Button Bits

Button Bits for Hard Rock Drilling

Oilfield, waterwell, HDD, and geothermal drillers extensively use button bits to drill hard rock.

Button bits offer several advantages over PDC drill bits. Two advantages are that they are 1) less expensive and 2) can drill through gravels, dolomite and very hard and abrasive limestone, making them able to withstand higher impact.

However,  the PDC drill bit has taken over more than half the market share of total footage drilled.

Button bits, or roller cones, have rotating cones that contain tungsten carbide inserts. Tricone, or three cone bits, are the most common; however, manufacturers continue to make two cone and one cone bits.

Tungsten Carbide Insert Design

Button bit inserts fall in two classifications: Chisel and Conical.

Chisel Inserts

Chisel inserts are more aggressive than conical, and some designs employ a mixture of both.

Conical Inserts

Chisel inserts are more aggressive than conical, and some designs employ a mixture of both.

Tungsten Carbide Inserts

Tungsten carbide inserts are individually manufactured and then press fit into pre-drilled holes in the cone of the bit.

Fluid Circulation

All rock drill bits require some type of fluid circulation. Drilling fluid may consist of water or oil, air, or foam. The circulation of drilling fluid serves several purposes:

• Drills new geological formations by clearing away cuttings
• Stabilizes the borehole wall
• Cleans and cools the bit

Bearing Types

Tricones have three main bearing types. They are:

Roller or open bearing

Well workover applications, shallow oil and gas drilling, waterwell drilling and mining applications commonly use this bearing type. This bearing type allows the cones to spin freely. The bearing itself consists of a row of ball bearings and a row of roller bearings. The drilling fluid in use cools these bearings.  Unless the race inside the cone becomes worn too badly, a roller bearing bit is oftentimes repairable.  However, these bits are best used in the application of waterwell drilling.

Open bearing bits are easily distinguished by its IADC code with the final number being either a 1, 2 or 3.  (Read more about the IADC Classification System)

Sealed Roller Bearing

A seal around the bearings to protect them from cuttings and debris marks the primary difference between a sealed roller bearing and open bearing drill bit.

Mining, natural gas drilling, geothermal and some oilfield exploration use sealed roller bearing bits. Once the seal is worn and is no longer good, the drill bit still has some life left and will perform like an roller bearing drill bit.

The IADC Classification easily distinguishes sealed roller bearing bits because the final number will either be a 4 or a 5.

Friction bearing or sealed journal bearing

Oil well and natural gas drilling most commonly use this bearing type. Although the cost is generally higher, this bearing type can withstand the most abuse.  A floating bushing replaces the roller bearings inside the cone.   Additionally, the bearing is also grease lubricated and sealed from outside debris.

Directional drilling applications frequently use friction bearing bits.  Gage inserts placed on the bit shank reduce extra shank wear. Rerun or rebuilt sealed bearing bits are mostly used in waterwell drilling and some oil and gas applications.

The IADC Classification System of Tricones easily distinguishes journal bearing bits just by identifying the final IADC number – it will either be a 6 or a 7.

Button Bits vs. PDC

Advantages

Cost – Smaller initial investment

Less Impact Damage – Impact is less damaging to the cutting structure

Better in Hard Rock Formations – The fracturing action of a button bit is better in non-homogeneous rock formations.

More Control Over Bit – In directional drilling applications, a button bit is easier to control thereby offering lower torque, greater flexibility on WOB and a more steady tool face.

***Note: Directional drillers have traditionally used button bits because they’re steerable. However, new technology has caused the pdc bit to out perform.

Why?

Special emphasis has been placed on the pdc cutter and design aspects of the pdc bit.

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Disadvantages

Lost Cones – Because tricones have moving parts, there is always the risk of losing a cone in the hole.

Lower ROP – The shearing action of a PDC bit is more efficient, resulting in higher ROP.

Higher Cost Per Foot – Lower ROP results in higher cost per foot. PDC bits can be repaired and run multiple times, tricones cannot.

Lower Bit Life – PDC bits have a longer life than tricones.