Formation of gas hydrate from dissolved gas in natural porous media

Migration of methane-bearing fluids from the depth is commonly proposed to explain the formation of marine gas hydrate. This formation mechanism is vi able only if the gas hydrate can nucleate and grow directly from an aqueous solution. While phase equilibrium calculations suggest that the gas hydrat e should crystallize from the dissolved gas, barriers associated with nucle ation may impede or even prevent hydrate formation. At present, experimenta l support for the phase equilibrium calculations is insufficient to confirm whether the hydrate can form in the sea floor from the dissolved gas. Here we report on a set of experiments in which the hydrate is crystallized fro m dissolved CO2 in natural porous media. The experiments demonstrate that t he hydrate crystals can nucleate in the absence of free gas, providing impo rtant experimental justification for models of hydrate formation that depen d on fluid migration. Our results are also relevant for other formation mod els that depend on in situ methane generation from biogenic sources. Experi mental evidence for hydrate formation at low gas concentrations implies les s stringent requirements on the biogenic sources of methane compared with s ome previous estimates. (C) 2000 Elsevier Science B.V. All rights reserved.