The most important function of the drilling fluids in rotary drilling is

• A. To lubricate the drill string
• B. To bring cuttings from the bottom of the hole
• C. To cool the bit
• D. To seal the borehole

Answer: (B)

Drilling fluids in rotary drilling are mainly tasked with lifting rock cuttings from the bottom of the hole to the surface. This transport keeps the borehole clean, prevents accumulation that would slow or stop drilling, and helps avoid sticking of the drill string. While lubrication, cooling, and sealing are important roles of the fluid, they support drilling operation only after efficient cuttings removal is ensured. Without effectively carrying cuttings to surface, progress halts regardless of those other functions, making removal of cuttings the most critical capability.