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Thermodynamic study of unsaturated hydrates formed from a gas mixture in marine sediments

Pasadakis Nikos, Marinakis Dimitrios, Varotsis Nikolaos, Yang J. , Tohidi, Bahman, Perissoratis, Constantine D

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Summary

Gas hydrates are crystalline compounds which can be formed when water or ice and suitably sized moleculesare brought together under favorable conditions. Small sized molecules, such as the natural gas ones fit suitablyin the hydrate lattice producing solid phase, which despite its resemblance to common ice, it contains substantialamounts of natural gas (1 m3 of gas hydrate can produce up to 175 Nm3 of natural gas). The pressuretemperatureconditions for their formation depend mainly on the composition of the gas and of the water-richphase. Conditions favorable for gas hydrate formation can be found in permafrost regions and mainly in marineenvironments. 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 naturalenvironments, the up-to-present research focuses primarily on single-gas-component hydrates. This may beappended both to the higher complexity of the thermodynamic behavior of the multi-component hydrates , aswell as to the fact that the most hydrates discovered so far were found either on the surface or at a depth of afew meters and were comprised of almost pure methane.The conducted research is part of a wider project targeted to evaluate the environmental conditions of the“Anaximander” mud volcanoes' seabed, an area of the East Mediterranean sea where gas hydrates were foundfrom previous exploration cruises. The aim of this study is to assess the thermodynamic behavior of a ternary gas(C1 to C3) of thermogenic origin at conditions well inside the hydrate formation region (13 to 20 Mpa pressureand 12 to 20 oC temperature). The fractionation of the enclathrated gas components has been experimentallyobserved and measured at these conditions and at different salinity levels.The results reveal that in hydrate undersaturated conditions such as the ones found in the “Anaximander” area,gas hydrates can be formed mainly from gas components dissolved in the pore water, even in the absence of anyfree gas phase. Moreover, the fractionation of gas components in the hydrate phase with respect to temperatureshould lead to a compositional variation with depth of the gas contained in the GH below seabed.

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