The bottomhole stress field analysis and the rate of penetration enhancement mechanism of a new type of central-grooved single-cone/ multi-blade hybrid bit for deep well drilling

In this work, a high efficiency rock-breaking idea of forming an isolated rock column in the center of the deep bottomhole, that is, a convex-stepped bottomhole and a new type of central-grooved single-cone/multi-blade hybrid bit (CSMHB) are proposed due to the problems of poor drillability of rock, low efficiency of rock breaking at the center of bit and low rate of penetration (ROP) in the deep hard formation drilling. Based on the poroelasticity theory, a numerical simulation model of the stress field of convex-stepped bottomhole is established, and the dynamic changes of the stress field of convex-stepped bottomhole with time are analyzed. Meanwhile the rock strength failure of the convex-stepped bottomhole under the stress field is analyzed based on the three-dimensional Coulomb criterion, which preliminarily reveals the ROP enhancement mechanism of the CSMHB. The research shows that after the convex-stepped bottomhole is opened, the convex area of the bottomhole is subjected to tensile stress in the circumferential, radial and axial directions. In the bottomhole area broke by PDC cutting structure, the part near the central rock column is also subjected to tensile stress in the circumferential, radial and axial directions, while the other area is subjected to compressive stress. The action range of tensile stress and the damage range of the central rock column at the bottomhole expand with the increase of the indent length and diameter of the single-cone bit respectively. The average effective stress and damage function at the center of bottomhole decrease with the increase of the indent length and diameter of the single-cone bit respectively.