Kinetics and Morphological Characteristics of Gas Hydrate Formation in Methane–Water–Sand Particle Systems

Natural gas hydrates are considered one of the most important clean resources with great development potential. Numerous studies have focused on flow assurance problems resulting from hydrate formation in gas–water–oil systems of transportation pipelines. However, there have been limited investigations on morphological evolution and hydrate formation kinetics in complex multiphase gas–water–sand hydrate slurry systems for wellbore fluids during gas hydrate production, considering the influence of sand particles. This study explored gas hydrate morphological characteristics, formation kinetics, and deposition phenomena in different sand (quartz sand, muddy silt, and natural sand)–water slurry systems. Morphological results indicate two patterns of hydrate formation: the wall-adherent hydrate layer and the gas–liquid interfacial hydrate layer. The amount of sand particles in the hydrate layer adhering to the wall could increase with the sand particle concentration in slurry. The strong dispersion of muddy silt in slurry caused the different morphologies of the formed hydrate slurry from other sand systems, resulting in a wider distribution range of the hydrate layer adhering to the wall and a more uniform hydrate slurry. The morphology of the hydrate layer highly depended on the sand type. All three types of sand particles reduced the induction period of gas hydrate formation, accelerating hydrate growth. The most remarkable promoting impact on the formation kinetics of hydrates was found in the natural sand–water slurry due to the small-sized particles providing more hydrate nucleation sites. The muddy silt–water system provided the longest induction period for hydrate formation since more water bound in muddy silt particles hindered the mass transfer for hydrate formation.