The methane stored in gas hydrates is of interest and concern as a potential alternative energy resource and as a potent greenhouse gas that could be released from sediments to the atmosphere and ocean during global warming. In continental margin settings, methane release from gas hydrates also is a potential geohazard and could cause submarine landslides that endanger offshore infrastructure.
Gas hydrate stability is sensitive to temperature changes. To understand methane release from gas hydrate, the U.S. Geological Survey (USGS) conducted a laboratory investigation of pure methane hydrate thermal properties at conditions relevant to accumulations of naturally occurring methane hydrate. Prior to this work, thermal properties for gas hydrates generally were measured on analog systems such as ice and non-methane hydrates or at temperatures below freezing; these conditions limit direct comparisons to methane hydrates in marine and permafrost sediment.
We form pure methane hydrate directly in the measurement chamber by slowly heating granular (180-250 µm) water (H2O) ice in a pressurized methane (CH4) atmosphere, as described by Stern et al. [1996,1998]. This reproducible technique produces methane hydrate with bulk composition: CH4·(5.89±0.01)H2O when quenched after high-pressure synthesis [Stern et al., 2000].
We check for full reaction to methane hydrate by lowering the sample temperature below the ice point. If more than 2% of the initial seed ice persists as water, a pressure transducer detects the pore gas pressure increase due to water expanding while freezing. Heat liberated when water freezes provides an additional, more sensitive indicator of unreacted water near the thermistor bead. In this work, ice volumes are below our detection levels.
Following hydrate synthesis, the sample porosity is ~34%. To reduce porosity, silicone oil is pumped between the vessel walls and Teflon sample liner, holding a compaction pressure of ~102 MPa at 14.4°C for 4 days. Cryogenic SEM images of the recovered sample show dense, fine-grained hydrate (~5-50 µm grain size). Porosity is difficult to assess from SEM images of the depressurized sample, but is expected to be less than 5% based on previous experience with samples compacted axially at 105 MPa [Helgerud, 2001]. The consistency of this synthesis and compaction method is evident in our thermal property results, which show a sample-to-sample variability below 3% for thermal conductivity. Thermal diffusivity and specific heat vary by ~5%. The fit equations for our measured results are given in the tables below.
Thermal Conductivity, λ: As with our measurements in Ice Ih, we conservatively estimate our uncertainty in thermal conductivity to be ±1%. Variations between thermal conductivity datasets is likely due partly to real differences between different types of hydrate, but also likely results from experiment-specific factors such as sample purity and sample preparation. A key result is the similarity between the thermal conductivity of water, and our measured thermal conductivity in methane hydrate. Implications of the ~10% difference in thermal conductivity between water and methane hydrate are described below in the Implications section.
Thermal Diffusivity, κ: The uncertainty in κ increases as the acquisition rate, and hence the number of available data points, decreases. Above 0°C, the needle probe's thermistor resistance drops below 10 kW, allowing a data acquisition rate of 27.8 meas./sec, compared to 18.2 meas./sec below 0°C. Though the scatter in repeated measurements above 0°C can reach ±3%, the measurement uncertainty is controlled by the sample-to-sample variability of approximately ±7% (error bar at 10°C for our work). Agreement between datasets, obtained using three different measurement techniques, provides a consensus result for κ in sI methane hydrate.
Specific Heat, cp: We calculate cp from our measurements of λ and κ using
for which density, ρ, is the only additional parameter required. We calculate ρ from the unit cell volume, Vo, and unit cell mass, assuming a stoichiometry of CH4·nH2O, with n = 6. methane hydrate has a cubic structure, so Vo is taken as the cube of the lattice parameter aI, measured as a function of temperature by Shpakov et al. . Extrapolating to our measurement temperature range of -20 to +17°C yields:
ρhydrate (kg/m3) = 926.45 - 0.239·T(°C) - 3.73x10-4·T(°C)2. (7)
Handa (1986) assumed a hydrate stoichiometry of CH4·6H2O, and measured a specific heat, cpHanda, given by:
cpHanda (J/kg·K) = 2100 - 7.07·T(°C) + 1.23x10-2·T(°C)2 + 5.08 x10-2·T(°C)3 . (8)
Measurement pressure differences between the 31.5 MPa used in our work and the ~3 MPa used by Handa (1986) account for half the 4% discrepancy between our derived values of cp and Handa's direct measurement of cp via precision calorimetry. Based on our sample-to-sample variability of ±5%, however, differences between our measurements and those of Handa are not considered significant.
Table 1: CH4 hydrate thermal property dependence on temperature, measured at 31.5 MPa confining pressure.
*The T-1 dependence of the κ fit requires input temperatures in Kelvin.
Table 2: CH4 hydrate thermal property dependence on pressure, measured at 14.4°C between 31.5 and 102 MPa confining pressure.
In nature, methane hydrates generally displace ice or water in the pore space of sediment when they form. Estimating how thermal properties in hydrate-bearing sediments differ from the surrounding sediment layers therefore requires understanding the extent to which methane hydrate thermal properties differ from those of ice or water
Thermal Conductivity, λ: The thermal conductivity of water is approximately equal to that of methane hydrate, so the thermal conductivity beneath permafrost or in marine settings is essentially independent of methane hydrate content. Methane hydrate is generally found in abundance below, rather than in, permafrost, but the presence of methane hydrates in ice-dominated permafrost can measurably increase the geothermal gradient because the thermal conductivity of ice is approximately four times that of methane hydrate.
Thermal Diffusivity, κ: Since the thermal diffusivity of water is about half that of methane hydrate, hydrate-bearing sediment can change temperature more rapidly than water-bearing sediment. This characteristic presents a potential geohazard for conventional hydrocarbon production from deepwater petroleum production sites overlain by hydrate-bearing layers. If warm hydrocarbons in the wellbore dissociate hydrate in the surrounding sediment, the sediment strength decreases, potentially causing well failure or localized submarine landslides.
Specific Heat, cp: Hydrate breakdown is an endothermic process, meaning heat is absorbed from the surroundings during hydrate dissociation. This heat can be supplied by the surrounding sediment formation. Since the specific heat of methane hydrate is about half that of water, hydrate-bearing sediment stores less heat that can then be made available to help fuel dissociation. When estimating the efficiency of hydrate dissociation, neglecting the contribution of methane hydrates to the specific heat of the formation results in an overestimate of the dissociation rate.
The results are published in: Waite, W.F., Stern, L.A., Kirby, S.H., Winters, W.J. and Mason, D.H., 2007, Simultaneous determination of thermal conductivity, thermal diffusivity and specific heat in sI methane hydrate, Geophysical Journal International, 169, p. 767-774, doi: 10.1111/j.1365-246X.2007.03382.x.