Dissociation behavior of sI and sII gas hydrates in response to environmental changes – Investigations on the self-preservation effect

Natural gas hydrates are ice-like solids occurring worldwide on continental margins and in permafrost regions. Their high methane (CH4) content makes them a potential energy source, but also a climate factor. Pressure and temperature changes in their environment may induce the decomposition of gas hydrates. Some hydrates, however, exhibit a so-called self-preservation effect which delays the decomposition process and is not yet sufficiently understood. In the present work, the decomposition behavior of simple and mixed sI and sII hydrates was studied via experiments (T = 267–271 K; p > 0.1 MPa) and numerical modeling. This combined approach led to new insights into which molecule-specific properties result in a self-preservation effect. The results show that CH4 and especially CO2 intend to participate in hydrate reformation whereas hydrates including heavier hydrocarbon molecules do not undergo a decomposition–reformation process, and thus these hydrates continued dissociating with no barrier. Under certain conditions, a liquid C4-hydrocarbons phase is preferentially formed in which C4-hydrocarbons are enriched. Generally, the dissociation rate seems to depend on the composition of the hydrates, and the behavior of CH4 molecules in the dissociation process is influenced by the presence of other gases in the mixed gas hydrate.