The effect of restrictive diffusion on hydrate growth

Abstract

Methane hydrate is formed naturally in a number of geologic settings around the world. The most predominant methane hydrate reservoirs are found in shallow oceanic basins at low temperatures and high pressures. A widely observed phenomenon in these oceanic sequences is extensive fine-grained sediments containing little to no hydrate interbedded with highly saturated sand bodies (20-60%). At Walker Ridge Block 313 in the Gulf of Mexico, one particular coarse-grained bed (approximately 3m-thick) is estimated to have methane hydrate occupying as much as 60% of the available pore space surrounded by hydrate-free clay. Here, I develop a numerical model that simulates methane hydrate growth in shallow oceanic basins in order to test whether diffusive transport of methane is a viable transport mechanism for forming highly saturated sand layers. I conclude that methane diffusion is likely responsible for the key identifying features of hydrate formation in interbedded sands and shales (i.e. greater hydrate saturations at the sand boundaries surrounded by hydrate-free zones in the fine-grained matrix). In addition, I show that the key parameters affecting the hydrate saturation profile include the amount of available methane for hydrate growth, thickness of the sand layer, and the radius of the fine grained pore space. I also discuss the shortcomings of the developed model and what complexities need to be added to more accurately reproduce hydrate growth throughout intricate hydrogeologic systems.