Bit Load

Geotechnical Core Bits

Click here to view or download the recommended operating parameters for bit load for geotechnical core bits in .pdf format. (398kb) Geotechnical core bits include:

  • Surface-set diamond (natural),
  • TSP – Thermally stable polycrystalline diamond (synthetic),
  • PDC – Polycrystalline diamond compact (synthetic) – also known as “Pax-set”,
  • Saw tooth tungsten-carbide,
  • “Carbide-chip” tungsten-carbide.

Bit Load Requirements for Surface-Set Diamond Core Bits

When drilling with surface-set diamond core bits, the lowest possible bit load or “weight-on-bit” (WOB) should be applied that will provide a satisfactory rate of penetration. Although surface-set diamond bits tend to drill with bit loads that are lower than diamond impregnated bits of similar dimensions, the surface-set type has a more specific range of application.

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

Bit Load Requirements for TSP-Set and PDC-Set Core Bits

Unlike diamond impregnated and surface-set diamond products, geotechnical bits that are set with either thermally stable polycrystalline diamond (TSP) elements or polycrystalline diamond compacts (PDC) generally require the application of high, sustained bit loads for optimum performance.

The exception to this high bit load requirement is saw-tooth TSP core bits due to the very low compressive strengths of the formations in which these products are applied. Some on-site experimentation may be required in order to establish an effective bit load setting due to the variable nature of the formations being drilled and to mitigate the potential for overfeeding the bit.

When properly applied, TSP-Set and PDC-Set core bits frequently provide rates of penetration that are considerably higher than either the diamond impregnated or surface-set types. In order to achieve these high rates of penetration, it is necessary to apply relatively high bit loads so as to maintain the sharpness of the individual TSP or PDC cutters. This is particularly true in medium formations. Insufficient bit loads will cause the cutter elements to become glazed and polished. As a result, the bit’s penetration rate will be reduced to virtually zero. In all cases, bit loads should not be so high as to exceed the manufacturer’s recommended maximum load rating for the core barrel and drill string in use.

Bit Load Requirements for Tungsten-Carbide Set Core Bits

These types of bits are normally used to drill extremely soft, abrasive and loose formations. As such, they require very low bit loads coupled with very low rotational speeds. Due to the variable nature of the formations in which these types of bits are used, great care should be taken in applying bit loads. The lowest possible bit load that will provide an acceptable rate of penetration should be applied.