mousehole, Oil & Gas Glossary

What Is a Mousehole?

A mousehole is a shallow hole drilled below the rig floor and lined with casing or pipe, into which a joint of drillpipe or drill collar is temporarily suspended in preparation for making a connection during drilling operations, allowing the rotary driller to quickly pick up and stab the next pipe joint without retrieving it from the pipe rack during each connection, reducing connection time and maintaining drilling efficiency on the rig floor.

Key Takeaways

The mousehole is typically 30-40 feet deep, sized to accept one standard joint of drillpipe standing vertically.
Pre-staging pipe in the mousehole eliminates catwalk retrieval time during each drillpipe connection.
The mousehole differs from the rathole, which holds the kelly or top drive sub during tripping operations.
Mousehole connections are standard in kelly-drive rotary drilling; top drive rigs handle connections differently without a mousehole.
A dropped mousehole pipe joint is a rig floor safety hazard and can damage bottomhole assemblies if it falls into the wellbore.

How the Mousehole Is Used in Drilling Operations

In a kelly-drive rotary drilling rig, the drill string is advanced by rotating the kelly (a four- or six-sided bar that passes through a matching bushing in the rotary table). When the kelly has been drilled down to the rig floor level — a sequence called "drilling down a kelly" — the driller must add another joint of drillpipe to continue advancing. The mousehole makes this connection sequence faster and safer. Before the kelly reaches bottom, a single joint of drillpipe is pulled from the pipe rack via the V-door and catwalk and placed vertically in the mousehole. When the connection is needed, the driller breaks out the kelly from the drillstring, swings it over the mousehole, stabs it into the waiting pipe joint, makes up the connection, then picks up the new joint and stabs it into the top of the drillstring at the rotary table.

Without the mousehole, the driller would need to retrieve pipe from the rack at the time of the connection, costing additional minutes per connection that accumulate significantly over a drilling programme. On a well making connections every 9-10 metres of drilling at a penetration rate of 30 metres per hour, connections occur every 18-20 minutes. Even a two-minute reduction in connection time saves approximately 90 minutes per 100 metres drilled — meaningful in terms of rig cost at USD 20,000-100,000 per day. The mousehole is therefore a simple operational efficiency tool whose value lies in the cumulative time savings across hundreds of connections over a well's drilling programme.

Mousehole Applications Across International Jurisdictions

In Canada, rig floor design requirements including mousehole and rathole dimensions are addressed in Alberta's Occupational Health and Safety regulations and Energy Safety Canada rig standards. WCSB drilling rigs are standardised for rotary drilling with kelly or top drive; land rigs operating in the Montney, Cardium, and Deep Basin formations make connections every 9.1 metres (30 feet, one joint of drillpipe). Modern top drive rigs that are now standard for horizontal well drilling in Canada do not use a mousehole for routine connections — the top drive's travelling block can pick up tubulars directly from the V-door without pre-staging in a mousehole.

In the United States, BSEE offshore platform drilling regulations address rig floor safety and equipment certification; mouseholes on jackup and semi-submersible rigs are inspected as part of the rig acceptance process before spud. Permian Basin land drilling uses a mix of kelly-drive and top-drive rigs; older kelly-drive rigs on lower-cost land programmes still use the mousehole for connection efficiency. In Norway, Equinor's NCS drilling operations are predominantly conducted on top-drive equipped semisubmersible and drillship rigs where the mousehole concept applies less directly; the rathole remains in use for storing the top drive sub during trips. In the Middle East, onshore Saudi Aramco drilling uses modern top-drive rigs for the majority of production drilling, with the traditional mousehole most relevant on older legacy rigs or rental units used in remote operations.

Fast Facts

The mousehole is typically fabricated from casing pipe set into the rig substructure at an angle approximately 3-5 degrees from vertical to allow easy pipe retrieval and reduce the chance of the pipe joint sliding back in. The mousehole casing extends below the rig floor approximately 9-12 metres (30-40 feet) — just enough to hold one standard drillpipe joint of 9.14 metres (30 feet) with the pin connection accessible above the rig floor. On a jackup or drillship, the mousehole is often hinged or located in a trough to the side of the rotary table to accommodate the confined deck space on an offshore drilling facility.

