How does the cost per foot of air rotary compare with mud rotary in unconsolidated formations?

• A. Air rotary is generally lower due to faster drilling; mud rotary may be more efficient in maintaining borehole stability.
• B. Air rotary is always more expensive per foot.
• C. Mud rotary is always cheaper per foot.
• D. They cost the same per foot in all conditions.

Answer: (A)

In unconsolidated formations, how fast you drill and what you have to spend on circulating and disposing of drilling fluids largely determine cost per foot. Air rotary tends to drill faster because it avoids the need to mix, pump, and manage drilling mud. The cuttings are carried out by a high-velocity air stream, so there’s less time and money tied up in mud handling, mud costs, and waste disposal. That speed and simpler fluid management usually make the per-foot cost lower.

Mud rotary, by contrast, uses drilling mud to lubricate, cool, and stabilize the borehole walls. While this can improve hole stability and reduce certain problems in some unconsolidated zones, it introduces costs for the mud itself, additives, pumps, filtration, and waste disposal. Those added expenses often raise the cost per foot, even if stability reduces some non-productive time.

So, air rotary is generally cheaper per foot in unconsolidated formations, with mud rotary offering stability benefits that can be advantageous in specific circumstances.