The drill bit is the cutting tool at the bottom of the drillstring that breaks rock and creates the wellbore. Bit selection, operation, and performance directly determine drilling efficiency—affecting rate of penetration, cost per foot, and hole quality.
Bit Types
Roller cone bits use rotating cones with teeth (milled tooth) or tungsten carbide inserts (TCI) that crush and gouge rock. IADC classification uses a 3-digit code: Series 1-3 for milled tooth, 4-8 for TCI, based on formation hardness.
PDC (Polycrystalline Diamond Compact) bits use fixed synthetic diamond cutters that shear rock. IADC classification uses a 4-character code indicating body material, formation type, cutter density, and profile shape.
Performance Comparison
PDC bits achieve 2-3× higher drilling rates than roller cone bits in suitable formations. South Texas operators reported completing wells in 70-80 days versus 120-130 days with conventional bits. The DOE FORGE geothermal program demonstrated over 400% ROP improvement in granite with physics-based drilling optimization (SPE/IADC 2022).
Cost Impact
While bit cost represents only 2-3% of total well cost, bit selection affects up to 75% of total well expenditure through its influence on drilling time, trips, and hole quality.
Automation Advantage
Autonomous drilling systems continuously optimize WOB and RPM based on real-time formation response, keeping the bit in its efficient operating envelope regardless of formation transitions. Autonomous weight-on-bit control extends bit life while maximizing penetration rate—demonstrated by 200%+ increase in footage per bit in hard rock geothermal applications.