Investigation on the cuttings carrying capacity of a novel retractable drill bit used in casing while drilling with air reverse circulation

An orthogonal experimental design method is employed to investigate the effect of the structural parameters of the drill bit used in the Casing-while-Drilling (CwD) technology on its rock cuttings carrying capacity and reverse circulation performance. The results of the range analysis indicate that diameter of inner jet nozzle has the most significant impact on the cuttings carrying capacity of the drill bit, followed by the diameter of flushing nozzle. While for the reverse circulation ability, the diameters of the flushing nozzles have a dominant impact, followed by the diameters of the inner jet nozzles. In addition, the cuttings carrying capacity and the reverse circulation effect of the drill bit can be improved by shrinking the inner jet nozzle or appropriately enlarging the flushing nozzle. Accordingly, the optimal structural parameters of the drill bit are determined. Through further experi-ments, the cuttings carrying efficiency and the entrainment ratio of the optimal drill bit are 99.57% and 20.36%, respectively. Moreover, the rotation speed of drill bit has a trivial effect on its performance. At any tested rotation speed, the entrainment ratio is greater than 20%, and all the rock cuttings can be carried out of the borehole bottom using the proposed drill bit with the optimal structure. To test its real performance, a large-diameter retractable drill bit with a diameter of 600 mm was manufactured and tested in field. The results demon-strated that this retractable drill bit had excellent cuttings carrying and reverse circulation ability. The average penetration rate of the drill bit reached about 1.7 m/h in the formation of weathered sandstone with argillaceous composition, which was 1.7 times greater than conventional mud drilling in the same area.