Until the 1900's, most oil wells were created using percussion drills - technology already in use by the Chinese in 1100 BC and still a very popular method of drilling for water (see www.avalon.net/~cmissen/wellsprn/drildesc.htm). A percussion drill is a pole with a heavy piece of metal on the bottom. It is lifted and dropped repeatedly into a hole, gradually making it deeper. Many men working together often took many years to make a single well this way. The work is now usually done with engines.

Throughout the 19th and 20th centuries, improvements in steel technology provided rotating drill bits that could quickly cut through soft rocks. Even in soft rocks, steel can be worn down very rapidly, so it is often covered with inserts, or a complete outside layer, of tungsten carbide, which is more brittle than steel, but has greater resistance to erosion. If this is not enough, synthetic diamonds are added, but the hardest rocks can only be drilled with the help of real diamonds.

Diamond, the hardest material known to man, is 10 times harder than steel, 2 times harder and 10 times more wear-resistant than tungsten carbide, and has 20 times the compressive strength of granite.

Geologists first used natural diamonds for drilling in about 1910 in hollow coring bits that cut doughnut-shaped holes and retrieved columns of rock for analysis. Diamonds were first introduced to full-hole bits for oil wells in the early 1920s and are widely used today. Natural-diamond bits use industrial-grade-not gem-quality-naturally occurring stones that are crushed and processed to produce specific sizes and regular, rounded shapes.

This drill bit has tungsten-carbide blades impregnated with tiny grit-like diamonds, enabling it to grind through very hard rocks. As the tungsten-carbide is worn away at the cutting surface, worn diamond grains fall out and fresh grains become exposed.

This drill bit cuts through hard rocks using natural diamonds set in patterns in tungsten-carbide blades

This man is holding a PDC
insert with tweezers

Natural diamonds form deep in the earth under intense heat and extreme pressure for thousands of years. In the early 1970s, the American company General Electric developed a process to make synthetic diamonds. Thin circular layers of alternating carbon graphite and cobalt are stacked in small cans and pressed to 2 million psi [13,733 MPa] followed by heating to 2732°F [1500°C] for five minutes. This process creates small crystals of synthetic diamond that bond together to become polycrystalline diamond compact (PDC). Unlike natural diamonds, the individual crystals are too small to carve into hard rock. Instead, PDC inserts are incorporated into the edges of drill bits to grind through the rock - rather like extremely hard sandpaper.

Many drill bits include a combination of steel, tungsten-carbide, PDC and diamond cutting and grinding edges. There are a wide variety of combinations, each engineered to optimally drill different types of rock.

The black discs in the
cutting edge of the
blades on this drill bit
are PDC inserts

Left: This "hybrid" drill bit contains both PDC cutting inserts and diamond impregnated studs. This view also shows the nozzles between the blades at the end of the bit. Special mud is pumped through these nozzles to cool the bit and wash pieces of rock away from the cutting edges, up the hole to the surface, where they are analysed by geologists.

Below left: Roller cone bits have metal cones that rotate independently. Each cone has teeth made of hard steel, tungsten-carbide, PDC, diamonds or a combination of these.

Below right: This side view of a roller cone bit shows PDC and diamonds inserted into all exposed parts of the bit, in addition to the cutting teeth. This is to reduce wear when drilling through very abrasive rocks.

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