In my last column, I discussed the most important part of our drill string: the drill bit. All parts of our string are important, but we go nowhere fast without a drill bit.
Under certain conditions, we may want to use a special drill bit — something different from the regular type. We may run these special bits all the time or on occasion in unusual drilling conditions. Three types of special bits come quickly to mind: twisted bits, star bits and underdigger bits.
A twist during manufacturing is the chief difference between a regular bit and a twisted drill bit. A normal twist — about 180 degrees — gives it the look of a drill bit one would use in a drill press or a hand-held battery drill.
Twisted bits spin more than regular ones, and drill a straighter hole. Think of a rifle bullet spinning as it goes down the barrel of the rifle to go in a straight line. People say a twisted cable tool bit drills a rounder, straighter hole than a regular bit. This helps in slanted rock formations, as it is less likely to follow the slant of the rock. This type of bit also tends to plaster the wall in unconsolidated formations, supposedly making it easier to drive and pull casing.
These bits come in the same diameters and weights as regular drill bits. I would think it difficult to get much twist in a drill bit 3-feet-6-inches long, but it I guess it is possible. Twisted bits can be built in either right-hand or left-hand twists. The recommended twists is opposite of the drilling cable, so for left-lay line (which tends to spin the tools to the right and keep the joints tight) we would want a right-hand twist. Looking at the bit, the body turns to the right as we look at it going from the cutting edge to top tool joint. I have never run a twisted bit but I can see their advantage in some drilling situations.
Next, I want to talk about the star or four-wing bit. A regular bit, from a worm’s eye view, looks like an H with the sides bent in a circle. A star bit will look like a cross one might find on a grave, but with four equal-length sides. This gives star bits four cutting edges rather than the two on a regular bit, though they are smaller. Just like a twisted bit, the design of a star bit helps it drill straighter in slanted formations. For extreme conditions, manufacturers can also twist star bits just like regular bits. Again, we would want a right-hand twist to use with left-lay drilling line. Star bits, despite their construction, weigh just about the same as a regular drill bit of the same length. Distance across the four wings is just about the same as the body diameter of a regular bit. Catalogs I have list star bits available for 4- to 18-inch hole sizes.
Fabricated star bits are available for drilling holes larger than 24 inches. Their design calls for welding blade sections onto a mandrel, or center section, which also has the tool joint. These bits can get pretty darn heavy, and one would need a good-sized rig to handle the weight. I have read of bits like these that will drill a hole up to 48-inches in diameter but, again, I have never run a star bit so I can only report what I have read.
Dressing a star bit is difficult. Some manufacturers provide a kind of die that fits over all four corners, which you would strike with a sledgehammer or bit ram to forge the bit. Certainly large-diameter bits would be easier to keep sharp with hard surfacing than forging, but both methods work. Actually if a star bit is used in areas where they can be hard-surfaced, I can see where that process would be easier than forging.
Although a special purpose bit, drillers could use it every day in some geologies and conditions. I have never seen a star bit for cable tools in my area, but I used a version back in the day when we drilled 2- and 3-inch wells by the hollow-rod method. Like any drilling tool, sometimes they helped a lot and sometimes not much at all.
I’ll hold off on discussing the underdigger bit until next time, but I do want to spend a few words on another type of bit: the underreamer. An underreamer expands below the casing for situations where you want to drill a larger hole than a bit going through the casing can do. This is another one I have never run, but its design includes several moveable parts, making it far less durable than a regular drill bit — or a twisted one for that matter. Even the manufacturer of one brand I know recommends you consider reducing the hole size or pulling the casing and starting over with a larger-diameter rather than using the underreamer. I’ve heard underreaming can prove slow and costly, with no guarantee of success. Certainly, running a underreamer requires care on the driller’s part. Again, I have never run an underreamer and, from what I understand, I’m glad I never have.
You will likely read this in 2022 with the holidays past. I sincerely hope you had a successful 2021. It seems that strange and difficult times continue. It sounds pessimistic, but I see the Covid-19 pandemic continuing for some time. Regardless, I wish you a happy 2022 and hope your drilling is going well.
As I write this in early December, we have had some snow but it all melted off and my infamous lawn is still quite green. If it continues to grow — and I don’t think it will — it is out of luck as I have long-since equipped my lawn tractor with a snow blade, chains and wheel weights for the inevitable snow coming. We did have a few flurries as I wrote this column, but no accumulation. To one and all, best wishes for 2022.
For more John Schmitt columns, visit www.thedriller.com/schmitt.