Methane Hydrate Formation and Decomposition Process in Clay-Silt Slurries

The gas-hydrate-sediment-water multiphase flow is often involved in drilling and exploitation of natural gas hydrate (NGH) reservoirs in seabed due to its weak cementation; therefore, understanding hydrate formation and decomposition in sediment-contained slurries is important. In this work, CH4 hydrate formation and decomposition in clay-silt slurries was explored, the effects of clay type and content, stirring rate, additives, experimental temperature, and pressure were systematically investigated. The presence of clay notably accelerated CH4 hydrate formation in slurries, and the montmorillonite exhibited a more obvious promotion effect than kaolinite. As the stirring rate increased, the induction time and reaction time were shortened, while the gas consumption rate and water conversion increased. The addition of 500 ppm sodium dodecyl sulfate (SDS) and 1.0 wt % fulvic acid (FA) significantly reduced the induction time and increased hydrate formation rate. During the interaction of SDS and sediment, CH4 hydrate formed in the slurries instantaneously. The CH4 hydrate decomposition rate in slurries increased as the experimental temperature increased or the pressure decreased. The temperature had a limited impact on the decomposition time above freezing point, while it was significant below freezing point. The presence of clay notably decreased the CH4 hydrate decomposition rate in slurries. Compared to the clay-free system, the ΔT d values (temperature decreased during hydrate decomposition) in the montmorillonite systems were decreased, while they were increased in the kaolinite systems. The findings provide valuable insights for the exploitation of marine NGH.