Strength characteristics of sediments from a gas hydrate deposit in the Krishna-Godavari Basin on the eastern margin of India

Knowledge of the strength of sediments overlying sub-seafloor gas-hydrate deposits is crucial for predicting borehole and seafloor stability during hydrate extraction. Ideally, sediment strength should be determined along a continuous downhole profile from the seafloor to the hydrate reservoir, but few such profiles have been obtained. In this study we used cores retrieved at Site NGHP-02-23 in the Krishna-Godavari Basin in unconfined penetration tests on split cores and in triaxial deformation experiments on hydrate-free sediment samples. Although penetrometer tests identified relatively low strength (70-250 kPa) likely due to hydrate dissociation in the hydrate-bearing interval 90-300 m below seafloor (mbsf), sediment strength exceeded 350 kPa in the intervals 140-170 and 250-270 mbsf, each of which lies just above a zone of high gas-hydrate concentration. The average stress ratio of triaxial strength at 4% axial strain to effective mean stress (q(epsilon=4%)/p') of hydrate-free silts was about 0.67 throughout the Hole. An exception to this trend was in fine sands from 280 mbsf in the deeper gas-hydrate zone, where the stress ratios were greater than 1.0. The stress ratios of hydrate-bearing sediments in the deeper gas-hydrate zone that were reported from the pressure-core measurements were far greater than those of any hydrate-free sediments for a given effective mean stress. The high-strength intervals in silty sediments we identified by the penetration tests could be associated with zones of high hydrate concentration. Because high-strength layers in fine-grained silty sediments commonly exhibit lower permeability than sandy layers (potential hydrate-host), they may act as seals that assist the precipitation of hydrate below them.