Friction law of compound drilling along the coal seam with super-long directional drilling

The internal friction is the main factor affecting the drilling rate and drilling efficiency in super-long directional drilling in coal mines. In order to meet the objective demand of gas drainage mode in big panels, the friction and its influence law in drilling process are studied. To provide a basis for the selection of friction reduction process parameters of compound drilling along coal seam with super-long directional drilling, the drill string motion characteristics and stress state under the condition of near-horizontal compound drilling in the coal mine are analyzed. With certain conditional assumptions, the mechanical models of axial frictional resistance and rotational frictional torque of composite drilling are established. Under different hole depths and matching drilling process parameters, numerical calculations are carried out to analyze the friction law characteristics of the common ø89 mm directional assembly. The results reveal that the mechanical drilling speed, drilling rotation speed and drilling depth are the main controlling factors of sliding friction resistance and rotating friction torque. Compared with the statistical analysis of actual drilling parameters of super-long directional drilling, the changes of propulsion resistance and rotating torque are consistent with the calculation of the model. The result is of important practical significance for guiding the selection of technological parameters and drilling conditions of compound directional drilling near horizontal holes. Furthermore, it has referential significance for the design of automatic and intelligent directional drilling control system in underground coal mines.