all granite specimens exhibited increased permeability reductions compared to those at room temperature. Coarse-grained granite showed a greater increase in permeability decline, while fine-grained granite exhibited a more moderate change. This is likely because coarse-grained granite, having lower strength and stiffness, is more susceptible to internal stress caused by the repeated expansion and contraction associated with ice-water phase transitions. These stresses promote pore propagation and the formation of new pores.  
In contrast, fine-grained granite, with its higher strength and more compact structure, primarily experiences widening of existing pores rather than the formation of new ones.  
Therefore, the freeze-thaw damage in coarse-grained granite is mainly manifested through the initiation and propagation of new pores, which results in an increase in smaller, interconnected pores. At low confining pressure (5 MPa), these new pores contribute  
Figure 10: Variation in granite intrinsic gas permeability with confining pressure.  
to permeability. However, at higher confining pressure (15 MPa), many of these small pores are compressed and closed, leading to a more significant reduction in permeability after freeze-thaw cycling. Conversely, for fine-grained granite, since the primary damage mechanism involves the expansion of existing pores with limited formation of new flow paths, the permeability decrease with increasing confining pressure is less affected by the freeze-thaw cycle.