Development and application of hole-bottom freezing drilling tool for gas-hydrate-bearing sediment sampling

Obtaining cores by drill sampling is necessary for assessing natural gas hydrates. Therefore, measures must be taken to ensure that they do not decompose during the coring process. In this study, the working principle of pressure and temperature preservation techniques as well as the history of the application of the drilling tool and some technical deficiencies are reviewed. Based on the phase equilibrium characteristics of gas hydrates, a hole-bottom freezing sampling technique (HBFS) is proposed, and a drilling tool is designed. The two key functions of cold source storage and freezing core of the drilling tool were tested. The results showed that the alcohol-dry ice mixture used as a cold source could be stored for 120 min with a cold energy loss rate of only 0.8%. The cold source froze the core to the lowest temperature of 241.9 K in 5 min and prevented the hydrate from decomposing within 60 min. The drilling tool test results showed that a coring success rate of 90% was achieved in four coring experiments. A final core temperature of 263 K was obtained four times in the entire functional experiment, which verifies the suitability of the hole-bottom freezing sampling technique.