Frank Hartley • Houston
Drill bits no longer the weak link
There's some good news in the drilling industry relating to making hole. Drill bits are no longer the weak link in the drilling process. Higher hydraulics, rotary speed, torque, and weight on bit (WOB) are now the limiting factors, even though average footage per bit has increased 60% in just two years.
"The original Hughes roller cone bit was invented in 1909, with some major improvements in hard facing on the steel developed in 1925, which resulted in increased lon-gevity with open bearings," says Evan Turner, director of Marketing Services, Baker Hughes Christensen. In 1933 Hughes realized that by using o-rings and grease sealed bearing bits would last longer. The first tungsten carbide insert bit came out in 1951, followed by the development and use of the first natural diamond bit in 1952.
In the 1970s, journal bearings came along, which lasted longer than roller bearings. These were major milestones that had a tremendous impacted on the performance of drill bits, which resulted in the first PDC bit run in 1975. We think that PDCs' are a new phenomenon, but looking back over time, they actually were developed over 30 years ago.
PDC bit rental
The accompanying charts track the average bit performance as related to the life of the product. Looking back at diamond bit usage indicates that in the 1980s PDC bits were a smaller part of the diamond business than natural diamonds. However, they have come into their own and increased the amount of footage per run. Performance in the past few years has led to the concept of rental and/or repairing the PDC bit so that more footage on average per unit can be generated. In particular, within the last two years the PDC bit business in the US and Canada has exploded. The bits last so long that they can be rented and then repaired and run again.
In the last two years, PDCs have made a major breakthrough in cutter design technology, allowing them to be used on harder formations. The original PDCs were used on soft shale, because a hard stringer would destroy the bit. Due to new technology, we now see them drill the hard stringers and long sections of fairly hard rock. So, they're becoming more and more the bit of choice. During this period the cutters have evolved and constantly gotten better.
(Above) Looking back at diamond bit usage indicates that in the 1980s, PDCs were a smaller part of the diamond business than natural diamonds.
If you were to compare the cutters of the 1980s to today's cutters, there's no comparison due to the magnitude of difference. The quality properties of the cutters today are better than they used to be as related to the diamond table. They changed the mix, the processes to manufacture, and made the bit much more abrasive and impact resistant.
A polycrystalline is made from synthetic diamond grit feed stock with a mix of cobalt as a catalyst in a HT/HP press that fuses them together for a diamond-to-diamond bond. This process has been perfected over time. The mix of diamonds and the way they are processed has created the diamond table, which is much stronger.
There has been a lot of work done on the interface between the diamond table and the substrate behind, which originally was a flat surface. What they found was that it created high stress concentrations in the diamond table, because when it cools down, the carbide behind it shrinks at a different rate than the diamond table. However, by changing the geometry behind the interface, they can alter the stress concentration. A lot of things learned over time have improved the product. The roller cone side of the business has been more directed to the hard, more difficult formations.
Early natural diamond bits were used in drilling hard abrasive rock at 3- 4 ft/hr by grinding, whereas the PDC bits actually shear the rock. With the changes and improvements in PDC bits, they are replacing the type five-bit roller by obtaining rates 30-40 ft/hr versus 20 ft/hr, which is quite a dramatic change. However, on a footage basis, there are still a lot of roller cones used because they don't drill as many feet per unit. But we're getting to the point where half of the rocks are being drilled with diamond bits. Currently, a major transition is going on in the drill bit industry.
Bits last long enough now that they can be rented, repaired, and run again.
In spite of this impressive performance, it could be a lot better. What's really happening is that we have developed bits to a point where the bit is no longer the weak link in the drilling process. If the rig can deliver more power from a rotary torque and hydraulic standpoint, we can drill wells farther and faster than we do today. The weak link is the rig capability, particularly in land drilling versus the offshore drilling arena.
