In homogenous, unfractured fine sediments separate phase gas flow is not possible. Diffusion will dominate over short distances and when the upwards flow rate of water is less than about 0.5 mm per year. Advection will likely dominate if the water moves upwards at more then a few mm per year, and particularly along fractures, pipe-like conduits or sand layers.
Capillary supersaturation increases the efficiency of diffusion and advection of the water dissolved methane through fine sediments, but exsolved methane bubbles can be trapped in pore bodies and in coarse sediment layers. In the hydrate system, capillary supersaturation enhances hydrate stability (Henry et al. 1999) and can drive a methane flux from fine layers where hydrate will not form, to coarse layers where hydrate will concentrate (Clennell et al. 1999).
Dispersed methane flux to the sea bed is limited by the oxidation process in the sulfate reduction zone (Borowski et al. 1999). This zone can be bypassed if flow is along hydrate-lined fractures, or if overpressure and buoyancy generate a rapid bubble stream (Martens and Klump 1980) or combined flow of sediment, gas and water in a diatreme or mud volcano (Ginsburg and Soloviev 1994, 1998). The time series of pressure and flux in fracture driven flow and during invasion percolation in a capillary network are remarkably similar. This is because both are critically-limited phenomena that generate a pulse of flow when a threshold of driving pressure is reached. Seabed vents commonly exhibit episodic discharge, but it is not a simple matter to deduce the mechanism responsible. Hydrate formation may be irregular in time and place as flow switches around, and in fractured sediments a stockwork of hydrate-filled veins may form (Ginsburg and Soloviev 1997; Soloviev and Ginsburg 1997).
Gas charged sediment is less dense than sediment with no free gas, will exert a buoyant thrust. This could lead to diapirism (gravitational instability, and overturn), or simply may cause arching and fracturing. Hydrate is also lighter than water and thick layers may be gravitationally unstable. Permafrost-like sediment deformation features are a dostinct possibility.
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