Capillary effects on hydrate stability in marine sediments

We study the three-phase (Liquid + Gas + Hydrate) stability of the methane hydrate system in marine sediments by considering the capillary effects on both hydrate and free gas phases. The capillary pressure, a measure of the pressure difference across a curved phase interface, exerts a key control on the methane solubility in Liquid + Hydrate (L + H) and Liquid + Gas (L + G) systems. By calculating the L + H and L + G solubilities as a function of water depth (pressure) and pore size (interface curvature), we show how the solubility requirements for forming both gas hydrate and free gas can be met in a three-phase zone. The top of the three-phase zone shifts upward in sediments as the water depth increases and the mean pore size decreases. The thickness of the three-phase zone increases as the distribution of pore sizes widens. The top of the three-phase zone can overlie or underlie the bulk three-phase equilibrium depth. At Blake Ridge, we predict that the three-phase zone is 27.7 m thick and that the top of the three-phase zone lies 13 m above the predicted bulk equilibrium depth. This reconciles the observation of the bottom-simulating reflector (BSR) at Blake Ridge that is shallower than the predicted bulk equilibrium depth. In contrast, at Hydrate Ridge where water depth is shallower, we predict that the three-phase zone is 20.4 m thick and that the top of the three-phase zone lies 0.7 m below the predicted bulk equilibrium depth. Our model, which predicts an upward shift in the top of free gas occurrence with increasing water depth (pressure), is compatible with worldwide observations that the BSR is systematically shifted upward relative to the bulk equilibrium depth as water depth (pressure) is increased.