Heavy steel casing in air rotary drilling primarily serves to:

• A. Stabilize unconsolidated or collapsing formations
• B. Increase drilling speed
• C. Improve sample quality
• D. Reduce torque on the drill string

Answer: (A)

When drilling through loose or unconsolidated formations, the borehole walls can collapse and close in on the drill string. Heavy steel casing provides a rigid lining that supports and stabilizes the hole, keeping the borehole open as you advance the drill. This stability is crucial for safe operation, prevents stuck pipe, and allows the air and cuttings to be correctly circulated to the surface. While casing can influence other aspects like sample handling or long-term well integrity, its primary role in air rotary drilling is to prevent hole collapse and maintain a stable opening. It doesn’t directly speed up drilling, improve sample quality on its own, or inherently reduce the drill string’s torque.