A Bit for Every Application
Is there a bit for every application? Well, pretty close. If you run a Spang or Acme conventional cable tool bit under a Bucyrus-Erie 22W, you can pretty well drill any formation on earth. But there are a few other things you should consider during the bit selection process. Whether or not a given bit will drill a formation usually is secondary to the economics of drilling. Lots of bits will eventually rub a hole in the ground, but if they're too slow, they aren't economically practical. Bit selection starts with the drilling process itself. Cable tool drilling, air rotary and mud rotary all have a vastly different set of bits to choose from.
Since I've spent the majority of my years in the business drilling with the mud rotary method, I'm going to limit this discussion to mud rotary bits.
Rotary bits come in three basic types - wing bits, roller cone bits and claw bits - and there are about a million variations in each group. Wing bits are probably the oldest type still used today. They usually feature three or four wings that are tipped with tungsten carbide inserts. For softer formations, the "scratcher" type is used. It has interrupted teeth to increase penetration, but sacrifices longevity. For stiffer formations, a step bit is used. It has the cutting structure in steps from the center to full gauge. This is the bit most drillers who run wing bits use. For the stiffest formations and highest weights, a chevron bit is used. It is most frequently a four-wing bit with solid tungsten carbide chevrons all the way across the face. It will run in pretty hard formations with high weights and bit speeds, although, in highly fractured formations, drill pipe torque can be a big problem. Wing bits all drill with a scraping action on the face of the borehole and use the fluid circulation for flushing, rather than jetting. This is why they are sometimes called drag bits.
The roller cone bit was developed around the turn of the century (not this one, the last one!) for use in the oilfield. It originally had from one to four cones, but most bits now have standardized to three cones. The roller cone bit comes in a bewildering array of tooth length, spacing, materials and cone offsets to match different drilling conditions. It drills with a combination of scraping, gouging, impact and jetting action, according to the particular bit. Generally, longer teeth with wide spacing and little or no cone offset best drill softer formations. Shorter teeth that are set more closely together with some cone offset are used to drill harder formations. Bits for the hardest formations have tungsten carbide inserts to maximize impact and scraping on the face of the borehole. Most roller cone bits need enough fluid circulation to provide a significant pressure drop at the bit to enhance jetting and keep the cones clean. This helps prevent bit balling and regrinding of cuttings.
Since roller bits are essentially flat bottomed, they have a tendency to "walk," or deviate, when drilling formations that are either bedded, have intermittent layers of hard and soft materials, or when drilling inclined formations. They deviate toward the bedding plane in formations that are only mildly inclined, and with the bedding plane in formations that are steeply inclined. For this reason, it is good practice to run some stabilization, such as a drill collar (or two), above the bit. Many drillers don't like the hassle of handling drill collars and rely on rig pulldown to get enough weight on the bit. When weight is applied from above, the flexibility of the drill pipe causes hole deviation. This can lead to all sorts of problems, such as inability to get the casing in the hole, inability to run close-fitting pumps or deviation so far out of tolerance that the customer refuses the well.
Due to the initial cost of new roller bits, most water well drillers run oilfield re-run or re-tipped bits. These are bits that, for one reason or another, have been pulled from the hole and sold on the used market. Most oilfield drillers run a new bit in the hole every trip, so there are a lot of good used bits available. The bit market fluctuates according to oilfield rig count and use, so finding a good bit supplier can be tricky. If you find a good one, stay with him - he'll take care of you.
Wear on roller bits naturally happens as they drill and can be classified as cutting structure wear, or bearing wear. Ideally, the cutting structure (i.e. teeth) wears out at the same time as the bearings, but this is rarely the case in the real world. Most water well drillers notice that the bearings fail before the teeth wear out. One or both of two things cause this. Not enough weight-on-bit causes the bearings to fail before the teeth, and poor mud control cause bearing erosion. When the teeth fail first, close examination often reveals flat spotting, indicating that the bit has balled up and been run with locked cones. Poor drilling practices cause this or, once again, poor solids control. Bits like clean, fresh mud, free of abrasive, unwanted solids.
The newest type of bit on the market is the claw bit. It uses tungsten carbide tipped "bullets" to drill. It drills by gouging action in softer formations and by impact in harder ones, so it will drill a wide variety of formations. Benefits to water well drillers are that it doesn't need a lot of weight to achieve good penetration, and it is the only bit that is quickly field rebuildable with nothing more than a hammer and punch. The downside is it is a little more expensive to buy, but it will typically drill five to 10 times the footage of other bits, so it easily pays for itself in bit costs over the long run. A claw bit doesn't usually ball up as badly as roller bits, but it can sometimes run a little rough in fractured formations. These bits like pretty high rotary speeds and lighter weights. Since they are a tapered design, they tend to drill a straighter hole.
Is there any one bit that will do it all? Nope. But there is a bit that will drill what YOU drill better than any other one. It's just a matter of finding it.