  We present the results of dynamic in-situ 3D X-ray imaging of methane
hydrates microstructure during methane hydrate formation and decomposition in
sand samples. Short scanning times and high resolution provided by synchrotron
X-rays allowed for better understanding of water movement and different types
of gas-hydrate formation. Complementing previous observations, we conclude that
the process of gas-hydrate formation is accompanied by the water movements
caused by cryogenic water suction that happens in short sequences with longer
equilibrium states in between (when the water is immobile). Based on the 3D
microstructure we identified two types of gas-hydrate formation: (i) into the
gas pockets and (ii) inside water volumes. During the decomposition in the
self-preservation mode (pressure drop at negative temperatures) the latter
remains more stable compared to the hydrate formed as growing into the gas
pocket. This means that the history of the gas-hydrate formation influences its
behaviour at the decomposition stage (e.g. gas-hydrate production).
