Bit Load Requirements for IMPREGNATED DIAMOND Core Bits

When drilling with impregnated diamond bits, the lowest possible bit load or “weight on bit” (WOB) should be applied that will provide a satisfactory rate of penetration. Bit load is a function of both the matrix type as well as the bit face bearing area, that is, the actual contact area of the bit face with the formation being drilled. In the case of softer matrices such as “HR11”, “HR12”, a high bit load is unnecessary as it will cause the matrix to abrade at a high rate and/or smear resulting in premature consumption. Harder matrices such as “D2”, “D3” and “D4” are more receptive to higher bit loads. The relationship between Dimatec matrix type and maximum bit load per unit bearing area is given in the following table:

It should be noted that unnecessarily high bit loads will not only damage the bit but will over time do considerable damage to both the drill string as well as the drilling machine. Applied bit loads may be reduced by changing to a softer (higher number Dimatec “D-Series” or "HR-Series") matrix and/or changing to a lower bearing area waterway configuration (for example from the “Regular” style to the “Turbo” style).

Conversely, insufficient bit load will cause the exposed diamonds on the bit face to become blunt as they will not micro-fracture to expose new cutting edges. As a result, the bit will not perform effectively due to a loss of penetration rate. The minimum recommended bit load per bit face bearing area for all Dimatec “D-Series” and “HR-Series” impregnated diamond bit matrices is 1280 lbs / inch2 (or 90 kg / cm2).

The following formulae may be used to calculate the maximum and minimum bit load or “weight on bit” (WOB) for any Dimatec “D-Series” or “HR-Series” impregnated core bit depending on the bit face waterway configuration.

Bit load formula for “Regular” (Style W), “Extra-Wide” (Style XW), “Extra-Extra Wide” (Style XXW) and “Face-Discharge” (Style FD) Waterway Configurations:

Bit load formula for "Turbo" (Style T) Waterway Configuration:

Bit load formula for “T-Regular” (Style TW) and “Slot Face Discharge” (Style SFD) Waterway Configuration or any other waterway configuration using trapezoidal shaped waterway canals:

Where:

“ F “ represents the calculated maximum bit load in “pounds force” (where the LMax values from the table above have been used) OR the calculated minimum bit load in “pounds force” (where the LMin value of 1280 lbs / inch2 has been used).

“ L “ represents the bit load per bit face bearing area in “lbs / inch2”. The maximum values are a function of the Dimatec matrix type and are taken from the table above while the minimum value is a constant of 1280 lbs / inch2.

“ D “ represents the outside set diameter of the bit crown in “inches”.

“ d “ represents the inside set diameter of the bit crown in “inches”.

“ n “ represents the number of full waterway canals across the bit face kerf. In the case of the “Turbo” (Style T) waterway configuration, where the number of waterway canals “n” equals the number of partial turbo canals “n”, use only the number of full waterway canals “n” across the bit face kerf in the formula; not the sum of waterway canals plus partial turbo canals.

“ w “ represents the typical width of the waterway canals (or partial turbo canals) in “inches”.

“ t “ represents the typical length of the partial turbo canals (where applicable) in “inches”. This value is measured from the outside set diameter of the bit across the bit face kerf.

“ x “ represents the typical width of the trapezoidal waterway canals at the outside set diameter of the bit crown (where applicable) in “inches”.

“ y “ represents the typical width of the trapezoidal waterway canals at the inside set diameter of the bit crown (where applicable) in “inches”.

Based on the equations given above and on the maximum and minimum recommended bit loads per unit bit face bearing area, the recommendations for bit load (F) or “weight on bit” (WOB) for some common impregnated diamond core bit size / waterway configuration combinations are given in the following table. The values given do not consider the limitations of the drill string in use.