# Formation Kinetics and Morphology Characteristics of Natural Gas Hydrates in Sandstone Fractures
## 1 Introduction
[32] investigated the dissociation rate of hydrates in quartz sand with varying particle sizes, concluding that the dissociation rate was governed by the sediment's heat transfer rate, while the influence of capillarity and hydrate distribution on dissociation was relatively minor. [PERSON] et al. [33] studied the dissociation process of hydrates in stacked quartz sand, finding that hydrate predominantly exists in the form of pore filling. [PERSON] et al. [34] employed a low-field nuclear magnetic resonance (LF-NMR) system to quantitatively study the heterogeneity of hydrate dissociation in quartz sand during geothermal intrusion, stressing that the dissociation rate was closely related to fluid resistance and heat replenishment. In conclusion, both domestic and international scholars have conducted extensive laboratory experiments and provided a comprehensive analysis of the kinetic characteristics of hydrates within the pores of porous media. In addition to hydrates within pores, hydrates found in fractures have been identified in several regions worldwide, including the hydrate ridge off the coast of Oregon, the Gulf of Mexico, the UJjin Basin in South Korea, the Krishna-Godavari Basin in India, and the South China Sea [35; 36; 37; 38; 39]. These fractures vary in shape, with some extending several meters, and the fracture widths range from submillimeter to millimeter scales [40], as shown in Figure 1. Hydrates within fractures are primarily formed in submarine sediments or permafrost regions. Sufficient spatial and flow pathways are afforded by fractures, enabling gas and water to react under appropriate thermobaric conditions. The formation and accumulation of hydrates within fractures are governed by various factors, including fracture connectivity, width, and type [41]. Hydrates are more commonly found in sediment fractures than in pores [41]. Furthermore, research has shown that once hydrates form in the pores and fractures, physical signals--such as electrical resistance and acoustic waves--of the sediments exhibit significant differences [42; 43; 44]. This indicates variations in the distribution and morphology of hydrates within the pores and fractures. The two differ significantly in terms of geometric properties, capillary action, their effects on gas-liquid flow, and the gas-liquid interface area [16]. Moreover, the Joule-Thomson effect caused by gas release and the endothermic effect of hydrate dissociation in sediments lead to a significant reduction in the local temperature of hydrate reservoirs. When the heat provided by the surrounding environment of the reservoir is insufficient, hydrate dissociation will cease. Under these conditions, hydrate reformation and freezing of water within the reservoir are likely to occur [45; 46]. Studies have shown that hydrate reformation is inevitable under all hydrate dissociation conditions [47]. Gas flow effectively facilitates hydrate reformation, primarily due to the hydrate's memory effect [48]. [PERSON] et al. [49; 50] simulated the memory effect of a hydrate in the presence of silica particles, indicating that the residual structure of the hydrate, the gas driving force, the particles in the porous medium, and the dissolved gas all influence hydrate reformation. The hydrate reformation affects their occurrence, thereby influencing production rates. Therefore, it is essential to study the formation and dissociation kinetics, morphological characteristics, and hydrate reformation phenomena in fractures. Existing research predominantly focuses on the formation and dissociation of hydrates in pore spaces, while relatively less attention is given to the behavior of hydrates in fractures. Specifically, there is a dearth of systematic studies examining the influence of fracture width, type, and roughness on the formation, dissociation, and reformation of hydrates. Current  
Figure 1: A schematic diagram of natural gas hydrate occurrence in fractures.