Experimental study of frozen gas hydrate decomposition towards gas recovery from permafrost hydrate deposits below freezing point

Abstract

Enormous natural gas hydrate reserves have been identified in the global permafrost and oceanic regions. How the existence of ice influences the hydrate dissociation is a fundamental issue related to the development of the permafrost-associated gas hydrate resources. The dissociation behaviors of frozen methane hydrate in hydrate-ice-gas saturated porous media below freezing point are studied in a high pressure reactor. The hydrate-bearing permafrost layer is simulated through decreasing the temperature of the methane hydrate samples to −0.60 °C, and then they are safely dissociated by depressurization and thermal stimulation. Results show that the disturbance on gas and water in the pores can induce secondary hydrate formation during the cooling of the system. The gas recovery rate by depressurization below freezing point is extremely slow, which is caused by the obstructive effect of ice on the diffusion of methane molecules. The self-preservation of frozen gas hydrate could be effectively broken under wellbore heating. Heat is primarily transferred by conduction mode across the hydrate and ice particles. The solid ice tends to absorb heat first and then melts into liquid water. The dissociated fluids play a role in promoting the heat transfer through thermal convection. Moreover, the net energy gain is found to be much higher than the energy consumption, and desirable energy efficiency has been obtained. Sensitivity analyses indicate that higher heat injection rate and lower ice saturation will be more favorable for the exploitation, while the production pressure seems to have negligible influence on the overall production efficiency.