What property makes bentonite especially useful in drilling fluids?

• A. High resistance to heat.
• B. Magnetic properties.
• C. Low density.
• D. Absorbency and swelling due to montmorillonite used in drilling fluids.

Answer: (D)

Bentonite is especially useful in drilling fluids because of its absorbency and swelling due to the montmorillonite its clay contains. When dispersed in water, montmorillonite hydrates and expands dramatically, increasing the mud’s viscosity and gel strength. This helps suspend cuttings when circulation slows and promotes a thin, low-permeability filter cake that reduces fluid loss into the formation. The other options don’t capture this behavior—heat resistance, magnetism, or low density are not the defining properties that make bentonite effective in drilling fluids.