# Formation Kinetics and Morphology Characteristics of Natural Gas Hydrates in Sandstone Fractures
## 1 Introduction  
Natural gas hydrate is a metastable substance formed by the interaction of water molecules with guest molecules [1, 2], primarily methane (CH4), under low-temperature and high-pressure conditions, resulting in a cage-like, solid crystalline compound [3, 4]. It is widely distributed across offshore areas and permafrost regions [5, 6]. As an unconventional energy source, the carbon reserves of natural gas hydrates are more than double those of conventional energy sources, and they release only small amounts of water and carbon dioxide (CO2) upon complete combustion [7, 8]. Based on these advantages, natural gashydrates are expected to become one of the most promising alternative energy sources as conventional energy gradually depletes [9; 10]. Therefore, the large-scale commercial exploitation of natural gas hydrates is of considerable importance in addressing the issues of insufficient conventional energy reserves and environmental pollution [11; 12]. The extraction of a gas hydrate is a complex process involving multiple factors, such as kinetics, thermodynamics, fluid flow, and reservoir mechanics [13]. Among these factors, the study of the formation/dissociation kinetics and morphological characteristics of hydrates forms a crucial foundation for improving the efficiency and stability of hydrate development [14; 15]. The primary factors influencing hydrate formation include temperature, pressure, disturbances, and additives [16]. Research has demonstrated that temperature and pressure conditions play a crucial role in hydrate kinetics [17]. Physical methods, such as disturbances, spraying, and bubbling, effectively increase the contact area between gas and liquid-phase water, thereby significantly enhancing the rate of hydrate formation [18]. Chemical additives, including kinetic additives like sodium dodecyl sulfate (SDS) [19], amino acid compounds [20; 21; 22], and nanoparticles [23; 24] have been shown to promote the kinetics of hydrate formation. These promoting techniques provide significant theoretical support for a deeper understanding of the dynamic characteristics of hydrates. Natural gas hydrates are typically found in porous sedimentary media, and the properties of these media significantly influence their formation, dissociation kinetics, and morphology. Water-saturated sandy sediments with high permeability are generally regarded as the most economically viable and feasible category for hydrate extraction [25]. [PERSON] et al. [26] observed the formation of hydrates in the pores of silica sand and activated carbon particles, emphasizing that the pore space and connectivity of the porous media play a crucial role in hydrate formation. [PERSON] et al. [27] used nuclear magnetic resonance (NMR) technology to observe that hydrate typically forms at the center of pores in sandstone, with the formation mechanisms in smaller pores differing significantly from those in larger pores. [PERSON] et al. [28] conducted hydrate formation experiments in glass beads and similarly observed that the water conversion rate in large pores was significantly higher than in small pores. [PERSON] et al. [29] propose that the formation mechanism of hydrates in sandstone pores is closely linked to the specific pore volume. [PERSON] et al. [30] similarly support this viewpoint and highlight that the rate of hydrate formation is determined by the effective gas-liquid contact area rather than the particle size of quartz sand. [PERSON] et al. [31], employing X-ray radiation imaging and 3D computed tomography (CT) technology, investigated the dissociation process of hydrates in sandstone and concluded that the dissociation rate of hydrates does not exhibit a linear relationship with their surface area. Additionally, they observed that the dissociation rate of hydrates in larger pores is significantly slower than in smaller pores. [PERSON] et al.