FORMATION AND ACCUMULATION OF GAS HYDRATE IN POROUS-MEDIA

Vast quantities of clathrate hydrate are found in the Arctic and in ma rine sediments along continental margins. The clathrate structure trap s enormous volumes of methane gas, which is both a possible source of global climate change and a potential energy resource. The growth rate and spatial distribution of gas hydrate in the shallow sediments are influenced by a variety of interacting physical processes. In order to quantify these processes, we develop mathematical models for hydrate formation in porous media. An analytical model is derived for the idea lized problem of hydrate growth in a porous half-space which is cooled on its boundary. Our calculations predict the growth rate of a hydrat e layer for a given rate of cooling and show that the volume of hydrat e is strongly dependent on the two-phase equilibrium between hydrate a nd seawater. For a representative phase diagram we find that the volum e of hydrate in the layer is less than 1% of the pore volume. Larger v olumes of hydrate observed in some locations demand a sustained supply of gas and a long accumulation time. Numerical calculations are used to investigate situations that are more representative of conditions i n marine sediments. A simple theoretical expression is derived for the rate of hydrate accumulation due to advection of methane gas from dep th. Using typical estimates of fluid velocities in accretionary enviro nments, we obtain an accumulation rate of 1% of the pore volume in 10( 5) years. The predicted vertical distribution of hydrate is consistent with geophysical inferences from observed hydrate occurrences along t he Cascadia margin. Similar distributions can arise from the combined effects of in situ methane production and warming due to ongoing sedim entation. Predicted differences between these two formation models may be detectable in geophysical and geochemical measurements.