What Drillers Should Know about PDC Bits
Polycrystalline diamond compact (PDC) bits have been around for years. Now, they are just getting popular in the water well industry. This is the way of most advances in our industry: The big boys have the money to do the initial research, and then it gets translated to our needs. PDC bits are no different.
At first, they were prohibitively expensive, because the companies that did the original research needed to get their money back. But, after a few years, the free market took over. The bits got into a price range available to all, while design improvements continued.
Although there are many different body styles, the “bullet head” shape is most common. PDC wing bits are available, although they don’t seem to be as popular.
PDC bits drill a little differently than tri-cones. The cutting structure is a thin wafer of synthetic diamond bonded to a stout shoulder. In many formations, a tri-cone bit drills by crushing. A PDC bit, however, drills by either scraping or gouging the formation, depending on hardness. You will notice that PDC bits have jets, while we usually take the jets out of our tri-cones. There is a reason for this. With the right pump and the right jets, there is a large pressure drop at the bit face. This impacts the bore face and also causes shear-thinning of the mud. This helps prevent bit balling. Properly designed mud will recover its original viscosity within a few feet, and help carry cuttings to the surface.
One of the original attractions of PDC bits was longevity. I’ve used several where we drilled over 10,000 feet, and they came out ready to go again. Of course, this depends on formation and bit design. In a sticky clay or loose sand, the wing style seems to work fine, but in rough running conditions it can be easily damaged. I’ve seen them lose whole blades. These formations like the “bullet head” style. They run smoother without chattering and usually last longer.
One part of the learning curve is how to drill balling clay with a PDC. Bit rpms are not such a big deal. Operators just need to find the sweet spot that the rig and the hole like, and get after it. One thing about soft, balling clay is the ability to out drill the bit. When this happens it balls up and pretty much quits drilling. Torque goes down and pressure goes up. This is a function of the clay. Most clays have a strong electrical charge on each particle. As that clay gets drilled, those charges push the clay to re-assemble itself as soon as it can. That’s a bit ball.
The cure is a combination of three things. First, the mud chemistry. A good tetraphosphate will strip the charge from the cuttings and help them stay apart, instead of trying to reassemble themselves. If you need this for most wells, Baroid makes a product called Bara-Phos. It works well, but can be pricey. If you only need a thinner once or twice a year, Calgon dishwasher detergent works well. Just pour about half a box in the drill pipe on connections until the mud looks right. I know I’m going to get some grief from the mud engineers, but it works well for me!
The other two tactics for heading off bit balling are adequate circulation and pressure drop at the bit. PDC bits like a lot of fluid to keep the bit face clean, as well as a significant pressure drop at the bit. In other words, run your pump as fast as you feel comfortable, and keep a pretty good pressure on your standpipe.
If you find yourself balled up anyway, pick up the bit off bottom, rotate fast and drink a cup of coffee. Often, this will easily clear the ball. If that doesn’t do it, rapidly stroke the pipe up and down the derrick, but don’t hit bottom! Free falling pipe is a lot easier on a rotary table rig than a hydraulic tophead rig, but it will help.
Hope this helps. You should be able to get thousands of feet of hole out of a good PDC bit. They are more expensive to buy, but the cost per foot is great. They can also be rebuilt — usually. Good luck, and keep ‘em turning to the right.
For more Wayne Nash columns, visit www.nationaldriller.com/wayne.