Rock-breaking performance analysis of worn polycrystalline diamond compact bit

Abstract

Polycrystalline diamond compact (PDC) bits always working in a worn state for a long time during the operation progress of exploratory well. To explore the cutting performance of the worn PDC bit, this study analyzes the stress state of the worn tooth, rock crack propagation law, and drilling characteristics of the drill bit by using a single tooth cutting experiment, numerical simulation, and full-scale worn drill bit drilling experiment. The results show that the worn teeth are more loaded than new teeth, and a blunt rubbing phenomenon is common in the rock cutting process after the cutting teeth are rounded. The load fluctuation increases with the increasing of wearing area of the PDC bits under the same cutting depth. The plastic flow of the rock at the worn PDC teeth blade is obvious, which is the initiation point of further wear. Moreover, the abraded surface of the PDC cutter can inhibit the propagation of longitudinal crack, and can reduce machining efficiency. The worn bit produces a smoother bottom hole than a new bit, which is not beneficial for PDC bits' teeth intruding into the rock. The penetration rate (ROP)decreases with the increasing of the wear height while the mechanical specific energy increases. When the wear depth is greater than 3 mm, the mechanical specific energy and ROP are insensitive to the weight of bit (WOB). As the wear depth increases, the oscillation of the system slowed, and the friction coefficient of the drill bit decreases. However, regardless of the drill bit's wear degree, the radial acceleration value is not a significant difference. The research can provide theoretical reference for the reasonable application of the bits and the evaluation of the working state of the drill bit.