We present an equilibrium model of methane venting through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon. Free gas supplied from below forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation. This system self-generates local three-phase equilibrium and allows free gas migration to the seafloor. Log and core data from Ocean Drilling Program (ODP) Site 1249 show that from the seafloor to 50 m below seafloor (mbsf), pore water salinity is elevated to the point where liquid water, hydrate and free gas coexist. The elevated pore water salinity provides a mechanism for vertical migration of free gas through the regional hydrate stability zone (RHSZ). This process may drive gas venting through hydrate stability zones around the world. Significant amount of gaseous methane can bypass the RHSZ by shifting local thermodynamic conditions.

During sea-level drop, water and gas pressures within oceanic hydrate systems can exceed the total vertical stress and this can drive slope failure and gas venting. We investigate this behavior with a multi-phase fluid and heat flow model that considers latent heat of hydrate formation/dissociation, poro-elastic effect of gas-filled sediments, sediment compressibility, and gas mobility. During sea-level drop, fluid pressures drop much less than the total stress due to both the high gas compressibility and hydrate dissociation. In permeable sediments, water expulsion and gas mobility combine to induce underpressure and downward water flow from the seafloor. This study provides a causal mechanism for slope failure and fluid exchange that occur in hydrate systems during sea-level fall. 1.

[1] During sea-level drop, water and gas pressures within oceanic hydrate systems can exceed the total vertical stress and this can drive slope failure and gas venting. We investigate this behavior with a multi-phase fluid and heat flow model. During sea-level drop, fluid pressures drop much less than the total stress due to both the high gas compressibility and hydrate dissociation. In permeable sediments, hydrate dissociation, water expulsion and gas mobility combine to induce underpressure and downward water flow from the seafloor. This study provides a causal mechanism for slope failure and fluid exchange that occur in hydrate systems during sea-level fall. Citation: Liu, X., and P. Flemings (2009), Dynamic response of oceanic hydrates to

Multi-phase fluid flow is critical to the formation and concentration of gas hydrate in marine sediments. A transient, multi-phase (hydrate, gas and liquid) fluid and heat flow model is presented to describe hydrate formation in porous media. Fluid flux and physical properties of sediment largely control the dynamics of gas hydrate formation and free gas migration. In fine-grained sediments, hydrate formation leads to rapid permeability reduction and capillary sealing. Free gas accumulates below the hydrate layer until a critical gas column builds up, thereby forcing gas upward to the seafloor. In coarse-grained sediments, large volumes of gas are transported into the hydrate region to produce a significant change in salinity. An interconnected three-phase zone with high hydrate concentration and elevated salinity develops from the base of hydrate stability to the seafloor. Both processes may drive gas venting through the hydrate stability zone. We also extend these models to demonstrate that the likely impact of climatic warming events on marine hydrate reservoirs. If hydrates are originally formed in the two-phase region, dissociated methane cannot be released to the ocean until the warming at the seafloor exceeds a critical value. However, all of hydrates formed within the threephase