Horizontal wellbore cleanout by rotating jets has been developed rapidly in the past decade. However there are few investigations on mechanism and characteristics of the high pressure water jet impacting and annular helical flow, which are two primary powers to break up the consolidated bed, lift and suspend particles, and increase the wellbore cleanout efficiency. To realize the characteristic of helical flow, the pathilines of rotating high pressure jets and annular velocity distributions were investigated firstly in this paper. The results indicated that the motive power to form the annular helical flow is the non-uniform cross flow generated by partial lateral jets which impact on the wall at an angle, while other jets provide the fluid sources. Further, base on the particle trajectory model and liquid-solid mixture model, the motions of particle, the annular distribution of sand velocity and concentration in annular helical flow were carried out. The pathlines of sand were spiral curves around the drill pipe in the annular helical flow, which can enhance the sand moving ability and increase the horizontal wellbore cleanout efficiency finally. The influences of flow rate, particle size, nozzle assemblies on the annular helical flow strength and sand transportation efficiency were analyzed comprehensively. Finally, the downhole tool was designed according the theoretical results and has been tested in hundreds wells in China. The test showed that it can greatly improve the oil and gas well production and reduce the operating duration. This paper is beneficial to investigate characteristic of sand sweeping by annular helical flow and optimize the tool structure, operation parameters.

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