Study on combined cutting and penetration of shaped PDC cutter under bottomhole condition

Abstract

A limited weight on bit (WOB) and the resulted low rate of penetration (ROP) are often encountered during horizontal well drilling due to high drags along drillstring. Hence, it is essential to develop PDC bits with great penetration ability. However, previous studies focus solely on cutting performance of PDC cutters, with many ignoring the influence of bottomhole pressures. Further insight into cutter-rock interaction is required to develop high-efficiency bits and reduce drilling costs. In this study, a physical model is developed to investigate rock failure characteristics experiencing combined penetration and cutting, in which the principal stresses and hydrostatic pressures are loaded simultaneously to simulate bottomhole pressurized conditions. Some indexes are defined and calculated to evaluate rock failure characteristics under different cutting parameters and formation conditions. The results indicate that the increase in hydrostatic pressures and in-situ stresses causes an overall decrease in the cutting depth and rock-breaking volume, with the former having a greater effect on rock failure efficiency. Under the same formation conditions, the stinger cutter has the greatest cutting depth, which can enhance the penetration ability of bits. The cylinder cutter has the biggest interaction area with the rock. However, the corresponding consumed energy and threshold penetration forces restrict its application. Relying on a pre-damaged zone and boundary effects, the cylinder cutter can be employed in conjunction with the stinger cutter to ensure the cutting depth, rock-breaking efficiency, and width of cutting grooves simultaneously. The axe-shaped cutter has the highest rock cutting efficiency and the largest rock breaking volume. Field tests validate that PDC bits equipped with axe-shaped cutters can increase the ROP and prolong their service life. This study provides an in-depth understanding of rock failure mechanisms and guides the application of shaped cutters.