Choosing a drill bit is one of the highest-leverage decisions in a drilling program. The bit decides how fast the section is drilled, how many trips are needed and how much vibration the rest of the string has to absorb. Yet the choice often comes down to habit: the bit that worked on the last well. This guide explains how the two main bit families work and gives a practical way to choose between them.
How a PDC Bit Cuts Rock
A polycrystalline diamond compact (PDC) bit has no moving parts. Synthetic diamond cutters are brazed into pockets on a set of blades, and as the bit rotates under weight, each cutter shears a thin layer off the rock, much like a lathe tool cutting metal.
Shearing is an efficient way to break rock that is soft to medium-hard and reasonably uniform. Because there are no bearings to wear out, a PDC bit can stay in the hole for very long runs, which is where most of its cost advantage comes from: fewer trips mean fewer rig hours.
The main design variables are:
- Blade count: fewer blades allow larger junk slots and more aggressive cutting; more blades spread the load and improve durability.
- Cutter size: larger cutters remove more rock per revolution; smaller cutters are more impact resistant.
- Back rake: the angle at which cutters meet the rock. Lower rake is more aggressive; higher rake is more durable.
- Gauge length and profile: affect stability and how easily the bit can be steered in directional wells.
How a Tricone Bit Cuts Rock
A tricone, or roller cone, bit has three cones mounted on bearings. As the bit rotates, the cones roll across the bottom of the hole and their teeth or inserts crush and gouge the rock. The cones are slightly offset so that they also scrape as they roll.
There are two main types. Milled tooth bits have steel teeth machined into the cones, usually hardfaced, and are used in soft formations and top-hole sections. Tungsten carbide insert (TCI) bits carry hard inserts pressed into the cones and are used in medium to very hard and abrasive rock.
Crushing is less efficient than shearing in soft rock, but it copes far better with hard, abrasive, fractured and interbedded formations, where impacts would chip a PDC cutter. The trade-off is the bearings: they wear with every revolution, which limits how long a tricone bit can stay in the hole.
Where Each Bit Performs Best
As a broad rule, PDC bits suit long sections of soft to medium-hard, consistent rock such as shale, claystone and many sandstones. Tricone bits suit hard and abrasive rock, broken or fractured intervals, hard stringers within softer formations, and sections where the bottom-hole assembly is prone to severe vibration.
A Practical Way to Choose
- Collect offset data: bit records, dull grades, rates of penetration and any logs from nearby wells. These tell you what the formation actually does to a bit.
- Characterise the section: rock strength, abrasiveness, interbedding and expected changes with depth.
- Define the job: hole size, section length, vertical or directional, and whether a motor or rotary steerable system will be used.
- Check the rig: available torque, weight on bit and pump capacity. An aggressive PDC bit is wasted if the rig cannot deliver the torque it needs.
- Compare cost per metre, not bit price. Include rig time, trips and the risk of failure, not just the purchase cost.
Reading the Dull Grade
Every pulled bit tells a story. The IADC dull grading system records wear on the inner and outer cutting structure, the main dull characteristic, gauge wear and the reason the bit was pulled. Recording dull grades carefully, and sharing them with your supplier, is the single best way to improve bit selection on the next well.
Get a Recommendation
If you are planning a section and want a second opinion, send us your well program and offset bit records with a quote request. A Kronborr engineer will recommend a bit and explain the reasoning in writing, so your team can check it.