The introduction of PDC cutters fundamentally changed the mechanics of drilling. Instead of gouging and crushing rock, as does the roller cone bit, PDC bits rely on a transverse shearing motion. To achieve the shearing motion, the PDC bit uses a special cutter called a compact - a synthetic diamond disk cemented to a carbide substrate. Diamond is one of the hard-est materials in the world. It is 10 times harder than steel, 2 times harder and 10 times more wear-resistant than tungsten carbide and 20 times stron-ger in compression than granite. Diamond also has the lowest coefficient of friction and highest thermal conductivity of any known material.
The PDC cutters use a polycrystalline diamond that is synthesized by sintering diamond grit - an aggregate of randomly oriented fine and ultra-fine synthetic diamond particles - with a cobalt [Co] catalyst under high pressure and temperature. Unlike natural diamond [C], which fractures along crystallographic planes, polycrystalline diamond has a randomly ori-ented matrix and no preferred cleavage planes, thus making the PDC cutter extremely hard and resistant to impact and wear.
The diamond table - that part of the cutter that comes in contact with the rock - is sintered to a tungsten carbide substrate. Unlike many sub-stances, tungsten carbide is able to bond to diamond; thus while the sub-strate gives structural support to the diamond table, it also provides a medium that can withstand brazingâ€”a process used to mount the cutter substrate to a blade on the bit. Some tables have a slight bevel that reduces stress concentration on the cutter as it makes initial contact with the rock at the instant it starts to cut. Although the beveled edge can reduce bit aggressiveness, it helps increase durability and impact resistance.
Diamond impregnated bits are sometimes used to cut formations deemed too hard or abrasive for roller cone or PDC bits. The diamond impregnated bit features diamond grit suspended in a tungsten carbide matrix for improved wear resistance. Instead of having individual cut-ters, the entire bit face contains industrial-grade diamonds, which grind hard formations while blade edges shear soft formations. The matrix abrades away to continually expose new sharp faces of the diamonds.
Coring bits also employ diamonds. These toroidal bits are used to cut core through zones of interest. Coring bits are hollow in the center to permit the cylindrical rock sample to be collected in a core barrel mounted above the bit. Rock cores are usually cut in about 9-m [30-ft] increments.
In certain formations, air drilling operations are required to make hole. When hard rock formations are encountered, the operator may call for a percussion hammer bit. These bits pound the rock vertically in much the same fashion as do air hammers used in construction. A piston above the bit imparts force through impact to drive the bit.
A New Type of Cutter
A recent step in the evolution of the fixed cutter bit is the development of the conical diamond cutter. With twice the diamond thickness of conventional PDC cutters, the conical diamond element (CDE) has demonstrated improve-ments in impact strength and wear resistance. When placed at the center of a conventional PDC bit, the CDE crushes and fractures rock for improved rates of penetration. Multiple CDEs can also be placed across the bit face for increased footage and improved toolface control compared with those of con-ventional PDC bits.
Occasionally, a new, disruptive technology transforms the way things are done. Wells were once constructed by hand but were later dug using cable tool rigs and then drilled by rotary rigs. The future will probably be defined by another technology - perhaps laser, ultrasonic, microwave or thermal technology - that relegates rotary drilling to a footnote in history. Until the next radical development in borehole construction is introduced, the bit by design occupying the front of the drillstring will remain at the cutting edge of drilling technology.
For help in preparation of this article, thanks to Wiley Long, Houston, Texas, USA.
Oilfield Review 2016.
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