Non-Fickian Diffusion Affects the Relation between the Salinity and Hydrate Capacity Profiles in Marine Sediments

On-site measurements of water salinity (which can be directly evaluated from the electrical conductivity) in deep-sea sediments is technically the primary source of indirect information on the capacity of the marine deposits of methane hydrates. We show the relation between the salinity (chlorinity) profile and the hydrate volume in pores to be significantly affected by non-Fickian contributions to the diffusion flux---the thermal diffusion and the gravitational segregation---which have been previously ignored in the literature on the subject and the analysis of surveys data. We provide amended relations and utilize them for an analysis of field measurements for a real hydrate deposit.Comment: 7 pages, 2 figures, 1 table, submitted to Compte Rendus Mecaniqu

Mass fractionation of noble gases in synthetic methane hydrate : implications for naturally occurring gas hydrate dissociation

This paper is not subject to U.S. copyright. The definitive version was published in Chemical Geology 339 (2013): 242-250, doi:10.1016/j.chemgeo.2012.09.033.As a consequence of contemporary or longer term (since 15 ka) climate warming, gas hydrates in some settings may presently be dissociating and releasing methane and other gases to the ocean–atmosphere system. A key challenge in assessing the impact of dissociating gas hydrates on global atmospheric methane is the lack of a technique able to distinguish between methane recently released from gas hydrates and methane emitted from leaky thermogenic reservoirs, shallow sediments (some newly thawed), coal beds, and other sources. Carbon and deuterium stable isotopic fractionation during methane formation provides a first-order constraint on the processes (microbial or thermogenic) of methane generation. However, because gas hydrate formation and dissociation do not cause significant isotopic fractionation, a stable isotope-based hydrate-source determination is not possible. Here, we investigate patterns of mass-dependent noble gas fractionation within the gas hydrate lattice to fingerprint methane released from gas hydrates. Starting with synthetic gas hydrate formed under laboratory conditions, we document complex noble gas fractionation patterns in the gases liberated during dissociation and explore the effects of aging and storage (e.g., in liquid nitrogen), as well as sampling and preservation procedures. The laboratory results confirm a unique noble gas fractionation pattern for gas hydrates, one that shows promise in evaluating modern natural gas seeps for a signature associated with gas hydrate dissociation.Partial support for this research was provided by Interagency Agreements DE-FE0002911 and DE-NT0006147 between the U.S. Geological Survey Gas Hydrates Project and the U.S. Department of Energy's Methane Hydrates Research and Development Program

Methane hydrate dissociation rates as 0.1 MPa and temperatures above 272K

We performed rapid depressurization experiments on methane hydrate under isothermal conditions above 272 K to determine the amount and rate of methane evolution. Sample temperatures rapidly drop below 273 K and stabilize near 272.5 K during dissociation. This thermal anomaly and the persistence of methane hydrate are consistent with the reported recovery of partially dissociated methane hydrate from ocean drilling cores

Simultaneous determination of thermal conductivity, thermal diffusivity and specific heat in sI methane hydrate

This paper is not subject to U.S. copyright. The definitive version was published in Geophysical Journal International 169 (2007), 767–774, doi:10.1111/j.1365-246X.2007.03382.x.Thermal conductivity, thermal diffusivity and specific heat of sI methane hydrate were measured as functions of temperature and pressure using a needle probe technique. The temperature dependence was measured between −20°C and 17°C at 31.5 MPa. The pressure dependence was measured between 31.5 and 102 MPa at 14.4°C. Only weak temperature and pressure dependencies were observed. Methane hydrate thermal conductivity differs from that of water by less than 10 per cent, too little to provide a sensitive measure of hydrate content in water-saturated systems. Thermal diffusivity of methane hydrate is more than twice that of water, however, and its specific heat is about half that of water. Thus, when drilling into or through hydrate-rich sediment, heat from the borehole can raise the formation temperature more than 20 per cent faster than if the formation's pore space contains only water. Thermal properties of methane hydrate should be considered in safety and economic assessments of hydrate-bearing sediment.Gas Hydrate Project of the U.S. Geological Survey’s Coastal and Marine Geology Program, in addition to Department of Energy contract DE-AI21–92MC2921