In unconsolidated formations, which statement about borehole stability is true?

• A. Mud rotary may be more efficient in maintaining borehole stability in unconsolidated formations.
• B. Air rotary always maintains stability better than mud rotary.
• C. They are equally effective in all cases.
• D. Stability is unaffected by rotary method.

Answer: (A)

In unconsolidated formations, the borehole is prone to collapse if the walls aren’t supported. Drilling with mud creates a hydrostatic pressure column that helps balance the formation’s pore pressure and physically supports the borehole walls. The mud also carries cuttings out and can form a wall-stabilizing filter cake, reducing sand entry and wall erosion as drilling proceeds. Because of these effects, mud rotary often provides more reliable borehole stability in loose or sandy formations than air drilling, which relies on low hydrostatic pressure and offers less wall support. Air rotary can be advantageous in some situations, but it doesn’t consistently maintain stability in unconsolidated formations. Thus, mud rotary may be more efficient at keeping the borehole stable in these conditions.