Abstract: This paper discloses a novel design of a roller cone and a polycrystalline diamond compact (PDC) hybrid drill bit constructed with two roller cones plus two blades. The design includes two types of constructions:cone-leading(CL) and blade-leading(BL) types. The physical processes for drilling tuff and granite using two differently constructed hybrid bits are numerically simulated using an explicit dynamic finite element analysis method. The experimental results demonstrate that the CL-type drill bit proves suitable for drilling hard formations, whereas the BL type can be used for soft earth formations. The axial forces exerted on the roller cone cutting teeth of the two bit types when drilling hard rocks remain larger than those in the case of drilling soft rocks. Furthermore, the CL-type bit exhibits larger imposed axial forces than the BL type when drilling the same rocks. The distribution pattern of the axial loads imposed on the PDC cutters along the radial direction of the bit is very similar to the shape of the blade crown segment, where the maximum axial load occurs at the conic crest. Moreover, the allocation of the weight-on-bit (WOB) on the roller cones and blades is related to the types of bit constructions. The above conclusion deepens the understanding of drilling processes using hybrid bits, which provides useful guidance for the design and on-site selection of drill bits.

Key words: hybrid drill bit, drilling process, LS-DYNA, finite element simulation, tooth load