Overview of Measurement Technique:

What thermal properties do we measure?

Methane hydrate, recovered during the USGS gas hydrate research cruise in the Gulf of Mexico in 2002. Hydrate's low thermal conductivity means hydrate transfers heat slowly and can therefore be handled with bare hands even after storage in liquid nitrogen. (Photo by William Winters, USGS. Click picture for larger image).

In each experiment, we simultaneously measure thermal conductivity, thermal diffusivity and specific heat.

Thermal conductivity, λ: The amount of heat passing through a body in response to a thermal gradient. A low thermal conductivity material is like insulation in a house, letting very little heat pass out of the house on a cold day. The units are (W/m·K).

Thermal diffusivity, κ: The rate at which heat spreads through a body. It is a function of the body's thermal conductivity and its specific heat. A high thermal conductivity will increase the body's thermal diffusivity, as heat will be able to conduct across the body quickly. Conversely, a high specific heat will lower the body's thermal diffusivity, since heat is preferentially stored as internal energy within the body instead of being conducted through it. The units are (m²/s).

Specific heat, c_p: The amount of heat required to raise the temperature of one kilogram of material one degree Celsius. The units are (J/kg·K).

Why are these properties important to know?

Methane hydrate stability depends sensitively on temperature, meaning thermal properties are important for modeling thermal fluctuations at local, regional and global scales [Ruppel, 2000; Xu and Germanovich, 2006].

• Local scale: Accurate heat flow models are required for controlled production of methane from hydrate [Ji, et al., 2003; Pooladi-Darvish, 2004], or conventional hydrocarbon extraction, which can destabilize overlying hydrate [Hovland and Gudmestad, 2001].
• Regional scale: Bottom water warming can destabilize methane hydrate, leading to seafloor slumping [Kennett et al., 2003]. Hydrate thermal properties are also used in heat flow measurements at local and regional scales [Grevemeyer and Villinger, 2001].
• Global scale: Methane is a potent greenhouse gas. As a result, hydrate thermal properties are needed to relate climate fluctuations to methane released from hydrate [Revil, 2000].

How do we measure these properties?

We have developed a method for simultaneously determining λ, κ, and c_p [Waite et al., 2006] based on the von Herzen and Maxwell [1959] needle probe thermal conductivity measurement and the analytical approach of Blackwell [1954]. Here we describe both the simultaneous measurement technique and the needle probe measurement apparatus.

Publication:

The measurement process described here is published in: Waite, W. F., Gilbert, L.Y., Winters, W.J., and Mason, D.H., (2006), Estimating thermal diffusivity and specific heat from needle probe thermal conductivity data, Review of Scientific Instruments, 77, 044904, doi:10.1063/1.2194481.