Editor’s note: Bit selection is a subject that often comes up, and as such, we believe that this informative piece on how bits work and how to choose the appropriate bit, which originally appeared in the June 2005 issue, is worth revisiting.
A customer of mine asked me the other day, “Exactly how does the bit drill?” I thought about it and gave him a half-baked answer, but his question got me thinking about it. I realized that I never had really thought about it much – it just does.
Obviously, the basic idea of a bit is to remove formation ahead of the drill string to advance the hole. That’s the simple answer, but I found out there is much more to it than that. The cuttings must be small enough to circulate out freely without balling or bridging the hole, but not so small as to force the bit to regrind them. Regrinding slows the progress of the bit and accelerates wear. The economy of the bit also is a factor. A bit that lasts forever, but doesn’t make much hole, is just as expensive as a bit that drills like a house-afire, but has to be changed before you reach T.D.
The first consideration in bit selection is the formation compressive strength. In general, formation compressive strength is classified as low, medium and high. Low compressive strength formations include unconsolidated sands and the softer clays. Medium compressive strength formations include formations such as shales and sandstone. High compressive strength formations include granite, basalt, dolomite and volcanic tuff. Knowledge of the area in which you are going to drill is very helpful in bit selection.
A bit removes formation and advances the hole in one, or more, of three ways – gouging, scraping or impact.
Gouging is an action where the cutting surface of the bit actually penetrates the formation and plows it up ahead of the bit. It works the same way as when you stick the knife in a jar of peanut butter to make a sandwich. You plow up the formation (in this case, peanut butter) ahead of the bit. Formation removal is ahead of the bit teeth.
Scraping is an action where the teeth of the bit scrape and compress the formation to failure. It works like scraping your knife across the top of a frozen stick of butter. Some of the formation is removed ahead of the tooth, and some is removed by the rebound effect behind the tooth.
Impact is an action where the tooth of the bit does not actually penetrate the formation, but transmits shock, which causes the formation to fail, and advance the hole. It is like when you whack a piece of peanut brittle against the counter to fracture it: The counter doesn’t penetrate the formation; instead, the impact causes it to fail structurally. In impact-mode, formation removal is behind the tooth of the bit.
Roller cone rock bits come with a bewildering array of options, all designed to enhance one feature or another. Tooth length, tooth spacing and tooth overlap all are design considerations.
Long teeth are important in soft formations to allow the tooth to gouge deeply into the formation. Tooth spacing is important because the number of teeth acting on the bore face dictates the amount of force, or energy, used. With fewer teeth, most of the energy is expended in a small area; more teeth, and the energy used to drill is expended in a larger area.
Tooth overlap also is important. When the teeth of each adjoining cone overlap, the bit is a lot more self-cleaning, and resistant to balling in sticky clays.
Another design consideration is skew angle. The skew angle is a relationship between the axis of the cones and the centerline axis of the bit itself. A large skew angle causes the teeth to gouge and scrape more. A small or missing skew angle depends on scraping and impact to drill.
After selecting a bit and drilling with it, close examination when you pull it can help a lot in the selection of your next bit. Wear patterns and failure analysis are crucial steps to proper bit selection. An ideal roller cone bit failure is when the cutting structure (teeth) wear out at the same time as the bearings. There are other factors, but simply put, if the teeth wear out first, it probably is because of too much weight and/or not enough rpms on the bit. If the bit is flat-spotted, it has balled up and a less-than-attentive driller has allowed it to fail.
Conversely, if the bearings fail before the teeth, it usually can be traced to not enough weight, rpms that are too high, or poor solids control. I’ve seen bits with the center cut completely out because the driller essentially was pumping “liquid emery cloth” through the system. Imagine what his pump, swivel and pipe must look like!
Just as the circulating system must circulate out the cuttings before they are reground by the bit, the solids control system must remove the cuttings from the mud before it is recirculated back to the hole. A pit that is too small or not properly designed will accelerate bit wear and slow penetration. Usually, the large cuttings will settle out fairly quickly; it is the finer, more abrasive contents of the mud that do the most damage.
Larger pits, or better still, mechanical separation, including desanders, will prolong bit life a measurable amount.
I hope this helps in your bit selection. As an exercise, why not go out behind the shop and take a look at that pile of used bits lying around? I’ll bet you can find a consistent failure in most of them. Recognizing that and fixing the problem – and maybe selecting a slightly different type of bit – should help raise your penetration rates and lower your bit costs, both of which go directly to the bottom line.