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 viable only if the gas hydrate can nucleate and grow directly from an aqueous solution. While phase equilibrium calculations suggest that the gas hydrate should crystallize from the dissolved gas, barriers associated with nucleation may impede or even prevent hydrate formation. At present, experimental 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 from dissolved CO2 in natural porous media. The experiments demonstrate that the hydrate crystals can nucleate in the absence of free gas, providing important experimental justification for models of hydrate formation that depend on fluid migration. Our results are also relevant for other formation models that depend on in situ methane generation from biogenic sources. Experimental evidence for hydrate formation at low gas concentrations implies less stringent requirements on the biogenic sources of methane compared with some previous estimates. (C) 2000 Elsevier Science B.V. All rights reserved.