| China has abundant coal bed methane(CBM)reserves,but the production of single well is very low,and the actual development efficiency is worse than expected.The Ultrashort-Radius Radial System(URRS)technique can form multiple radial branch wells with hydraulic jetting,bulid many high-conductive channels between wellbore and reservoir,so as to effectively improve the production of single well.The high-pressure jet nozzle plays an important role in URRS technique.Its structure directly affects the efficiency of radial drilling.This paper adopts the method of combining theory analysis with experiment.The breaking coal rock efficiency of the self-rotating multi-orifice nozzle is studied.The rock mechanics parameters and the anisotropy of coal rock samples are researched.The characteristics and particle size of natural rock samples cuttings are analyzed.In this paper,the experimental results show that the natural coal rock has strong anisotropy.The uniaxial compressive strength in the vertical and parallel bedding direction is respectively 30.8MPa and 10.9MPa,the shear strength is respectively 11.5MPa and 4.2MPa.The rock breaking volume and rock breaking-hole diameter of the self-rotating multi-orifice nozzle are 1.5~2.0 times that of the multi-orifice nozzle.The rock breaking threshold pressure of the self-rotating multi-orifice nozzle is 6.0~10.0MPa and the rock breaking effect of the nozzle increases with the increase of pressure in the range of 10.0~26.5MPa of nozzle pressure drop in the range of 26.5~32.5MPa nozzle pressure drop,the rock breaking efficiency is relatively close.When the jet distance is 6~7 times the diameter of the nozzle,rock breaking efficiency is the best.The cutting particle size in the direction of vertical stratification is concentrated in 4~10mm;in parallel bedding direction,the cutting particle size is more than 30 mm.The results of this paper demonstrate the breaking coal rock efficiency of the self-rotating multi-orifice nozzle and provide theoretical and technical support for the development of CBM reserves by using the URRS technique. |
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