No abstract

Over the last 10 years, marine gas hydrate drilling expeditions utilizing advances in pressure coring techniques and imaging have routinely encountered gas hydrates residing in fine-grained sediments. The hydrate typically occurs as fracture-filling, near vertical, veins that displace the sediment, potentially leading to increased sediment strength that may prevent normal consolidation of the sediment thus leading to underconsolidation. Destabilization of this hydrate, through climate change or human activity on the seafloor, may cause dramatic loss of strength of the sediment and pose a significant geohazard. To assess the impact of hydrate veins on sediment behaviour, this paper reports on a series of consolidated (CU) and unconsolidated undrained (UU) triaxial tests carried out on fine-grained soil specimens hosting simplified, vertical, cylindrical tetrahydrofuran (THF) hydrate veins of varying diameters. The results show that increasing hydrate vein diameter significantly increases strength and stiffness, including the development of post-peak strain-softening. The mode of failure of the hydrate veins influenced the specimen strength, but did not affect the specimen stiffness. Hydrate dissolution during CU tests prevented quantitative comparison with UU tests. However, CU test results on the soil specimens containing the largest hydrate vein suggest that increasing lateral confining stresses increase the sediment strength.

The strength of hydrate‐bearing sediments is an important input parameter for numerical simulations for evaluating the long‐term future gas production from these sediments and the risks associated with such activities on the environment. Studies on laboratory synthesized, and natural, hydrate‐bearing coarse‐grained soils exhibit similar behavior, where increasing hydrate saturation increases specimen strength and stiffness with the corresponding development of peak stress, postpeak strain softening and tendency for sample dilation, which is suppressed with increasing effective stress, although strength is increased. For synthesized specimens, hydrate growth at grain contacts leads to “cementing” behavior and the largest increase in strength, which is subdued when hydrate growth is prevented at these locations. Sample disturbance in natural samples lead to lower strength and stiffness compared to laboratory synthesized samples. Unconfined compression shear tests on natural samples highlight the strong “cementing” effect of gas hydrates on coarse‐grained soils. The strength of natural sediments appears strongly correlated with particle size and clay content, with smaller particle and increasing clay content reducing strength for a given hydrate saturation. The strength parameters, friction angle, and cohesion appear to depend on sample type. For synthesized specimens, friction angle was reasonably independent of hydrate saturation while cohesion increased in an exponential manner, with the largest increase occurring when hydrate growth is at particle contacts. In contrast, friction angle appeared to increase with a corresponding reduction in cohesion for natural hydrate‐bearing sediments. However, these observations may be related to sample disturbance and stress conditions under which the data were acquired.

No abstract

The last few decades have seen considerable interest in methane gas hydrates, an ice-like compound comprised of methane gas and water that forms in deep-water marine sediments on continental margins and below permafrost in Arctic regions (Kvenvolden & Lorenson, 2001), as a future energy resource (Boswell & Collett, 2011). This is especially true for coarse-grained sediments where the high intrinsic permeability and high hydrate saturations, up to 90% of available pore space (e.g., Boswell et al., 2016), make them potentially commercially exploitable using current production technologies (Moridis et al., 2009). Although recent estimates suggest upwards of ∼500 Gt of C are stored as hydrates (Wallmann et al., 2012) the vast majority of hydrate located in marine sediments are formed within fine-grained sediments (Klauda & Sandler, 2005).