Investigation on rock-breaking characteristics of hard sandstone by non-planar PDC cutters under high confining pressure

Abstract

Due to high hardness, strong heterogeneity, and low drillability in deep formations, conventional Polycrystalline Diamond Compact (PDC) bits with planar cutters usually get bit worn, short drilling footage, and low rate of penetration (ROP) when drilling into such formations. To improve the aggression and the impact resistance of PDC bit, a series of non-planar PDC cutters have been proposed and developed by scholars and oil & gas industries. However, the in-situ stress and hydrostatic pressure significantly weaken the rock-breaking performance of PDC cutters, which is a paucity in related studies. Consequently, this paper conducts rock-cutting experiments in hard sandstones by planar cutters and three types of ridged cutters under confining pressure and atmospheric conditions. The influences of cutting parameters on the rock-breaking performances of different cutters are investigated, focused on the changes in cutting force, mechanical specific energy, and rock cuttings distribution. Additionally, visual cutting experiments are conducted to compare the differences in the rock-breaking mechanisms of different cutters. The results indicate that in-situ stress and hydrostatic pressure exert a noticeable inhibitory effect on rock fragmentation, resulting in smaller cuttings and lower rock-breaking efficiency than those under atmospheric conditions. Furthermore, compared to the three types of ridge cutters, planar cutters exhibit a lower rock-breaking efficiency. Ridged cutters, which leverage the efficient stress concentration caused by their ridges, demonstrate a higher rock-breaking performance by effective and efficient rock failure near the ridges. Additionally, under different cutting parameter conditions, each cutter have a corresponding optimal rock-breaking efficiency range. Specifically, for the triple-ridged concave cutters, the highest rock-breaking efficiency is achieved when the cutting angle is approaching 20°. As the cutting angle increases, the axe-shaped cutter exhibits the highest rock-breaking efficiency. In addition, the triple-ridged concave cutter and the axe-shaped cutter possess unique rock-breaking advantages, with the former being more stable and the latter producing larger volumes of cuttings. The selection of appropriate cutter parameters can enhance the service life and the drilling speed of PDC bits. The experimental results and optimization charts have significant implications for selection on cutter shapes and the design of hybrid-cutter parameters in deep hard formation drilling.