Erosion characteristics of hydrate-bearing sediments exposed to a cavitation jet

Natural gas hydrate (NGH) is a potential alternative source of energy for the future, and the low mining efficiency severely restricts its commercial utilization. Recently, the cavitation jet technology has been emerging as a prospective way of drilling rate improvement for the industrial development of NGH. The breaking and mining efficiency of cavitation jet is strongly associated with the nozzle structures. In this research, the structure parameters of angular cavitation water jet (ACWJ) nozzle are optimized to maximize the cavitation initiation range and thereby the cavitation damage effect, using the computational fluid dynamics (CFD) method. Then, the erosion experiments of synthetic hydrate-bearing sediments (HBS) samples under various standoff distances, inlet flow rates, and erosion times are conducted with the optimized ACWJ nozzle. Finally, the grey relational analysis method is utilized to reveal the sensibilities of the nozzle structure parameters to the cavitation clouds initiation, and the hydraulic conditions to the erosion performance of ACWJ. Results show that divergent part structure greatly enhances the cavitation erosion ability of ACWJ, and the optimized divergent length and angle are 2d and 34 degrees, respectively. Under present experimental conditions, the volume and depth of erosion pits achieve the highest values when the standoff distance is about 5 times the throat diameter of ACWJ nozzle. The threshold velocity of the ACWJ breaking HBS with 50 % hydrate saturation is estimated as 20 m/s similar to 25 m/s according to the fitting curves. The development process of erosion pit erosion by an ACWJ is revealed, and an erosion pit enlargement stage occurs because of the damage effect caused by cavitation bubbles. This work gives some meaningful findings and provides promising guidance for the field application of cavitation jet in NGH drilling and mining.