Deep sea NMR: Methane hydrate growth habit in porous media and its relationship to hydraulic permeability, deposit accumulation, and submarine slope stability

Abstract

Review of the literature reveals that the nature of pore‐scale interactions between gas hydrates and porous media remains a matter of controversy. To clarify the situation, nuclear magnetic resonance (NMR) measurements have been made on methane hydrate‐bearing sandstones. The samples were synthetically prepared within the gas hydrate stability zone, at or near the seafloor in Monterey Bay, California. The method simulated natural hydrate deposition by gas flows that are not in thermodynamic equilibrium with the surrounding earth. The efficiency of hydrate production was variable, as has been observed elsewhere. When substantial hydrate saturations were achieved, NMR relaxation time measurements indicated that hydrate tended to replace water in the largest pore spaces. The relative permeability to water, as determined by an NMR‐based correlation, was significantly reduced. The magnitude of this reduction was also consistent with formation of hydrate in the centers of pores, rather than with hydrate coating the grains. The growth habit suggested by these results is consistent with creation of hydrate nodules and lenses in coarse, unconsolidated sediments. It is also consistent with scenarios in which methane gas is delivered efficiently to the atmosphere as a result of seafloor slope failure, thereby strengthening global warming feedback mechanisms.