Production Characteristics of Natural Gas Hydrate in Muddy Marine Sediments of Different Moistures by Depressurization

Marine natural gas hydrate (NGH) is an important source of energy for the future, while the experimental simulations on NGH production are rarely conducted in real marine sediments. This study employed the real sediments from South China Sea and put upward a method of remolding NGH sediments by hydrate formation after injecting water into a dry sediment with high-pressure methane gas. A 2 degrees C temperature increase was observed in the middle layer of the sediment due to the exothermic formation of hydrates. The production characteristics of NGH sediments with different moistures (28.3-73.7%) were evaluated by depressurization. A uniform decomposition stage during the production process was defined, and the apparent uniform decomposition rates in all cases were around 2.1 mmol/min. Intrinsic decomposition rate of hydrate crystal and limited heat transfer outside of sediments were likely to be the main deciding factors. When the sediment changed into the water-saturated condition, the excess water improved the transfer and greatly increased the decomposition rate of hydrates. In addition, the complex response of sediment temperatures to the pressure change, hydrate decomposition, heat transfer, as well as sometime ice formation and melting were analyzed from the perspectives of both thermodynamic and heat transfer. The results of this study are of great significance for future pilot production of real marine NGH.

Automated summary

This study conducted experimental simulations on the production of marine natural gas hydrate (NGH) using real marine sediments. The study developed a method of remolding NGH sediments by injecting water into dry sediments and forming hydrates. The production characteristics of different moisture levels of NGH sediments were evaluated, and the complex response of sediment temperatures to pressure change, hydrate decomposition, heat transfer, and ice formation and melting were analyzed. The results have great significance for future pilot production of real marine NGH.