Naturel Diamonds Drilling Bits

Diamond bits were used in the early days for coring extremely hard formations. The first diamonds bits were very expensive compared to the roller bits which made them less economical for drilling operations. Later improvement helped the diamond bits to be more effective for drilling.

Fig 01- Diamonds Drilling Bits

I. Diamond Bits Application

The decision of running a diamond bit is based on the cost analysis, but there are some situations when the diamond bit can be an economical application.

Very short life of roller cone bit: If the roller bit fails due to tooth wear, bearing failure of even tooth breakage, diamond bit can be a good alternative because it has no bearings and the diamond compressive strength is very high.
Low Penetration rate: In some cases, high mud weight or rig hydraulics becomes an obstacle to drill with roller cone bits that gives low rate of penetration. Using the diamond bit can give better results due to the moderate hydraulic horse power per square inch needed for cleaning the bit face.
Lower weight on bit: Less weight on bit is needed to drill with a diamond bit compared to other types of bits.
Motor application: Bearings can fail under the high rotary speed of motor. The diamond bits have very long life for such conditions.
Casing windows: Diamond bits are used for cutting casing windows for sidetracking a well.
Coring: Diamonds core heads allow for longer cores. The increased on bottom time helps to get better cores.
Fig 02- Diamond Coring Bit

II. Diamond Bit Operating Parameters


Circulation rate and pressure loss have to be considered when designing the hydraulic program. Cooling the diamond bit and cleaning the area below it are real issues which have to be performed effectively. 

The diamonds are heated continuously while drilling, so they have to be well cooled to keep their performance. Sufficient flow rate is the only way to cool the diamonds and prevent an early damage. 
The diamond bit has to be cleaned to prevent bit balling and keep the formation exposed directly to the diamonds. Pressure is needed to force the drilling fluids which can provide high velocities to reach adequate cleaning and cooling. 

Weight on Bit 

Generally, the required weight on bit for the diamond bit is less than the required weight on bit for other types of drilling bits. As a rule of thumb, weight on bit of 350 to 700 pounds per square inch of bit area can be a good operating parameter. In some cases, it is needed to slack off more weight on the bit to get higher rate of penetration. The weight on bit can be increased by steps till the level when no rate of penetration increase is recorded. Higher weight on bit is not always an advantage; bit life is another parameter which has to be considered when adjusting the weight on bit.

Rotating Speed

Drilling with diamond bits required high rotary speed. The level of 200 to 1000rpm is normally applied by using down hole motors (turbine or PDM). Good hydraulics design with high rotary speed can provide high rate of penetration. As general rule, rate of penetration increases with an increase of rotary speed, may be the only limit for this is the safety consideration for the drill pipe.

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