| Recently,as oil&gas exploration and exploitation have been heading for deep and ultra-deep formations,efficiency of conventional jet drilling technology decreases sharply,whose effect in drilling rate raising is quite limited.Therefore,it is quite essential to develop new technologies and methods so as to increase drilling efficiency in deep formations.Cavitation is a common phenomenon in the nature,which can cause heavy damage to hydraulic machines.But if cavitation can be introduced into well drilling,it should increase drilling efficiency.In this paper,the self-resonated cavitating nozzle put forward before is studied further through numerical simulation and experimental methods.First,through theoretical analysis and experimental research,characteristics of hydroacoustics in the resonator of the self-resonated cavitaing nozzle are studied.Results show that resonant frequency is the inherent property of the resonator,which has nothing to do with jet operating parameters.In all influencing factors,the length of the resonator is the most significant one.With increasing the resonator length,its inherent frequency decreases,showing negative power law.Besides,nozzles with different structural parameters have their own reasonable jet operating parameters.After that,large eddy simulation is adopted to investigate the influence of nozzle out velocity pulsation amplitude and frequency on the jet flow field.It is found that periodic pulsed velocity helps the jet stay its concentricity and the jet doesn’t diverge easily.It is especially obvious when the Strouhal number is 0.3.Pulsation helps those vortexes develop further in the flow field.As for macroscopical numerical simulation,the turbulent model,the multiphase model and the cavitation model are adopted to simulate the flow field of the self-resonated cavitatig nozzle and the influence of upstream pressure ahead of the nozzle,nozzle diameter,nozzle length,divergent angle of the nozzle outlet and ambient pressure on the jet flow field is investigated.The results say that the optimal dimensionless stand-off distance is 7 to 14 and pump pressure has no effect on it.When the convergent angle of the nozzle outlet is 30°,the jet velocity is higher and its attenuation rate is lower,with higher volume fraction of vapor near the outlet and higher crossflow velocity on the bottom.When other conditions remain constant,ambient pressure inhibits the velocity and vortex magnitudes in the flow field.Finally,the erosion tests indicate that there are three types of pitting distribution pattern on the surface under different ambient pressure and stand-off distance.In the first pattern,a large number of pits exist in a circle with only a few ones scattering around,which is denoted by the filled circle pattern.In the second pattern,pits distribute evenly on the surface of specimen.The last pattern is denoted circular ring pattern where a ring full of pits appears.With stand-off distance increasing,the cavitating erosion intensity decreases.However,there is no obvious law to demonstrate how ambient pressure influences the cavitating erosion intensity.This thesis further investigates the cavitation mechanism of the self-resonated nozzle,its flow field and cavitating erosion intensity,which can provide basis for the nozzle design and modification and drilling efficiency improving. |
