The powdery or laminar cuttings that are produced by a conventional polycrystalline-diamond-compact (PDC) bit are not suitable for geological logging. To solve this problem, this paper introduces a new PDC bit with suction-type minicore drills. The basic working principle of this new PDC bit is to remove the main cutters in the center of the PDC bit. The new PDC bit can generate minicores of formations with a certain diameter and break these cores in a timely manner during drilling. By use of a special hydraulic design, the broken minicores are sucked from the bottom of the well through the internal coring channel of the bit.
Combined with the theory of rock-breaking simulation and solid/liquid two-phase flow, the effects of different jet-nozzle diameters (6, 8, and 10 mm) on the suction in the coring channel were investigated by considering the quality of the minicores, cuttings, and drilling fluid. The numerical-simulation data show that the cores, cuttings, and drilling fluid that are discharged from the discharge hole have the highest quality if a jet nozzle with a diameter of 8 mm is used. In other words, the suction force generated from the negative-pressure cavity is stronger than that generated with the other two jet-nozzle sizes (6 and 10 mm) investigated.
A test bit was subjected to laboratory and field tests to verify the coring. The experimental results show that the collected cores are generally columnar, and both the integrity and the collection rate of the core are high.
This study demonstrates the feasibility of the concept of the new PDC bit with suction-type minicore drills and provides a design scheme to reduce the coring cost and improve the quality of geological logging.