Methane hydrates are believed by some in the scientific community to be the answer to the planet’s energy needs. These ice crystals are a combination of water and methane and form in areas of high pressure and low temperature, they are found in ocean sediments close to continental margins and under the permafrost in the arctic (Figure 1). Scientists predict that the amount of carbon held in these areas exceeds the rest of the world’s carbon energy stores combined. Therefore, they have a massive energy potential, but their collection poses large risks to the climate of the planet due to the fact that methane is a stronger greenhouse gas than carbon dioxide. Currently, extensive research is being performed to learn more about these deposits, and how they could be safely removed. Thermal properties are a crucial part of this research, as they influence the stability of the deposits, extraction process, and the role that they could play in the carbon cycle and climate change. Muraoka et al. (2014) used the Hot Disk transient plane source technique to study the thermal conductivity of sediment cores taken from the Nankai Trough off the coast of Japan to determine how it is influenced by composition of the sediment containing the methane hydrate deposits.
Figure 1. Schematic detailing where methane hydrate deposits form and potential methods of accessing them1.
The cores used for this research were collected from Tokai-oki test wells using a pressure temperature core sampler (PTCS) which maintains the sediment cores at their original pressure after collection. The cores contained layers of sand and mud, the majority of the methane hydrate deposits were contained within the sand. Muraoka et al. (2014) specifically focused on porosity and methane hydrate saturation as the two parameters of investigation in relation to effect on thermal conductivity. Both the porosity and the methane hydrate saturation of the cores varied.
The thermal properties of the sand layer containing the methane hydrate and the mud layer were tested. The Hot Disk technique is capable of measuring thermal conductivity, thermal diffusivity and specific heat in a single test. Thermal conductivity measurement with the Hot Disk TPS can be done by using either a two-sided or one sided sensor. Measurements completed using the two-sided sensor require that the sensor is sandwiched in between two identical samples. The single-sided sensor requires only one sample piece, which is ideal for researchers with limited samples available. In this example, the researchers were careful to maintain the samples at the appropriate pressure during testing to prevent the dissociation of the methane hydrates. The thermal conductivity of the mud samples was measured over a range of temperatures. Once the experimental testing was completed, the researchers did extensive work with models which predict thermal conductivity to determine if they matched with their results.
Figure 2. Diagrams showcasing the test set-ups for thermal conductivity measurement using the two-sided and one-sided sensors.
Thermal Conductivity of Sediment Cores
The thermal conductivity of the sediment core samples containing methane hydrates decreased slightly as porosity increased, and increased slightly alongside the methane hydrate concentration. Thermal diffusivity also decreased from as porosity increased, but did not change with methane hydrate concentration. Thermal conductivity data obtained from the mud layer of the cores revealed that it did not change with variation in temperature.
Comparison of their data and that obtained through predictive models enabled Muraoka et al. (2014) to conclude that the distributive model, in which the individual thermal conductivities of the components of the sample (in this case sand, methane hydrate, and sea water) are used to calculate the product thermal conductivity, was a good representation of their experimental results. By comparing the thermal conductivity of the sand/methane hydrate samples (which had larger grain sizes) and the mud samples (which had smaller grains), the researchers also concluded that small grain size influences thermal conductivity. This paper is the first in a series focusing on the thermal properties of methane hydrates. Muraoka et al. (2014) plan on continuing their research to further understand how vertical stress and the dissociation process affect thermal conductivity. This paper is an excellent example of how the Hot Disk Transient Plane Source technique can be a valuable tool in research and development of energy sources.
Note: For comprehensive results and an in depth discussion, please follow the link to the scientific paper in the reference section.