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Hydromechanical Properties of Gas Hydrate‐Bearing Fine Sediments From In Situ Testing

The hydromechanical properties of gas hydrate‐bearing sediment are key in assessing offshore geohazards and the resource potential of gas hydrates. For sandy materials, such properties were proved highly dependent on hydrate content (Sh) as well as on their distribution and morphology. Owing to difficulties in testing gas hydrate‐bearing clayey sediments, the impact of hydrates on the behavior of such materials remains poorly understood. Hence, to provide insight into the characterization of clayey sediments containing hydrate, this study relies on a unique database of in situ acoustic, piezocone, and pore pressure dissipation measurements collected in a high gas flux system offshore Nigeria. Compressional wave velocity measurements were used as means of both detecting and quantifying gas hydrate in marine sediments. The analysis of piezocone data in normalized soil classification charts suggested that contrary to hydrate‐bearing sands, the behavior of gas hydrate‐bearing clays tends to be contractive. Correlations of acoustic and geotechnical data have shown that the stiffness and strength tend to increase with increasing Sh. However, several sediment intervals sharing the same Sh have revealed different features of mechanical behavior; suggesting that stiffness and strength of gas hydrate‐bearing clays are influenced by the distribution/morphology of gas hydrate. Pore pressure dissipation data confirmed the contractive behavior of gas hydrate‐bearing clays and showed that at low hydrate content, the hydraulic diffusivity (Ch) decreases when Sh increases. However, for Sh exceeding 20%, it was shown that an increase of Ch with Sh could be linked to the presence of fractures in the hydrate‐sediment system.