Polycrystalline Diamond: Manufacture, Wear Mechanisms, and Implications for Bit Design
Authors: Sneddon M.V. , Hall D.R. 
Published: December 2007

This paper focuses on the failure modes of polycrystalline-diamond-compact (PDc) cutters and discusses efforts to improve impact resistance, thermal stability, and hydraulic cooling. Composite-transition-layer technology has made possible a new generation of polycrystalline-diamond (PCD)-enhanced inserts that can be used in percussion and roller-cone applications.

Introduction

This paper deals with the application of PCD in drilling and gives a brief overview of the manufacturing process used in synthesizing PCD. Discussion focuses on the failure modes of PDC cutters, which include residual stress failure, mechanical fatigue failure, thermal-physical failure, and thermal-chemical failure.

Efforts to improve the operating performance of PDC shear bits are evaluated, with specific attention given to improving PCD's thermal stability and to cooling PDC shear bits through better hydraulics. Curved PDC cutters offer potential benefits for drilling applications because they enhance cleaning, reduce cuttings buildup, and probably improve cooling.

It is doubtful, however, that much can be done to improve the formations, such as granite or quartz. PDC cutters have limited impact resistance in hard formations and fail under repeated loadings. Also, hard, abrasive formations can generate too much friction at the cutter/rock interface for the convective cooling of drilling mud to keep operating temperatures within theoretical 730deg.C [1,346deg.f threshold. PCD more effectively fails rock through compressive impact loading rather than through shear. This paper discusses (from the perspective of a PCD manufacturer) the current state of high-pressure, high-temperature (HPHT) technology, failure mechanisms of PDC cutters, limitations that restrict the use of PCD in drilling, and recent developments in PCD technology.

Overview

For more than a decade, the oil industry has used synthetic diamond drill bits to increase drilling performance and to lower drilling costs. However, several problems that result from material properties, quality control, and limitations in drilling technology remain with PDC cutter bits.

Problems with materials and bit design includespalling, delamination, and chipping of the diamond layer;bit balling in plastic formations;stud breakage in harder formations;excessive cutter wear in abrasive formations;inadequate hydraulics; andpoor mechanical cleaning action.

Quality control issues in manufacturing includeunsatisfactory bonding of PDC cutters to tungsten carbide (WC) stud;bonding of WC stud to bit; andpositioning of PDC cutters on bit.

PDC cutter bits also cause problems in drilling operations because theyare troublesome to operate-e.g., greater care is needed during drilling, rig crews need specialized instructions and training, and bits frequently need further refinement in design before they will perform in a given formation;are unable to drill medium and hard formations and are unknowingly used in unsuitable formations like pyrite or quartz;often deviate with high weight on bit (WOB);contain inadequate solids-separation equipment to keep up with high rates of penetration (ROP's); andneed shock absorbers in formations that cause drillstring bounce.