Rathole: Junk Storage, Perforating Gun Debris, and Rig-Floor Kelly Storage
A rathole is an extra length of hole drilled below the deepest interval of interest so that expendable downhole equipment has somewhere to go after it has done its job, most commonly the spent carriers from perforating gun assemblies, drift rabbits, broken cones, and other small junk that would otherwise sit across or above the pay zone. When a wireline or tubing-conveyed perforating string fires, the depleted gun carriers and any debris drop to the bottom of the wellbore; if that bottom coincides with the top of the producing sand, the junk can mask perforations, snag later completion strings, or interfere with a packer set. By drilling the wellbore a few metres deeper than the lowest perforation, the operator creates a sump that swallows this material and keeps the production interval clean, which is exactly the sense captured in the source definition: an extra hole drilled at the bottom of the hole to leave expendable completion equipment such as the carriers for perforating gun charges. The word rathole carries a second, entirely separate meaning that every rig hand knows, namely the cased, near-vertical hole bored into the rig substructure beside the rotary table that holds the kelly and kelly bushing when they are not in the string during a connection or trip; alongside it sits the shallower mousehole that parks the next joint of drillpipe. Both senses share the idea of a short auxiliary hole that exists to hold something out of the way, but the downhole completion rathole and the rig-floor rathole are unrelated pieces of the operation and should never be confused. In Western Canadian Sedimentary Basin practice, completion ratholes are routine on cased-hole completions in the Cardium, Viking, and Montney where perforating guns are run on wireline; a typical sump is 5 to 15 m (16 to 49 ft) of additional drilled depth, sometimes more in deviated or horizontal wells where the toe must accommodate longer tubing-conveyed perforating assemblies. The depth chosen balances the cost of drilling and cementing extra hole against the operational risk of a congested wellbore. On the surface side, a rig-floor rathole is typically 10 to 12 m (33 to 39 ft) deep, lined with surplus casing, and drilled by a small rathole rig or auger truck during lease preparation before the main drilling rig arrives, work that in Alberta falls under standard well-servicing and site-preparation practice rather than the AER well licence drilling phase itself.
Key Takeaways
- Two distinct meanings: A rathole is either a short sump drilled below the pay to hold spent perforating gun carriers and junk, or the cased hole in the rig floor that stores the kelly during connections. The completion sense and the rig-floor sense are unrelated; context decides which applies. The source definition refers specifically to the downhole junk-storage sump.
- Keeps the pay zone clean: Firing a perforating gun drops depleted carriers and debris to the wellbore bottom. A 5 to 15 m (16 to 49 ft) rathole below the lowest perforation gives that material somewhere to fall, preventing it from covering perforations, snagging a packer, or obstructing a later tubing string in WCSB Cardium and Viking completions.
- Rig-floor version drilled first: The kelly rathole and the adjacent mousehole are bored during lease preparation by a small truck-mounted rathole rig, usually 10 to 12 m (33 to 39 ft) deep and lined with surplus casing, before the primary rig moves onto location. This work is governed by Alberta well-servicing and site-prep rules.
- Top-drive rigs changed the rig-floor need: Conventional kelly-and-rotary rigs require a rathole to park the kelly; top-drive rigs eliminate the kelly, so many modern WCSB pad rigs no longer drill a kelly rathole, though a mousehole for staging pipe is often still cut.
- Cost is small but planned: Drilling 10 to 15 m of extra hole for a completion sump adds only modest rig time and cement volume, on the order of a few thousand CAD, but it must be designed into the well plan and casing depth up front, since deepening total depth after casing is set is impractical.
Completion Rathole as a Perforating Debris Sump
On a cased-hole completion, the perforating program drives the rathole length. A WCSB operator perforating a 12 m Cardium interval with a tubing-conveyed gun string might run 18 to 24 m of gun assembly below the packer; after firing, the entire spent carrier drops to bottom. If total depth sits only a metre below the lowest shot, the carrier can stand back up into the perforated interval. Designing 15 to 20 m of rathole below the deepest perforation guarantees the carrier clears the pay. The same logic applies to drift rabbits, junk baskets, and bridge plugs that are later milled, since the cuttings settle into the sump rather than packing off across the producing sand and choking inflow.
Rig-Floor Rathole and the Kelly Connection Cycle
On a conventional rotary rig the rathole holds the kelly and kelly bushing while the crew makes a connection. After drilling down a joint, the driller picks up off bottom, sets the kelly into the rathole, breaks it out, then uses the elevators to pick up the next stand. The mousehole, drilled shallower and closer to the well centre, holds the single joint that gets added to the string. A small rathole rig drills both before the main rig arrives, typically reaching 10 to 12 m (33 to 39 ft) for the kelly hole. Top-drive rigs, now standard on Montney and Duvernay pad operations, remove the kelly entirely and often skip the kelly rathole.
Fast Facts
The dedicated rathole rig is one of the oldest specialized units in the oilfield, predating most modern drilling automation. These small truck-mounted or auger rigs would arrive on a fresh lease days before the main rig, drill the conductor hole, kelly rathole, and mousehole, then move on, sometimes drilling ratholes for three or four locations in a single day. In Alberta they remain a distinct WCB rate-classified industry, separate from primary drilling, a reflection of how routine and high-volume this small-bore preparatory work has always been across the basin.
Related Terms
A rathole only makes sense alongside the equipment it serves. Perforating generates the spent gun carriers a completion rathole is meant to swallow, while a packer set just above the pay relies on a clean wellbore below it to seat properly. The kelly is the very component the rig-floor rathole parks during connections, and a mousehole is its shallower companion that stages the next joint of drillpipe, making the two surface holes a matched pair on conventional rotary rigs.
Real-World WCSB Scenario: Cardium Recompletion near Pembina
An operator recompleting a vertical Cardium well in the Pembina field planned to add lower perforations across a 9 m sand at roughly 1,650 m. The original well had been drilled to a total depth only 2 m below the proposed lowest shot. Before perforating, the completions engineer flagged that the 6 m tubing-conveyed gun carrier would land back inside the new perforations after firing. Rather than risk masking the pay, the team ran a small workover rig to clean out and drill an additional 12 m of rathole at roughly CAD 1,800 per metre including cement and rig time, about CAD 22,000 total.
With the deepened sump in place, the perforating run fired cleanly and the spent carrier dropped well below the new perforations. The well returned to production at the expected rate with no debris interference, and the modest rathole cost was recovered within the first weeks of restored Cardium oil production.