Methane hydrate decomposition and sediment deformation in unconfined sediment with different types of concentrated hydrate accumulations by innovative experimental system

Methane hydrates are regarded as a potential source of energy supply. Geological features of different types of concentrated gas hydrate accumulations show great variations. In this study, methane hydrate decomposition in unconfined sediment with different types of concentrated hydrate accumulations are firstly investigated by experiments, and the influence of hydrate decomposition on sediment deformation is analyzed. Two types of concentrated hydrate accumulations are selected, which are grain-displacing hydrate (nodules) and pore-filling hydrate in sediment. An innovative high pressure set-up with a quick-opening top cover is applied to investigate hydrate decomposition under the geological conditions of the hydrate reservoir in the South China Sea. Experimental results indicate that the influence of hydrate morphology and hydrate distribution on gas production is not obviously. The average heat transfer rates during grain-displacing hydrate dissociation and pore-filling hydrate dissociation are also similar. However, the sediment deformation characteristics for different types of concentrated hydrate accumulations are totally different. Structure collapse of porous media is firstly observed in the experiments within the grain-displacing hydrate, which indicates that the sediment deformation cannot be ignored during the gas recovery from grain-displacing hydrate. Meanwhile, the radial shrinkage effect of sediment is found during pore-filling hydrate dissociation, due to the cementation effect of hydrate.