EFFECT OF GAS HYDRATE STABILITY ON CLIMATIC CHANGE. THE CASE OF 'ANAXIMANDER' MUD VOLCANOES.

摘要

Gas hydrates are crystalline compounds which can be formed when water or ice and suitably sized molecules are brought together under favourable conditions. Small sized molecules, such as the natural gas ones can be accommodated in the hydrate lattice producing solid phase, which despite its resemblance to common ice, it contains substantial amounts of natural gas (1 m3 of gas hydrate can produce up to 175 Nm3 of natural gas). The pressure-temperature conditions for their formation depend mainly on the composition of the gas and of the water-rich phase. Conditions favourable for gas hydrate formation can be found in permafrost regions and mainly in marine environments. Over 95% of the estimated gas hydrates globally are considered to be found below the seabed. While both single and multi component natural gas can be enclathrated in the hydrate phase in natural environments, the up-to-present research on the stability of the hydrate reserves focuses primarily on single-gas-component hydrates. This may be appended both to the higher complexity of the thermodynamic behaviour of the multi-component hydrates , as well as to the fact that most hydrates discovered so far were either found on the surface or at a few meters of depth and were comprised of almost pure methane. The research that is presented in this paper is part of a wider project targeted to evaluate the formation of hydrates at natural subsea environment in absence of any free gas phase. Such hydrate formation conditions prevail on the "Anaximander" mud volcanoes' seabed, an area of the East Mediterranean where gas hydrates were found from exploration cruises. The behaviour of the gas hydrates in this area was experimentally simulated through a ternary gas mixture (C_1 to C_3) at conditions well inside the incipient hydrate formation region (8 to 20 MPa pressure and 5 to 24°C temperature). The fractionation of the enclathrated gas components has been experimentally observed and measured at these conditions and at different salinity levels (0 to 12%wt). Based on the data from a mud volcano (MV) of the specific site as a case study, simulations were performed on the stability of the gas hydrates, when conditions shift from the in situ ones. The results indicate that temperature rise bears the most profound effect on hydrate stability and that the methane release from a potential dissociation of hydrates should be taken into account in any detail study on global warming effect. Estimations were made over the total amount of the gas in place, which is stored in the gas hydrate (GH) phase, as well as over the total gas emissions due to GH dissociation.