Structure optimization and drilling capacity analysis of a self-propelled straight-swirling mixed jet bits

As the core part of hydro-jet radial drilling technology for coal-measure gas extraction, the self-propelled jet drill bit has a decisive impact on drilling efficiency. In order to obtain the optimal parameters of the direct rotating mixed jet, the velocity characteristics of the three-dimensional flow field with different bit structure parameters are analyzed by the combination of Fluent numerical simulation and specific experiments, and the optimization criteria are given. The optimal parameters of the drill bit are obtained in the drilling test: the central hole of diameter 1.2 mm, the impeller groove of width 0.7 mm, the impeller of thickness 4.5 mm, radial length 3.5 mm, and inclination angle 45°, and the mixing chamber of length 6 mm. Through theoretical analysis, the reasons why different parameters change the bit drilling speed are explained. That is, the three-dimensional change of the jet is caused mainly by affecting the flux of the direct flow and rotating jet in the direct rotating mixed jet. The sensitivity coefficients of each parameter structure on drilling displacement are obtained by using gray correlation analysis from high to low are as follows: center hole diameter, impeller groove width, impeller radial length, mixing chamber length, impeller inclination and impeller thickness. The results show that the optimized jet bit has higher drilling efficiency. In the correlation analysis, the direct flow and rotating jet flux have the greatest impact on the drilling efficiency, and the jet energy distribution is more reasonable when the direct rotating flux ratio is 0.54. The research has guiding significance for the design and structure optimization of bits in the radial drilling of coalbed methane, and natural gas production.