Formation of natural gas hydrates in marine sediments gas hydrate growth and stability conditioned by host sediment properties

摘要

The stability conditions of submarine gas hydrates (methane clathrates) are largely dictated by pressure, temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of the host sediments also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our model presumes that gas hydrate behaves in a way analogous to ice in the pores of a freezing soil, where capillary forces influence the energy balance. Hydrate growth is inhibited within fine-grained sediments because of the excess internal phase pressure of small crystals with high surface curvature that coexist with liquid water in small pors. Therefore, the based of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lowever temperature, and so nearer to the seabed than would be calculated from bulk thermodyamie equilibrium. The growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, sheelts, and lenses in muds; cements in sand and ash layers) can be explained by a requirement to minimize the excess of mechanical and surface energy in the system.