Viscous torques characteristics in a self-rotating jet bit with silicone oil-controlled rotational speeds

Rock-breaking drilling using a self-rotating jet bit with a controllable speed can improve the efficiency of coalbed methane development using the water jet tree-type borehole technology. The viscous torque generated by filling silicone oil inside the bounded annular column of the bit can control the speed, but its characteristics are unclear. The rheological properties of silicone oil with different viscosities were quantitatively analyzed. The viscous torque at various rotational speeds and inner diameters was tested by a self-developed fluid-bounded annular column forced flow dynamics device. The flow state of silicone oil was elucidated, and viscous torque models were derived and discussed for applicability. The research findings are as follows: when the viscosity of silicone oil is below 5 Pa·s, the silicone oil can be approximated as a Newtonian fluid, however, at viscosities exceeding 5 Pa·s, the silicone oil exhibits a shear-thinning behavior that can be described by the Cross rheological model. The apparent viscosity of the Newtonian-type silicone oil is unchanged when it is forced to flow in the bounded annular column, thus its viscous torque increases linearly with its viscosity and rotational speed, and the growth rate is accelerated with the increase of the inner diameter, which is favorable for the rotational speed control under multiple pressure gradients. However, the shear stress of the Cross-type silicone oil with the shear rate shows a trend of increasing, decreasing, and then increasing, resulting in the existence of the peak in its viscous torque that is synergistically controlled by rotational speed and inner diameter, and the greater the viscosity of the silicone oil, the more the combination of rotational speed and inner diameter. To prevent a decrease in the viscous torque of the Cross-type silicone oil, leading to a loss of control over the bit speed, it is necessary to select the viscosity and inner diameter according to the speed control range. At 84% of the tested rotational speed and inner diameter combinations, the Cross-type silicone oil viscous torque is greater than that of the Newtonian-type silicone oil under the same conditions, suggesting that the former is better suited for rotational speed control at high pressures, and the latter is better suited for low pressures. Under the present experimental conditions, the forced flow of silicone oil is laminar, and the derived Newton-type and Cross-type viscous torque models have a high reliability with a maximum error of 14.3% under the inner diameters up to 14.5 mm and equal to 14.5 mm, respectively.