What is hydrate?

In nature, water comes in three different phases, solid (ice), liquid and vapor. But dissolving certain gases into the water allows for a new phase - namely hydrate. Hydrate is a solid, just like ice, but the detailed structure is different. In hydrate, the water molecules formes a lattice with holes in which the guest molecules reside. Without the guest molecules, this lattice structure would not be stable. But not any gas can provide the guest molecules necessary to stabilise the structure - the gas molecules cannot be very large. Typical examples of gases which form hydrate is CO₂ and methane.

Why is hydrate interesting?

There are several reasons why hydrate is of interest. In the beginning, hydrate was not much more than a curiousity. But later, it has become interesting for more practical purposes. Hydrate may be formed at temperatures a little above the freezing point. In gas pipes on the sea bed hydrate may form from the gas which is transported together with the small amount of water which is impossible to avoid being there. If the hydrate grows large enough, it may clog the pipe line. To avoid this it is necessary to have knowledge about formation and growth of hydrate.

Another reason for interest in hydrate is the discovery of large amounts of methan stored in hydrates in Siberia and Alaska as well as in the sea bed. Noone knows how much methane thereis in these kinds of reservoirs, but estimates suggest that there is more then all other known fossile fuel reserves combined. This is of course interesting from a commersial point of view, but the methane hydrate may also pose a serious envrionmental threat. When the temperature is increasing, the hydrate will melt and the methane will be released to the atmosphere. Being a very active green house gas, this methane will increase the green house effect and thus give even more temperature increase.

But hydrate may also prove to be useful in the fight against global warming. If CO₂ is extracted from for instance gas or coal fueled powerplants, their environmental impact will be greatly reduced. The extracted CO₂ will of course have to be stored somewhere, and one much considered choice is to store the CO₂ in the see bed. In cold areas hydrate may form and make a seal so that the CO₂ does not leak out. Another proposal is to release the CO₂ in the sea water at large depths. Here the CO₂ will be compressed so much that it will be heavier than water, and will thus sink. But with this large pressure hydrate will form even if the temperature is several degrees above the freezing point of water. So in both these proposals hydrate is a key player, and therefore we need more knowledge about it.

What do we do?

To try to understand more of how the hydrate form, we describe it with a mathematical model which we use to make computer simulations. Most of our work are based on Phase Field Theory which is based on a set of coupled differential equations describing the system consisting of CO₂/CH₄, water and hydrate. The Phase Field Theory gives a fairly accurate description of the physics of hydrate growth on a microscopic scale. But unfortunately the computer simulation based on Phase Field Theory requires a lot of coumputing time. Therefore we have also tried to use a simpler model based on cellular automata. This is a less stringent model, but the results look promising and the simulations do indeed go much faster.

Relevant publications

Two Approaches for Modelling Hydrate Growth
T Buanes, B Kvamme, A Svandal
(in press)

Exploitation of natural gas hydrate reservoirs combined with long term storage of CO₂
B. Kvamme, A. Graue, T. Kuznetsova, T. Buanes, G. Ersland
WSEAS Transactions on Environment and Development (in press)

Multi-scale approach to CO₂-hydrate formation in aqueous solution: Phase field theory and molecular dynamics. Nucleation and growth
G. Tegze, T. Pusztai, L. Grànàsy, A. Svandal, T. Buanes. T. Kuznetsova, B. Kvamme
Journal of Chemical Physics, Volume 124, 2006

Computer simulation of CO₂ hydrate growth
T. Buanes, B. Kvamme, A. Svandal
Journal of Crystal Growth, Volume 287, 2006