Abstract

Polycrystalline diamond compact (PDC) bits have been widely used in drilling process due to their high efficiency and great reliability. To investigate rock failure mechanism and further improve the performance of PDC bits, a series of single cutter tests have been conducted in previous studies. However, most of them were performed under atmosphere conditions. That is, the effects of temperature and in-situ stress were ignored due to the limitation of experimental facility, which restricts the developing of novel bits. To solve the aforementioned problem and deeply study the rock failure mechanism of PDC bits, numerical simulation researches were conducted based on the discrete element method (DEM). Firstly, a numerical triaxial test model was established to calibrate microscopic parameters of rocks that collected from Sichuan basin Ziliujing formation. Then, boundary conditions such as bottom hole temperature, surrounding pressure and hydrostatic pressure were loaded to rocks, thus simulating the actual bottom conditions. Based on this model, rock failure characteristics under the impacts of cutters with different shapes (cylindrical cutter, triangular prismatic cutter, ridge cutter, V-cutter) and different cutting parameters (cutting depth, back rake angles, etc) were studied. The results indicate that the triangular prismatic cutter has the greatest rock-breaking efficiency in our studies, followed by V-cutter and cylinder cutter. The triangular prismatic cutter can obtain the highest rock cutting efficiency when the cutting depth, back rake angles and the diameter of cutter are equal to 2 mm, 20 degrees and 16 mm respectively. The present study provides an in-depth understanding on rock failure mechanism and a basis for the development of efficient PDC bits.

Introduction

The Sichuan Basin is rich in natural gas resources and is one of the major gas production regions in China. However, the corresponding geological conditions are complex. Under the impacts of high temperature and high pressure at the bottom hole, the formation rock becomes highly abrasive and poorly drillable, resulting in a significant decrease in the drilling speed. To improve the drilling efficiency and enhance the performance of PDC bits, it is necessary to carry out studies on cutting and investigate rock failure mechanisms under the actual bottom hole temperature and pressure.

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