Bit Load Requirements for SURFACE SET DIAMOND Bits

As is the case when drilling with impregnated diamond bits, the lowest possible bit load should be applied to a surface-set diamond bit that will provide an acceptable rate of penetration. Although surface-set diamond bits tend to drill with bit loads that are lower than impregnated diamond bits of similar dimensions, they have a more specific range of application than impregnated diamond bits.

Several variables must be considered when determining the appropriate bit load for a surface-set diamond bit, these include:

• The relative size of the diamonds set on the bit face,
• The number of diamonds set on the bit face,
• The grade of the diamonds set on the bit face (relating to the average strength and particle shape of the individual diamond crystals),
• The compressive strength of the formation being drilled,
• The maximum load rating of the core barrel and drill string to be used

The maximum bit load or “weight on bit” (WOB) to be applied to a surface-set diamond bit is calculated by multiplying the number of diamonds set on the bit face by a maximum load factor. The relationship between the maximum load factor and the diamond size / diamond grade are illustrated in the table below:

SPC = “Stones per carat”

Example:
The maximum permissible bit load for a T2-76 Surface-Set Core Bit with 354 diamonds of 50 SPC, Dimatec “Grade AA” natural diamond set on the bit face is given by: (354 diamonds) x (9.7 lbs / diamond) = 3434 lbsF or 15.2 kN.

Naturally, the characteristics of the formation being drilled has a large influence on the actual bit load applied. Some on-site experimentation may be required to find the optimum bit load required to achieve a satisfactory penetration rate