Flow field investigation of a straight–swirling integrated jet for radial jet drilling

Abstract

Radial jet drilling (RJD) technology is applied to low permeability oil-gas reservoirs, coalbed methane, and shale gas. A self-propelled bit is an integral component. The external flow fields of a new-type of straight–swirling integrated jet (SSIJ) were tested under non-submerged conditions and the key parameters of the jet were studied in this paper. A rock-breaking experiment at a specific target distance was conducted to verify the PIV test results. It is indicated that, from the introduction of a central straight jet, the central low-axial-velocity zone can be eliminated, and the maximum radial and tangential velocities are not at the jet axis. Second, the hydraulic parameters do not affect the velocity distribution law, but significantly affected by nozzle structures. The straight-to-swirl ratio and the dip-angle of an impeller slot should be 0.8 and 45°, respectively, to obtain a flow field with both a higher central velocity and larger coverage area. Through the rock-breaking experiment, it was found that when the dip-angle is 45° (swirl number of 0.45), both the borehole depth and diameter are larger, and the bottom is the flattest, simultaneously, the correctness of the PIV test is verified. Finally, since the two jets (straight & swirling) have interacted and mixed after ejected from the nozzle, there is no constant velocity core, the SSIJ has no initial and basic sections compared with the conventional combined jet, and could be divided into a transition section and uniform-mixing section along the axial. By analyzing the swirl numbers at different positions, it was found that the SSIJ has a straight, weak-swirl, strong-swirl, and periphery jet zones along the radial. The results can guide rock-breaking mechanism revelation and the optimal design of an SSIJ bit for the RJD technology.