Thermal Property Measurements in Tetrahydrofuran (THF) Hydrate Between -25 and +4\deg C, and Their Application to Methane Hydrate

Using a "hot wire" needle probe, we make simultaneous thermal conductivity and diffusivity measurements of pure THF hydrate between -25\deg C and the THF hydrate stability boundary near +4\deg C. Combining these measurements with published pressure and temperature dependent THF hydrate density data, we derive the sample's specific heat capacity. Over the measured temperature range, there are two distinct thermal property behavioral regimes. Between -25 and -7.5\deg C, all three thermal properties depend linearly on temperature. Thermal conductivity rises from 0.489 to 0.496 W/mK, in agreement with our previous measurements at 0.1 MPa confining pressure. Diffusivity rises from 2.50x10^-7 to 2.66x10^-7 m²/s, and the specific heat falls from 2020 to 1930 J/kgK. Both results are in agreement with estimates calculated from published THF data. Above -7.5\deg C, the thermal conductivity rises nonlinearly, increasing to 0.58 W/mK at +3\deg C. The diffusivity also increases nonlinearly to 3.2x10^-7 m²/s, resulting in a nearly linear specific heat decrease to 1865 J/kg·K at +3\deg C. The thermal conductivity rise is less dramatic than the nearly 350% increase observed in the same system at 0.1 MPa confining pressure, but indicates a behavioral change in the thermal properties of THF hydrate for temperatures approaching the stability boundary near +4\deg C. THF hydrate has been used as a thermal property analog for methane hydrate. In contrast to our structure II THF hydrate results, our thermal conductivity measurements in porous, pure structure I methane hydrate are linear at least to +15\deg C, approximately 3\deg C below the methane hydrate stability temperature at 24.8 MPa. The nonlinear thermal properties of THF hydrate above 0\deg C do not accurately represent those of methane hydrate at the same temperature. For thermal conductivity, a closer comparison between THF and methane hydrate is achieved by extrapolating the linear THF hydrate property trends from below -7.5\deg C to positive temperatures. This procedure yields results within ±0.01 W/m·K of published estimates for methane hydrate up to 20\deg C.