Analysis on the hole-forming rules of hydrate-bearing sediments broken by cavitation jet

Cavitation jet has emerged as a drilling rate improvement technology in oceanic natural gas hydrate (NGH) exploitation. Previous studies have primarily used jet flow field data to analyze the effects of cavitation jets, while the hole-forming details of hydrate-bearing sediments (HBS) erosion by cavitation jet are still not clear. In this work, the jet breaking experiments for synthetic HBS samples are performed using conical and angular nozzles. The morphological characteristics of various erosion pits are obtained and the corresponding dimensions, such as diameter, depth, and volume are recorded. Then, the process of HBS erosion by conical jet and angular cavitation jet are analyzed with fluid-solid coupling simulation, to identify the temporal evolution of erosion pits. The experimental results verify that the angular cavitation jet has stronger erosion performance than conical jet. The depth and volume of the erosion pit formed by the angular cavitation jet are 1.3 times and 1.2 times that of the conical jet, respectively. And the angular cavitation jet shows superiority in aperture enlargement. The simulated results reveal that the hydraulic high-efficiency drilling of angular cavitation jet mainly benefits from the enormous power emitted by cavitation bubbles. For angular cavitation jet, as the jet migrates and impacts on the HBS model, a V-shaped hole is formed. Subsequently, the cavitation bubbles reach the bottom and side of the erosion pit, collapse, and destroy the cemented structure of HBS. The erosion pit expands rapidly and appears cylindrical. Meanwhile, a reverse jet is formed after the main jet hits the bottom. The reverse jet constantly crushes the side wall of erosion pit and enlarges the erosion pit. Finally, a spindle shaped hole is obtained. This study reveals the hole-forming rules of HBS broken by cavitation jet and provides a reference on the structure optimization of cavitation jet nozzle applied in NGH recovery.