Study on the non-in situ measurement method for hydrate thermal conductivity

A non-in situ measurement method for gas hydrate thermal conductivity was established, including the sample preparation, sample transfer, sample loading, and thermal conductivity measurement. Taking tetrahydrofuran (THF) hydrate and methane hydrate as examples, the detailed process was investigated by many experiments. THF hydrate samples were prepared by the combined method of THF aqueous solution + temperature oscillation + aging, while methane hydrate samples were prepared by the combined method of compacted ice powder + temperature oscillation + supplementary gas + aging. The sample box, quick-opening reactor, and sample mold were developed to improve the sample quality, shorten preparation time, and reduce exposure time. It was found that the time of sample transfer and loading can be controlled within 1 minute and 15 minutes for THF hydrate and methane hydrate, respectively. The thermal conductivities of THF hydrate and methane hydrate are 0.5200 ~ 0.5700 and 0.4803 ~ 0.6197 W•m⁻¹•K⁻¹ in the studied temperature range, respectively. The thermal conductivity of THF hydrate shows a positive dependence on temperature. The thermal conductivity of methane hydrate shows no dependence on temperature, but negative dependence on pressure. The errors of THF hydrate mainly come from the frost layer and micro airflow while the errors of methane hydrate mainly come from the ice film and micro gas flow. The precision, accuracy, and uncertainty of THF hydrate are 0.09% ~ 0.75%, 0.0450 ~ 0.0650 W•m⁻¹•K⁻¹ (absolute difference), 8.65% ~ 12.01% (relative error) and 0.00007 ~ 0.00017, respectively. Similarly, the precision, accuracy and uncertainty of methane hydrate are 0.04% ~ 5.98%, 0.005 ~ 0.0619 W•m⁻¹•K⁻¹(absolute difference), 1.02% ~ 12.12% (relative error) and 0.00016 ~ 0.00032, respectively. Based on the analysis, the established non-in situ measurement technique in this work is completely feasible and can be used to measure the obtained naturally occurring samples.