Texture Change of Ice on Anomalously Preserved Methane Clathrate Hydrate

We used a confocal scanning microscope to observe growth and texture change of ice due to the dissociation of methane gas clathrate hydrate (CH4 hydrate). The experiments were done under CH4 gas atmospheric pressure and isothermal conditions between 170 and 268 K. Above 193 K, the dissociation of CH4 hydrate resulted in many small ice particles that covered the hydrate surface. These ice particles had roughly the same shape and density between 193 and 210 K. In contrast, above 230 K the ice particles developed into a sheet of ice that covered the hydrate surface. Moreover, the measured release of CH4 gas decreased when the sheet of ice formed at the surface of the hydrate. These findings can explain the anomalous preservation behavior of CH4 hydrate; that is, the known increase of storage stability of CH4 hydrate above 240 K is likely related to the formation of the ice that we observed in the experiments.