Experimental Study of the Gas Hydrates Dissociation Effect on the Properties of the Host Marine Sediment

Gas hydrates consist of small molecules trapped in crystal lattices of water molecules in a form resembling to common ice. The hydrate forming components are abundant in subsea sediments at conditions inside the hydrate stability zone. The vast majority of natural gas hydrates is expected to be hosted at the subsea sedimentary geo-environment. Despite the fact that such hydrates are not exploitable under the current technology, they can pose an environmental risk in case of uncontrolled dissociation. In this work, the effect of gas hydrates dissociation on the mechanical properties of a muddy clayish sediment retrieved from the "Anaximander" subsea mud volcano area and on its ability to conduct flow, was studied. A marine sediment was reconstituted in vitro with in-situ retrieved mud, water and hydrocarbon gas mixtures of compositions similar to the ones measured just above the seabed and was brought at conditions favouring the formation of hydrates. Geomechanical tests were conducted on the sediment before and during the dissociation of the guest hydrates, by isobaric heating and by isothermal depressurization. Permeability was found to increase only moderately during hydrate dissociation, possibly due to the high effective stress. The consolidation of the formation was increased up to a maximum just before the hydrates start to decompose and then collapsed when the free gas was produced. The bulk sediment volume was reduced by around 15% when the liberated gas was released from the formation in sudden "bursts" which occurred sporadically. The experimental results presented in this study seem to support the possibility of subsea landslides and the hydrate gun hypothesis for such areas as a result of hydrate dissociation.