Which statement best describes the cost per foot relationship between air and mud rotary in unconsolidated formations?

• A. Air rotary generally lower cost per foot, but mud rotary may be more efficient at maintaining borehole stability.
• B. Mud rotary always yields faster drilling per foot.
• C. Air rotary usually costs more per foot and offers no stability benefits.
• D. Costs are unrelated to the rotary method.

Answer: (A)

In unconsolidated formations, the main trade-off is cost versus hole stability. Air rotary uses only air to lift cuttings, so you avoid the expenses of drilling mud—costs for mud, mixing, pumping, and disposal—which generally makes the cost per foot lower. Mud rotary, on the other hand, provides hydrostatic pressure and a viscous column that helps hold the borehole open and carry cuttings more effectively, which can reduce non-productive time and improve drilling efficiency in unstable formations. So the statement that air rotary typically costs less per foot, while mud rotary can be more efficient at maintaining borehole stability, best captures the real balance. The other options are too absolute or incorrect about stability and cost relationships.