MIDAS. Complete geo-mechanical properties of gas hydrate bearing sediments from in situ geotechnical measurements

The present report aims at providing details about the overall approach, tool and processing techniques deployed to determine geo-mechanical properties of gas hydrate-bearing sediments. It focusses on two piezocone soundings carried out in the Romanian sector of the Black Sea during the GHASS cruise (September 2015). One of them was selected as core sampling at the same site provided ground truth for the presence of gas hydrates in clay sediments. The other sounding was selected to serve as a reference for comparing geo-mechanical properties of sediments with and without gas hydrates. Two distinct classifications were used to identify the behaviour type of sediments during piezocone penetration. Each of them pointed out that, at the reference site, penetration occurred fully undrained as in typical clay soils. Besides, the ‘unusual’ characteristics of gas hydrate-bearing sediments was highlighted by the discrepancy between the two classifications. The discrepancy has been ascribed to the fact that gas hydrate-bearing sediments behave undrained as stiff or sensitive clays while they have lower friction ratio. Accordingly, similar empirical correlations were applied to estimate some geo-mechanical properties of reference and gas hydrate-bearing sediments. This allowed to point out that the presence of gas hydrates tends to increase the effective yield stress, peak undrained shear strength and strength sensitivity of sediments while it tends to decrease their compressibility. Although the extents to which gas hydrates affect these properties could have been estimated, the lack of experience and statistical correlation studies with gas hydrate-bearing sediments warrants caution in their use.