# Formation Kinetics and Morphology Characteristics of Natural Gas Hydrates in Sandstone Fractures
## 3 Results and Discussion
### Hydrate Formation
#### 3.1.3 Effect of Roughness  
This section primarily modifies the specific surface area of fractures by adhering glass sand with varying particle sizes to the fracture walls. By analyzing the hydrate induction time, total formation duration, and gas consumption curves during hydrate formation, as depicted in Figure 12, it was observed that in Type I fractures, compared to smooth fractures, an increase in the specific surface area of the fractures leads to a decrease in both the hydrate induction time and total formation duration. Furthermore, as the specific surface area decreases, both the hydrate induction time and total formation duration further decrease when the particle size of the glass sand increases from 2.5 mm to 3.5 mm. Based on Figure 13, this may be because, in smooth fractures, hydrate growth primarily depends on the infiltration of water molecules, which gradually extend upward. However, the addition of glass sand creates more irregular structures on the fracture walls, increasing the storage sites for liquid and altering the distribution of surface energy. In areas with protrusions, surface energy is higher, making these sites active locations for heterogeneous nucleation, thus promoting the induction of hydrate nucleation. However, the complex geometric structure of the rough surface also limits the diffusion of gas molecules. As a result, it was observed that as the specific surface area decreases, both the induction time and the duration of hydrate formation begin to decrease as well. Figure 11: An observation diagram of the fracture hydrate growth under different fracture shapes. Figure 12: Type I fracture under different roughness: (**a**) hydrate induction time; (**b**) total formation duration; (**c**) gas consumption curve. By analyzing the gas consumption curve during the hydrate formation process, as shown in Figure 12c, it was observed that, compared to smooth surfaces, an increase in the fracture surface's specific surface area does enhance the rate of hydrate formation. Interestingly, although increasing the specific surface area promotes hydrate formation, an analysis of the final hydrate formation quantity reveals that hydrate formation on smooth surfaces reaches approximately 0.383 mol, while fractures with increased roughness show a final hydrate formation quantity in the range of 0.323-0.331 mol. Moreover, the variation in roughness does not have a significant impact on the final hydrate formation quantity. This may be due to the difficulty of heat dissipation on rough surfaces, where the continuous release of heat during hydrate formation causes local temperature increases that suppress further formation of hydrates. Additionally, the rough surface limits the mass transfer of gas, and the combined effects of these factors result in the inhibition of hydrate formation at later stages, despite the increase in the fracture's roughness. Although the particle size of the glass sand increased from 2.5 mm to 3.5 mm, this change had little effect on local heat transfer and mass transfer processes, so the final hydrate formation quantity did not exhibit any significant changes. Further investigation of the hydrate formation process in type X fractures with varying roughness reveals that hydrate formation is more pronounced in the raised regions. Additionally, multiple data points from the Type X fractures exhibit patterns similar to those of the Type I fractures, as shown in Figure 14. The duration of hydrate formation decreases as smooth fractures are replaced by rough fractures, and increasing the fracture's specific surface area leads to a reduction in the final quantity of hydrate formation. However, unlike the Type I fractures, the hydrate induction time in Type X fractures is significantly prolonged after increasing the roughness. The cause of this phenomenon can be attributed to the inherent instability of gas flow within the Type X fractures.