Experimental investigation of coolant selection and energy efficiency analysis during gas hydrate-bearing sediment freeze-sampling

The prerequisite for the exploration and exploitation of natural gas hydrate (NGH) is ensuring that the hydrate sample does not decompose during the sampling process. In order to solve this problem, the hole-bottom freezing sampling technique (HBFS) method was proposed, which uses a cold-alcohol to reduce the temperature of the hydrate sample and prevent its decomposition. However, the low heat exchange between the cold-alcohol and the hydrate sample results in a large loss of energy, and the sample cannot be frozen to the required temperature due to the size limitations of the sampler. In this study, three different heat transfer modes were used to increase the energy efficiency, namely heat conduction (cold-alcohol), phase change heat transfer (liquid nitrogen), phase change convection (alcohol-dry ice mixture). The test results show that the freezing efficiency of the alcohol and dry-ice mixture is 11.38%, which is the highest. The alcohol-dry ice mixture can cool the average temperature of the sample from approximately -17.81 to -23.37 °C, in which the hydrate sample does not decompose rapidly under normal pressure. The alcohol–dry ice mixture is more suitable as a coolant for the HBFS.