A bentonite-cement mix is commonly used for which purpose in mud rotary drilling?

• A. Grouting the surface area around the well
• B. Mixing cement for cementation of surface casing
• C. Grouting the mud rotary well
• D. Lubricating the drill pipe

Answer: (C)

Grouting the mud rotary well relies on a bentonite–cement mix to create a solid, low-permeability seal in the borehole. The bentonite adds viscosity and swelling action, so the slurry can be pumped down the hole and push cement into the annulus around the drill string (and any casing). Once set, this grout seals off pathways for fluids from surrounding formations, helps prevent lost circulation, and stabilizes the borehole walls. In mud rotary drilling, this use of grout is the practical way to immobilize and isolate the wellbore, making future cementing and overall well integrity more reliable. The other options describe lubrication or surface-area cementing, which serve different purposes and are not what this grout is primarily used for.