Self-preservation of methane hydrate revealed immediately below the eutectic temperature of the mother electrolyte solution

Abstract

To elucidate the phenomenon of self-preservation of gas hydrates, the decomposition rates of methane hydrates prepared from dilute electrolyte solutions were measured while the temperature was increased from 233 to 273K at atmospheric pressure. Decomposition was significantly suppressed near the eutectic temperature (TE) of the electrolyte+water system. When the concentration of the electrolyte was relatively high, however, a transitory upsurge in the decomposition rate occurred at TE. We speculate that the presence of electrolyte crystals increases the mobility of water molecules. This promotes the formation of a contiguous layer of ice that acts as an obstacle to diffusion of methane. This mechanism becomes marked as the temperature increases. When the temperature exceeds TE, however, the occurrence of stable liquid causes local melting of the ice layer with attenuation of the barrier for methane diffusion.