Underreaming procedures vary from tool to tool. However, it shall be verified that:
- the tool is function tested with the mud pumps on surface before running in
- the cutters are secured inside the tool body with a small piece of rope or tape to ensure a safe trip in the hole without damage to the cutters
- pump pressure gradually increases while the tool is rotating until the cutter arms cut to the maximum diameter
- a constant drilling weight is maintained while underreaming. Do not slack off intermittently and do not spud the tool. A good rule of thumb for maintaining constant drilling weight is 100 - 200 lbs per inch of body diameter
Note :
- In soft formations, the initial cut may only take a few minutes but in hard formations, it can take in excess of one hour.
- In hard formations, apply time-drilling, allowing cutter arms to expand into the newly drilling over-sized hole.
- A penetration rate that is too high will result in poor hole cleaning and possibly under gauge or spiral hole being drilled.