Gas hydrate growth, methane transport, and chloride enrichment at the southern summit of Hydrate Ridge, Cascadia margin off Oregon

The mechanics of uncemented soft sediments during bubble growth are not widely understood and no rheological model has found wide acceptance. We offer definitive evidence on the mode of bubble formation in the form of X-ray computed tomographic images and comparison with theory. Natural and injected bubbles in muddy cohesive sediments are shown to be highly eccentric oblate spheroids (disks) that grow either by fracturing the sediment or by reopening preexisting fractures. In contrast, bubbles in soft sandy sediment tend to be spherical, suggesting that sand acts fluidly or plastically in response to growth stresses. We also present bubble-rise results from gelatin, a mechanically similar but transparent medium, that suggest that initial rise is also accomplished by fracture. Given that muddy sediments are elastic and yield by fracture, it becomes much easier to explain physically related phenomena such as seafloor pockmark formation, animal burrowing, and gas buildup during methane hydrate melting.

https://www7430.nrlssc.navy.mil/facilities/CTScanner/GEOLOGY_Bubbles.pdf

Bubble growth and rise in soft sediments

A critical state framework for unsaturated compacted clay is proposed. Five state variables are included: mean net stress, deviator stress, suction, specific volume and water content. The proposed framework includes three equations to define isotropic normal compression states, three equations to define critical states and two equations to define a state boundary.
Support for the proposed critical state framework is provided by results from a series of controlled suction triaxial stress path tests on samples of unsaturated compacted kaolin. Unsaturated 50 mm diameter samples were produced by static compaction in 9 layers, prior to consolidation to virgin states at selected values of mean net stress and suction. Shear tests were conducted with a variety of different stress paths and drainage conditions, in a double-walled triaxial cell with independent computer control of cell pressure, pore air pressure and pore water pressure.
The experimental results indicated that all test paths originated on a unique isotropic normal compression hyperline and terminated on a unique critical state hyper-line. To a first approximation all stress paths traversed a unique state boundary that could be defined by the expansion of a yield surface with elliptical cross-sections at constant values of suction.
Possible applications of a critical state model for unsaturated soil include: providing a qualitative framework to aid understanding of unsaturated soil behaviour; guiding the choice of strength parameters to be used in conventional stability calculations; and providing a formalized mathematical stress-strain model for incorporation in a finite element program.

A critical state framework for unsaturated soil.

Seabed Fluid Flow

Understanding the nature and extent of methane production and flux in aquatic sediments has important geochemical, geotechnical, and global climate change implications. Quantifying these processes is difficult, because much of the methane flux in shallow sediments occurs via ebullition (bubbling). Direct observation of bubble formation is not possible, and bubbling is episodic and dependent upon a number of factors. Whereas previous studies have correlated methane flux with surface wind intensity, detailed study of Lake Gatun in Panama and Lago Loiza in Puerto Rico suggest that methane flux is more closely correlated with the shear stress in sediments caused by bottom currents. Bottom currents in turn are a complex function of wind, internal pressure gradients, and lake bathymetry. A simple physical model of bottom currents and sediments in these lakes suggests that most methane ebullition originated from the upper 10–20 cm of the sediment column. Our data reaffirm previous studies showing that ebullitive methane flux is minor in water deeper than ∼5 m.

/pdf/physical-controls-on-methane-ebullition-from-reservoirs-and-8x4tmtr0w1.pdf

Physical Controls on Methane Ebullition from Reservoirs and Lakes

An extensive programme of research into the influence of undissolved gas bubbles on the behaviour of fine-grained onshore soils is reviewed. The programme has been based on the development of a laboratory technique for the preparation of reconstituted soil samples containing a uniform distribution of gas bubbles. The structure of these samples is similar to that observed in sediment recovered from the sea bed, and consists of large gas-filled cavities surrounded by a matrix of saturated soil. It is found that surface tension effects limit the difference between gas pressure and pore water pressure, and that the overall void size is effectively a function of the strength of the matrix, so that changes in void volume may be modelled by cavity expansion and contraction in an ideal plastic medium, leading to limits on the difference between gas pressure and mean total stress. A new parameter, operative stress, is shown to influence both the consolidation and the strength of these gassy soils. Thus, during con...

T. N. Gardner

Papers

Behaviour of offshore soils containing gas bubbles