Mechanism and Test Validation of a Self-Locking Jet-Crushing Tool for Marine Natural Gas Hydrate Exploration and CO₂ Sequestration

This study proposes a novel self-locking pressure-controlled jet-crushing tool (SPJT) designed for solid-state fluidization mining of natural gas hydrate (NGH) and cavity creation during deep-sea CO2 sequestration. The SPJT allows efficient switching between jet crushing and normal drilling, reducing the operation time in NGH reservoirs. The opening and closing of the jet-crushing nozzles are controlled by the movement of the slide core, which is regulated by the fluid flow rate from the offshore pump. Once the jet-crushing nozzles open, the slide core can achieve self-locking to maintain stability despite fluid fluctuations. Numerical simulations were conducted to analyze the impact of structural dimensions on the slide core’s internal pressure, axial force, and the motion laws of self-locking mechanisms under various flow-loading modes. Based on simulation results and actual working conditions, the optimal structural parameters of the slide core were identified: 45 mm inlet diameter, 30° inlet angle, and 470 L/min inlet flow rate. The self-locking mechanism can achieve both self-locking and unlocking under different flow-loading schemes with a rapid increase in the drilling fluid flow rate for self-locking and a gradual decrease for unlocking. An engineering prototype of the SPJT was fabricated, followed by indoor tests and land well experiments to verify the feasibility of the design and its performance. The test results showed that the nozzles remain closed under conventional borehole drilling flow rates but are fully open when the flow rate reaches 455 L/min. The self-locking mechanism reliably switches between the locking and unlocking states. The relative error between the tool’s self-locking flow rate and the design flow rate of 470 L/min is only 3.19%, within acceptable limits. This study provides new insights for designing remote-intelligent hydrate mining tools, optimizing the mining process, and enabling controllable seabed CO2 sequestration, thus accelerating the commercial exploitation of gas hydrates.