# Formation Kinetics and Morphology Characteristics of Natural Gas Hydrates in Sandstone Fractures
## 3 Results and Discussion
### Hydrate Dissociation  
As shown in Figure 15, during the early stages of hydrate dissociation in fractures with varying widths, dissociation initiates at the surface in contact with the fracture and gradually propagates inward. This may result from the higher thermal conductivity of the solid, which preferentially triggers hydrate dissociation. As dissociation progresses, the morphology of the hydrate becomes irregular, especially as the fracture width increases. This phenomenon is more pronounced in wider fractures, which may be attributed to the more uniform heat transfer in narrower fractures, where heat diffuses from the fracture walls toward the interior. This leads to the formation of residual hydrate distributions that resemble the shape of the fracture cavity. In contrast, in wider fractures, due to the larger cavity volume, heat diffusion is uneven, and the hydrate dissociates more in the hotspot areas, resulting in irregular residual hydrate formations. Upon examining the microscopic morphology of the residual hydrate, a wave-like undulating surface is observed, as shown in Figure 16. This may result from the uneven pressure gradient changes inside the fracture, where hydrate dissociation occurs more slowly in areas of higher pressure and more quickly in areas of lower pressure. Additionally, temperature gradient variations induced by hydrate dissociation further accelerate the development of this phenomenon.  
Furthermore, hydrate dissociation is more significantly influenced by the fracture width. In narrower fractures, the hydrate possesses a larger specific surface area, and its thermal conductivity is more effective. In this case, thermal transfer efficiency dominates, which explains why the gas production rate is the highest in fractures with a 3 mm width, resulting in the production of 0.34 mol of gas. Additionally, when the gas production region gradually reaches equilibrium, residual hydrates continue to be observed in the fracture beneath the window (as shown in Figure 15(D1-D3,E1-E3)). This suggests that, during the dissociation process, hydrates exhibit a self-protective effect, which is consistent with previous studies [53].  
Figure 15: Observation images of hydrate dissociation under different fracture widths: (**A1–EI**): 3 mm; (**A2–E2**): 6 mm; (**A3–E3**): 10 mm.  
The changes in environmental temperature during hydrate dissociation are analyzed, as shown in Figure 17. It was observed that the temperature initially increased briefly and then decreased rapidly. This phenomenon may be attributed to the endothermic process during the early stage of hydrate dissociation, in which the released water rapidly freezes at low temperatures, releasing heat and forming concentrated thermal energy. Additionally, the gas produced during hydrate dissociation may expand and contract, potentially causing localized temperature increases. As the dissociation process progresses, the released methane and water molecules transport heat away from the surrounding environment. Meanwhile, heat exchange between the system and its local environment further reduces the overall temperature. Therefore, the temperature rapidly decreases after a brief increase.