Cavitation jet drilling has been extensively employed for the exploitation of geo-energy resources. The dynamics of cavitation bubbles in close proximity to the solid boundary have been a subject of great interest during jet drilling, as they play a crucial role in determining the cavitation performance. In present work, the dynamics of a single cavitation bubble near a solid surface is numerically investigated by using the axisymmetric Navier-Stokes equations and the volume of fluid method with considering the surface tension of gas-liquid interface, liquid viscosity and compressibility of gas in bubble. The simulated profiles are qualitatively and quantitatively consistent with the experimental images, which proves the reliability of employed numerical model. The effects of stand-off distance on the bubble profiles, bubble volume and collapse time have been analysed. Moreover, the cavitation erosion patterns towards the solid wall are also revealed for different dimensionless stand-off distances. The simulation results reveal two distinct collapse patterns for the bubble profiles. The solid wall significantly impedes the shrinkage rate of the bubble, resulting in the longest collapse time when the dimensionless stand-off distance is 1.0. Three erosion patterns of cavitation bubbles towards the solid wall are observed, with the shock wave and micro-jet both contributing significantly to the damage caused by cavitation erosion. The shock wave sweeps the wall resulting in circular corrosion pits with a severely eroded centre, while the micro jet penetrates the wall leading to small spot corrosion pits.

Cavitating jet is a promising drilling rate improvement technology in both the marine natural gas hydrate (NGH) fluidization exploitation method and the integrated radial jet drilling and completion method. In present study, we aim to improve the efficiency of jet erosion and extracting NGH. With a computational fluid dynamics (CFD) method, the pressure, velocity and cavitation field characteristics of organ-pipe cavitating jet (OPCJ) are analysed. The divergent angle, throat length, and divergent length of OPCJ nozzle are preferred to obtain stronger jet cavitation erosion effect. Laboratory experiments of gas hydrate-bearing sediments (GHBS) erosion by OPCJ and conical jet (CJ) are conducted to compare and validate the jet erosion performance. The impinging models of OPCJ and CJ are constructed to study the impact characteristics. Results show that the preferred values of divergent angle, throat length, and divergent length are 15°, 1d, and 3d, respectively, in present simulation conditions. For GHBS, the OPCJ possesses the advantages of high efficiency and low energy consumption. Moreover, the OPCJ has higher penetration efficiency, while showing equivalent penetration ability compared to CJ. During the impinging process, the OPCJ can induce stronger impact pressure and turbulence effect, and also shows stronger chambering effect and bottom cleaning ability compared to CJ. This study presents the erosion performance of OPCJ and CJ on GHBS, and provides preliminary insights on the potential field applications in NGH exploitation.