Which additive is commonly used to improve viscosity in mud rotary drilling fluids?

• A. Sodium bicarbonate
• B. Calcium carbonate
• C. Bentonite
• D. Polyacrylamide

Answer: (C)

Viscosity in mud rotary drilling fluids is largely built by hydrating a clay mineral that swells and forms a thick, dispersed network in water. Bentonite does this best: when sodium bentonite is mixed with water, it swells many times its dry volume and creates a gel-like suspension of plate-like particles. This network increases both the plastic viscosity and the yield point, meaning the mud resists flow under low shear but can still carry drill cuttings up the hole efficiently when sheared by pumping. That carrying capacity and improved suspension are exactly what you want for effective drilling fluid performance, especially in water-based muds.

Sodium bicarbonate is mainly used for pH adjustment and buffering, not for thickening. Calcium carbonate is a weighting material that adds density rather than viscosity. Polyacrylamide is a polymer that can modify rheology, but the most common and reliable viscosifier in muds is bentonite because of its strong, water-absorbent swelling behavior.