Mousehole Versus Rathole

The mousehole and rathole are two distinct rig floor fixtures that are frequently confused. The mousehole, as described above, is used to pre-stage the next joint of drillpipe during connections. The rathole is a separate hole below the rig floor, also lined with casing, used specifically to store the kelly and its subs (kelly cock, saver sub) when pipe is being tripped out of the hole. When the drill string is being pulled out of the hole on a trip, the kelly must be unscrewed from the drillstring and set aside so that the elevators can be latched around the drillpipe without the kelly in the way. The kelly is placed in the rathole for storage during the trip. On top drive rigs, the equivalent is placing the top drive crossover or pup joint in the rathole while the main body of the top drive is retracted to the top of its stroke. The rathole is slightly deeper than the mousehole because it must accommodate the kelly plus its subs.

Tip: When setting up a kelly-drive rig for efficient drilling operations, ensure the mousehole pipe is made up to the correct torque specifications before the joint is staged — the makeshift threads at the top of the mousehole joint must be fully made up to the kelly or it will spin off during the connection sequence. Also confirm the mousehole pipe joint has been visually inspected for thread damage and corrosion before staging; a damaged joint discovered at connection time delays operations more than the time saved by pre-staging. Assign a dedicated floor hand to the mousehole pre-staging task so that the next joint is always ready before the current kelly is drilled down to connection depth.

Mousehole is also referenced as:

Mouse hole — the two-word spelling variant; both spellings appear in industry documentation and rig design manuals; the single-word form is more common in recent usage
Pre-stage hole — a descriptive term used in rig operations training materials; emphasises the function (pre-staging the next connection) over the physical description
Rathole — frequently confused with mousehole in informal usage; the rathole is a separate rig floor fixture used for kelly storage during trips, not for connection pre-staging

Related terms: kelly, rotary table, drillpipe, connection, rathole

Frequently Asked Questions

Do top drive rigs use a mousehole?

Top drive rigs do not use a mousehole for routine drillpipe connections in the same way that kelly-drive rigs do. The top drive unit hangs from the travelling block and rotates the drillstring directly from above, without the need for a kelly that must be drilled down and then repositioned for each connection. On a top drive rig, single joints are added by picking them up with the top drive's integrated pipe handler directly from the V-door or from a singles tray on the drill floor, then stabbing the joint into the drillstring at the rig floor level. However, many top drive rigs retain a rathole for storing the top drive pup joint or crossover during trips, and some designs incorporate a mousehole-like storage position for the pup joint used in the drillstring makeup sequence. The fundamental concept of pre-staging the next piece of pipe for fast connection remains relevant even on top drive rigs, but the implementation differs.

What safety hazards are associated with the mousehole?

The primary safety hazards associated with the mousehole are dropped pipe and pinch points during handling. A joint of drillpipe staged in the mousehole is typically secured only by gravity; if the slips or the mousehole casing angle do not retain it securely, the joint can slide or fall back into the hole during the connection sequence, potentially striking floor hands working nearby. The connection itself involves manual handling of heavy tubulars with spinning connections under tension, creating pinch point hazards at the thread engagement. On offshore rigs where the mousehole may be located near the edge of the drill floor with limited guard rails, a dropped mousehole joint could fall to a lower deck or into the sea. Rig safety procedures require that floor hands maintain position awareness during mousehole connections and that the mousehole joint be secured or hand-braked before personnel work adjacent to it.

Why the Mousehole Matters in Oil and Gas

Drilling efficiency is measured in metres drilled per day, and every minute of flat time (time when the bit is not advancing) reduces that metric. Connection time — the time to stop drilling, make up a new joint of pipe, and resume drilling — is a major component of flat time on any drilling programme. Across a 3,000-metre well making hundreds of connections, even minor connection time improvements accumulate to hours of additional drilling time. The mousehole represents the evolution of rig floor design toward minimising these connection times through smart pre-positioning of equipment. While the mousehole is a simple and inexpensive fixture, its contribution to connection efficiency has been replicated and refined in modern top drive and automated rig designs where robotic pipe handling systems extend the mousehole concept to fully automated connection sequences with near-zero manual handling time — the direct operational descendent of the simple mousehole concept that has been standard on rotary drilling rigs for over a century.

Mousehole: Definition, Rig Floor Operations, and Drillpipe Connection Procedures