Ostwald ripening changes the pore habit and spatial variability of clathrate hydrate

Clathrate hydrate is a solid crystalline compound that consists of water cages with trapped gas molecules. Methane hydrate in permafrost and marine sediments is estimated to contain 2 x 10(3) to 4 x 10(6) Gt of carbon, which makes it both a potential energy source and a liability to global warming. The transport and mechanical properties of hydrate bearing sediments (HBS), and therefore hydrate dissociation through depressurization, thermal stimulation, or CO2 injection, depend on hydrate pore habit and spatial distribution in sediments. However, the evolution of hydrate pore habit and spatial distribution in HBS are not well understood yet. Here we show experimental evidence of Ostwald ripening of gas hydrate crystals in pores and porous media. We find that (1) hydrate growth rate (under nearly isothermal conditions) versus the degree of overpressurization fits an Arrhenius-type equation, and (2) Ostwald ripening gradually changes hydrate pore habit from grain-attaching to pore-filling and increases hydrate saturation heterogeneity at both pore and core scales. The latter ultimately creates significant spatial variations of permeability and sediment strength. Our results provide insights into kinetics and pore-scale physics that determine the geophysical signature from HBS at various scales. Based on our findings of hydrate occurrence in both gas and gas-water interface, we argue that hydrate growth in natural HBS is not limited to the water liquid phase. Our observation of hydrate pore habit alteration by Ostwald ripening explains patchy saturation and heterogeneous distribution of hydrate in natural HBS, which gives physical insight to improve geophysical hydrate models in porous media.