A multi-orientation system for characterizing microstructure changes and mechanical responses of fine-grained gassy sediments associated with gas hydrates

Fine-grained marine sediments containing veiny and nodular gas hydrates will evolve into fine-grained gassy sediments after hydrate dissociation due to climate-driven ocean warming. The mechanical properties of the fine-grained gassy sediments are basically acquired by ocean engineering design. However, they have not been fully understood, largely due to the lack of microstructure visualization. In this paper, a new system is developed to jointly conduct x-ray computed tomography scans, oedometer tests, and seismic wave testing on a single specimen with temperature being well controlled, allowing varieties of experimental data to be acquired effectively and automatically. The results show that stress history can hardly affect the undrained stiffness of fine-grained gassy sediments, while the drained stiffness of fine-grained sediments without gas bubbles is stress history dependent. After being unloaded, many microstructure changes remain, and examples include the free gas distribution being more concentrated and the connectivity among gas bubbles becoming much better. The multi-orientation system lays the foundation for further studies on the microstructure changes and mechanical responses of fine-grained gassy sediments associated with gas hydrates.

Methane Bubble Ascent within Fine-Grained Cohesive Aquatic Sediments: Dynamics and Controlling Factors

Methane (CH₄) is a potent greenhouse gas. Its release from aquatic sediments to the water column and potentially to the atmosphere, is a subject of great concern. A coupled macroscopic single-bubble mechanical/reaction-transport numerical model was used to explore the ascent of a mature CH₄ bubble toward the seafloor in muddy aquatic sediment. Two bubble ascent scenarios were demonstrated: stable and dynamic. For small effective overburden loads (≤11 kPa), stable ascent is followed by dynamic ascent (which has not been previously demonstrated to the best of the our knowledge). This ultimately leads to the bubble being released to the water column. Higher effective overburden loads induce only stable bubble ascent, which stops at the gas horizon frequently observed below the seafloor. The depth of the gas horizon increases, while bubble rise velocity decreases with an increase in the overburden load. It is shown that the bubble migration scenario is managed predominantly by inner bubble pressure, which defines a bubble solute exchange with ambient porewaters. Predicting a bubble ascent scenario in muddy sediment will further allow estimation of CH₄ emission to the atmosphere and evaluation of changes in the effective mechanical properties of aquatic sediment due to the ascending bubbles.

Non-Fickian diffusion and the accumulation of methane bubbles in deep-water sediments

In the absence of fractures, methane bubbles in deep-water sediments can be immovably trapped within a porous matrix by surface tension. The dominant mechanism of transfer of gas mass therefore becomes the diffusion of gas molecules through porewater. The accurate description of this process requires non-Fickian diffusion to be accounted for, including both thermal diffusion and gravitational action. We evaluate the diffusive flux of aqueous methane considering non-Fickian diffusion and predict the existence of extensive bubble mass accumulation zones within deep-water sediments. The limitation on the hydrate deposit capacity is revealed; too weak deposits cannot reach the base of the hydrate stability zone and form any bubbly horizon.