Which of the following is a disadvantage of air rotary drilling in loose formations?

• A. Faster drilling
• B. Cleaner samples
• C. Less fluid handling
• D. Less control in loose formations

Answer: (D)

In air rotary drilling, the lifting and removal of cuttings rely on a stream of air rather than a heavy drilling mud. In loose formations, such as unconsolidated sands or gravels, that lack of a viscous liquid column means there’s little hydrostatic pressure behind the bit to stabilize the borehole walls. The air can carry cuttings away quickly, but the hole walls are not supported, so the borehole can wash out, cave in, or collapse more easily. This makes maintaining a straight, steady path and applying consistent weight on bit much harder. The result is reduced control over the hole in loose formations, which is the primary disadvantage of air rotary drilling in those conditions.