Thick-walled, heavy tubulars placed directly above the bit. Provide the primary source of Weight on Bit (WOB) and contribute to directional control.

· Slick Drill Collars: Smooth outer diameter, cost-effective for vertical wells.
· Spiral Drill Collars: Machined spiral grooves reduce differential sticking risk by minimizing wall contact area.
· Non-Magnetic Drill Collars (NMDC): Manufactured from high-quality austenitic stainless steel (P550, P650). Used near the Measurement-While-Drilling (MWD) tool to prevent magnetic interference with directional sensors.

Standard: API 7-1, NS-1.