In mud rotary drilling for unconsolidated formations, how do higher RPMs relate to feeds?

• A. Higher RPMs require lower feeds to avoid borehole collapse, while lower RPMs can tolerate higher feeds for better penetration
• B. Higher RPMs require higher feeds to maintain cutting rate
• C. RPM has no effect on feed
• D. Lower RPMs require no feed

Answer: (A)

In mud rotary drilling through unconsolidated formations, keeping borehole stability is the main constraint. When you run the bit at higher speeds, the cutting action is quick but the formation wall is more easily disturbed and the mud has to carry away cuttings rapidly. If you also push a large feed at that high speed, the chip load becomes excessive and the hole can become unstable or collapse because the cuttings aren’t cleared fast enough and the wall isn’t supported properly. So, at higher RPMs you want a lighter feed to maintain control and allow the mud to flush cuttings and support the walls. Conversely, with slower RPM, you can apply a heavier feed per revolution to improve penetration without overwhelming the hole, since the rotation is gentler and the mud can handle the debris more effectively.