What is Underreaming?
Underreaming is defined as the process of enlarging a wellbore past its originally drilled size. Underreaming is a simple concept but involves complex expandable hydraulic tools. In principle, underreaming is the same as drilling with a bit to remove formation to create a full gauge hole.
Underreaming is done for many reasons, which may include safety, efficiency or necessity. Most underreamer applications are used to expand an existing borehole where there are restrictions. There are two types of underreaming applications that apply to the oilfield, which will be explained below.
In its simplicity, the operator trips in hole (TIH), with the pumps off, and sets his depth to where the cutter blades are roughly 6 -10 feet below the casing shoe. Once below, the drill pipe is filled with drilling fluid and pumps are engaged, forcing a hydraulic piston in a certain direction which will open the cutter blades. Once the operator performs tests to ensure the underreamer is open, it is time to start reaming. Drilling weight and hydraulics from the pump pressure keep the tool open while multiple jet nozzles facing the cutting structure keep them clean and cool. Upon reaching desired depth the pumps are disengaged, causing the tool to close and pulling out of hole (POOH) can commence.
The most common form of underreaming is where there is a pre-existing pilot hole. In this instance the operator will use either a bullnose or the bit below the underreamer to help guide it through the pre-existing bore hole. The second form of underreaming, which is becoming increasingly popular, is drilling and reaming simultaneously. This allows the operator to make one trip to drill a pilot hole and underream it to a larger diameter at the same time. This type of underreaming is run in complex bottom hole assemblies (BHA) and usually is 90 - 120 feet above the bit.
A successful underreaming job is defined as a job which results in a full gauge hole, underreamed in the least amount of time without leaving junk in the hole. All three of these aspects of the job must be weighed against each other. If a tool is left in the hole too long, the chances of leaving junk in the hole increase. If penetration rates are too high, a corkscrewed, under-gauged hole may result.