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.
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