Abstract Radial jet drilling technology can realize efficient and economic development of the coalbed methane (CBM). The self-rotating multi-orifices nozzle, one of the core devices, has highly rock breaking efficient. The characteristics of coal breaking by a self-rotating multi-orifices nozzle are studied by experiment, the influence of jet pressure, standoff distance, and exposure time is also studied. Particularly, its ability of continuous rock-breaking and hole-forming is tested. Rock breaking diameter, depth, and volume are selected as evaluation criteria. Results show that the self-rotating multi-orifices nozzle can drill large circle hole in the vertical bedding direction of coal while elliptical hole in the plane bedding direction, efficiently. The rock breaking diameter, depth and volume increase with the jet pressure. The rock breaking threshold pressure is 10–15 MPa. Due to high speed rotation and low strength of coal, when the jet pressure is greater than 25 MPa, the rock breaking diameter is stable within the range of 35–40 mm. Similarly, standoff distance has little influence on the rock breaking diameter, which indicates that the self-rotating multi-orifices nozzle is very suitable to the coal formation for being difficult to control the standoff distance during drilling process. The nozzle can fully break the coal rock in 2min which is recommended for experiment study. Under continuous feeding in condition, there is a negative linear correlation between the rock breaking diameter and the feeding rate. The recommended drilling speed is 0.1 m/min. The research results can promote the application of RJD technology in CBM development.

中文翻译：

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煤层气径向射流钻井自旋多孔喷嘴破岩特性

摘要 径向喷射钻井技术可以实现煤层气（CBM）的高效经济开发。核心装置之一的自旋转多孔喷嘴具有很高的破岩效率。通过实验研究了自旋转多孔喷嘴破煤的特性，并研究了射流压力、间隔距离和暴露时间的影响。特别是对其连续破岩成孔能力进行了测试。选择破岩直径、深度和体积作为评价标准。结果表明，自转多孔喷嘴可在煤层垂直层理方向钻大圆孔，而在平面层理方向可钻椭圆孔，效率高。岩石破碎直径、深度和体积随着射流压力的增加而增加。破岩阈压力为10-15 MPa。由于煤体转速高、强度低，当射流压力大于25 MPa时，破岩直径稳定在35-40 mm范围内。同样，间隔距离对破岩直径的影响很小，这表明自旋转多孔喷嘴非常适用于在钻孔过程中难以控制间隔距离的煤层。该喷嘴可在2min内完全破碎煤岩，推荐用于实验研究。在连续给料条件下，破岩直径与给料速度呈负线性关系。推荐的钻孔速度为 0.1 m/min。研究成果可促进RJD技术在煤层气开发中的应用。