Microscopic measurements on the decomposition behaviour of methane hydrates formed in natural sands

Abstract

In this work, the decomposition behaviour of methane hydrate in porous media was investigated microscopically using powder X-ray diffraction, cryogenic scanning electron microscopy and in situ Raman spectroscopy. The effect of grain sizes on the decomposition of methane hydrate was measured. The results showed that bulk hydrates could exist stably at 223 K and atmospheric pressure because of the self-preservation effect. However, hydrate formed in sands was relatively easier to decompose because it had a higher equilibrium pressure compared with bulk hydrate at the same temperature. In this case, there would be a higher decomposition driving force. Interestingly, the complete decomposition time for hydrate formed in sands did not decrease with the decrease in particle size. The shortest decomposition time was observed for the sands with the particle size range of 38–55 μm, which was less than 30 minutes. Moreover methane hydrate was found to decompose faster in the porous medium containing 3.5 wt% NaCl, which suggested that there was almost no self-preservation effect. In situ Raman measurements showed that the integrated intensity ratio of methane in large and small cages (A_L/A_S) did not change during the decomposition process, suggesting that the methane hydrate crystal units decomposed as an entity in sands. This study provided important data as a basis for drilling fluid technology in hydrate mining.