Recent developments in polycrystalline diamond (PCD) technology have led to breakthroughs in PCD bearing designs and PCD insert cutting structures. The authors suggest that the next major step in bit technology will be to combine these two innovations in a new generation roller cone bit, capable of sustained operation with a PDM or turbine.
Other potential areas of innovation are now made possible with this new technology. One of the first areas will be an integrated high-speed downhole drilling system consisting of the next generation roller cone bit, turbine, and MWD. Another area for development will be a downhole mudhammer for percussion drilling of petroleum wells. A more distant potential development is called the Robotic Advanced Drilling System (RAD), which uses a flexible drillstem and a downhole thrusting mechanism to give WOB and real-time drilling control.
(Review our companion paper, SPE 17006, to understand the wear mechanisms of PCD and the limitations of PCD shear bits.) This paper focuses on recent developments of PCD technology, which will overcome these limitations. Our discussion includes the following: a proposed Superbit with PCD bearings and PCD enhanced inserts; PCD bearing systems for PDM's and turbines; and a downhole hammer bit with PCD enhanced inserts.
Falling Rock through Compression Improves Wear of PCD
Crushing rock, rather than shearing it, changes the wear characteristics of PCD inserts. Two things occur. First, crushing rock does not generate as much friction as shearing rock. Second, the heat generated dissipates faster because there is less residual build up of rock cuttings on the face of the insert. This combination of less heat and faster heat dissipation keeps PCD well below the thermal-physical threshold of 700–730deg. C. Finally, the PCD at the rock/cutter interface is not subject to the same wear mechanism of mechanical fatigue as the PCD cutting in shear. Of course the prerequisite is to create an insert with low residual stresses and sufficient toughness to withstand repeated impact loadings. Because of its brittle nature, traditional PCD cutters have not been successfully applied to drilling in an impact mode.
Polycrystalline Diamond Enhanced Insert Development
Two different concepts have been combined in order to develop an enhanced insert that gives greater impact resistance while still retaining the wear resistance of traditional PCD inserts:
a composite material composed of polycrystalline diamond and pre-sintered WC/Co particles (Ref.1), and
the use of this composite material in transition layers between the PCD and WC/Co substrate.
(Ref. 2) The composite polycrystalline diamond is formed by mixing, in different volumes, individual diamond crystals and particulates of precemented carbide. The mixture is subjected to the same HP/HT process used for sintering PCD, creating intercrystalline bonds between diamond crystals and chemical bonds between tiny regions of PCD and the presintered Wc/Co particles.
Sintered to the tungsten carbide substrate are several such composite layers with varying concentrations of PCD and cemented carbide (Fig. 1). The outer layer, which is the cutting surface that penetrates the rock, has the highest volume percent of PCD, and may, in fact, be nearly identical in composition to the PCD used in drag bit cutters. Each inner layer has decreasing concentrations of PCD, until the final layer directly above the tungsten carbide substrate is nearly all cemented carbide.