Diamond Core Drill Bits
A core bit is a drilling tool with a hole through the center that removes sediment rock and allows the core pedestal to pass through the bit and into the core barrel. The Ocean Drilling Program (ODP) employs different coring systems and bits to obtain continuous cores in all types of oceanic sediments and igneous basement.
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Once a coring system is selected based on the expected lithology, the engineer determines which type of core bit to use. As coring conditions change, the coring bit can be changed in an attempt to improve the recovery and rate of penetration with that coring system. The type of bit used depends on the expected lithology and past bit performance in the area or in a similar lithology. Once a bit is removed, it is “graded” (i.e., it is examined to determine wear on the cutting structure, gauge, bearings, etc., based on industry standards) to optimize coring performance.There are three coring systems. Each uses different bottom-hole assemblies (BHA) and types of core bits
- Advanced Piston Corer (APC)
- Rotary Core Barrel (RCB)
- Advanced Diamond Core Barrel (ADCB)
Core Bit Types
Core bits are classified according to the cutting structure and type of bearings. There are five basic types of bits used based on their function or structure: drag, scraper, abrasive, roller cone, and hammer.- Drag-type bits have a flat chisel-like surface to plane away soft formations (i.e., clay and chalk).
- Polycrystalline diamond compact (PDC) bits use multiple tungsten carbide studs with artificial diamond cutting surfaces in a claw like scraping action to remove soft formations (e.g., clay and chalk) up to hard claystone and limestone.
- Diamond bits use either surface-set or impregnated diamonds to abrade (i.e., sanding-like process) hard formations like shale or basalt.
- Roller cone bits rotate cone-shaped rollers encrusted with teeth to remove soft to hard formations through a combination of scraping and crushing processes.
- Hammer bits use percussion to crush the hard rock around the core.
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