Vertical migration of gas through fractures due to salinity-buffered hydrate formation within the hydrate stability zone

Abstract:

Seafloor gas vents within the hydrate stability zone along continental margins are frequently associated with high saturations of gas hydrate that increase upwards towards the seafloor (e.g. NGHP 01 Site 10A, UBGH2 Site 3, ODP Site 1249A). One model for how gas migrates through the hydrate stability zone proposes that salt exclusion during hydrate formation elevates the in-situ salinity, which allows for the coexistence of gas, hydrate, and water. In some portions of these sites, however, the hydrate saturations derived from pressure cores are insufficient to elevate the bulk salinity enough to induce three-phase conditions. Commonly, hydrate in these zones exists in individual, high-angle fractures within mudstone. An alternative gas migration model, applicable to fractured regions, proposes that the fractures act as highly permeable gas conduits for upward fluid flow. We present a model to describe gas flow and hydrate formation in these fractures. Hydrate begins forming at the fracture wall, where the components (water and gas) are available. Water is continuously drawn from the mudstone to the fracture by capillary forces. Due to hydrate formation, salinity is elevated to three-phase conditions at the fracture wall. Simultaneously, salt diffuses away from the fracture into the bounding mudstone. At this point, hydrate formation is controlled by the chemical diffusion of salt away from and the advection of less-saline water towards the fracture wall. Hydrate formation is ultimately limited by either insufficient water flow to the hydrate front or elevation of the bulk salinity to three-phase conditions. We explore the timescale, nature, and limitation of hydrate formation in vertical fractures.