Underreamers and Hole Openers – Enlarging the Hole
Most of the time, the bit sets the size of the hole. But sometimes the hole you drilled isn't the hole you need. The next casing string needs more room, the rock is squeezing in, or the well design calls for a bigger hole below a smaller pipe. That's when drillers reach for tools that make an existing hole wider. So what's the difference between an underreamer and a hole opener, and when is each one used?
An underreamer is an expandable tool that passes through a smaller casing or hole, then opens its cutters downhole to enlarge the wellbore below, while a hole opener has fixed cutters and enlarges a hole to a set size without needing to fit through anything smaller. Drillers use an underreamer when they need a hole larger than the casing above it, which is common in deepwater, salt and tight-clearance well designs. A hole opener is simpler and stronger, so it's the usual choice for opening a pilot hole from the top down, especially in shallow, large-diameter sections.
Both tools exist for the same basic reason. They make room for casing and cement, lower the risk of stuck pipe and give the well more clearance than the bit alone would provide.
Why Enlarge a Wellbore at All?
Drillers enlarge a wellbore when the hole drilled by the bit doesn't leave enough room for what comes next. Here are the most common reasons:
- Casing clearance: a bigger hole lets the next casing run in more easily and leaves room for a good cement sheath around it.
- Tight well designs: in deep wells with many casing strings, each string gets smaller. Underreaming below each shoe slows that shrinking and saves a hole size.
- Moving formations: salt and some shales slowly creep into the hole. Extra diameter buys time.
- Lower pressure while circulating: more annular space can reduce friction pressure, which helps in wells with a narrow margin between pore and fracture pressure.
- Pilot holes: drilling a small hole first to check for shallow gas or to steer accurately, then opening it up.
Casing is the main driver behind all of this. Rigzone's explainer on how casing works shows why each string has to fit inside the last one.
What Is a Hole Opener?
A hole opener is a fixed-diameter enlarging tool with cutters set around its body, often with a pilot section or bullnose leading the way. It follows an existing smaller hole and cuts it out to the tool's full size.
Hole openers have used roller cone cutters for decades, and PDC cutter versions are also common. Because nothing on the tool has to fold up, it can be built very strong. The catch is simple: the hole opener's full diameter has to fit through everything above it. So it's used in sections where the casing or hole above is already at least as big as the target size.
Here's an everyday comparison. A hole opener is like a step drill bit for sheet metal. The small tip follows the existing hole and the larger step cuts it wider.
What Is an Underreamer?
An underreamer is an enlarging tool with cutter arms or blades that stay folded in while the tool is run through casing, then expand once it's below the casing shoe. That's the key idea, and it's the whole reason the tool exists. It can make a hole bigger than the pipe it just passed through.
Underreamers are activated in a few different ways depending on the design:
- Hydraulic: pump pressure or a pressure drop across the tool pushes the cutters out.
- Drop ball: a ball pumped down the string lands on a seat and shifts the tool open.
- Electronic or RFID: a signal sent downhole tells the tool when to open or close.
When the job is done, the cutters retract so the tool can come back out through the casing.
Underreamer vs. Hole Opener: How Do They Compare?
| Feature | Underreamer | Hole opener |
|---|---|---|
| Cutter design | Expandable arms or blades | Fixed cutters |
| Passes through smaller casing? | Yes | No |
| Hole below casing shoe | Can exceed casing inside diameter | Limited to what fits through the casing |
| Complexity | Higher, with moving parts | Lower, very rugged |
| Typical use | Deepwater, salt, tight-clearance designs | Opening pilot holes, shallow large-diameter sections |
What Is Reaming While Drilling?
Reaming while drilling means running an underreamer in the BHA above the bit, so the well is drilled and enlarged in the same run. This saves a separate trip, which is a big deal when rig time is expensive.
There's one catch to plan around. Because the reamer sits some distance above the bit, the last stretch of hole at total depth is left at pilot size. This is called the rathole. It may need a separate cleanup run, or a second reamer placed close to the bit, to bring the whole section up to size.
Where Do Roller Reamers and Bicenter Bits Fit In?
A roller reamer is a different animal. It has rolling cutters set close to the hole size, and its main job is to keep the hole in gauge and smooth out doglegs, not to make it much bigger. It works like a stabilizer that also trims the wall, which is useful in hard, abrasive rock where fixed stabilizer blades wear fast.
A bicenter bit is another way to drill a larger hole below a casing. Its body is offset so that it fits through the casing, but when it rotates on bottom it cuts a hole larger than its pass-through diameter. Bicenter bits avoid moving parts, but they can be harder to steer and more prone to vibration. If you want a refresher on cutter styles and how bits differ, our article on drill bit types lays out the main options.
What Can Go Wrong When Underreaming?
Underreaming adds steps and moving parts, so it adds risk. The common problems include:
- The tool fails to open, leaving a pilot-size hole that the casing won't fit through.
- The tool fails to close, making it hard or impossible to pull back into the casing.
- Cutter or arm damage in hard or broken formations.
- Vibration, especially when the bit and reamer are cutting different rock at the same time.
Crews often confirm the tool opened by watching pressure signals, torque changes or caliper measurements from LWD tools. A planned test pull against the casing shoe is another common check.
Which Do You Need: an Underreamer or a Hole Opener?
Choose an underreamer when the hole below a casing shoe needs to be bigger than the casing it came through. Choose a hole opener when there's nothing smaller above and you want a simple, rugged tool to enlarge a pilot hole.
Both exist for the same reason: to give casing, cement and the drill string more room. When reviewing a well plan, look at the hole and casing sizes side by side. Wherever the hole is bigger than the pipe above it, an underreamer or a bicenter bit is almost certainly involved.