dart, Oil & Gas Glossary

A dart is a purpose-built projectile dropped or pumped through a tubing, drillpipe, or coiled tubing string to actuate a downhole tool by landing on an internal seat and creating a pressure differential across it. Functionally it is the elongated cousin of the drop ball: where a ball is a sphere that rolls to a spherical seat, a dart carries a shaped nose and sealing shoulder that centralize it as it travels and seat it squarely regardless of wellbore inclination, which matters in the horizontal laterals that dominate Montney and Duvernay development in the Western Canadian Sedimentary Basin. Once the dart occludes the seat, surface pumps raise pressure above the tool's shear or actuation rating, and the resulting force shifts a sleeve, opens a circulating port, sets a packer, releases a hydraulic disconnect, or fires a jetting sub. In coiled tubing work the dart must survive a punishing journey: it is injected into the surface plumbing, pumped through the tightly wound wraps still on the reel, forced around the gooseneck, and driven the full measured depth of the string before it ever reaches the tool, so modern metal or composite darts are engineered to resist premature shearing and extrusion under aggressive pump rates. This robustness is the reason darts have displaced balls in many coiled tubing applications, because a dart can be pumped hard through 5,000 metres of 60.3 mm coil without deforming, whereas an elastomer or phenolic ball may hang up in a wrap or shear early. Darts are central to plug-and-perf and ball-drop multistage fracturing, where a graduated series of increasing-diameter darts or balls sequentially isolates and treats each stage from toe to heel, and to milling, fishing, and stimulation strings that rely on a single dart to switch a tool between run-in and working modes. Material selection ranges from aluminum and cast iron through PEEK composites to fully dissolvable magnesium alloys that disappear after treatment so no post-job cleanout trip is needed, a decisive advantage on long horizontal wells where a coiled tubing drillout of dozens of seats would cost days of rig time and hundreds of thousands of Canadian dollars.

Key Takeaways

Seat-and-pressure actuation: A dart works by landing on a machined seat and blocking flow so pumped pressure builds a differential across it; that differential, typically 7,000 to 21,000 kPa (about 1,000 to 3,000 psi) depending on the tool's shear pin rating, shifts a sleeve or releases a mechanism. Without a competent seal the tool cannot be actuated, so dart-to-seat clearance and elastomer condition are checked before every run.
Dart versus drop ball: A ball is spherical and simple but can roll past a seat or hang in coiled tubing wraps; a dart carries a nose and sealing shoulder that self-centralize and seat reliably in deviated and horizontal wells. Churchill Drilling Tools and others commercialized robust extrudable darts specifically so they can be pumped aggressively through coil without shearing prematurely, improving tool cycling reliability.
Coiled tubing gauntlet: In coiled tubing operations the dart is pumped from surface through the reel wraps, around the gooseneck, and down the full string before reaching the tool. This demands high compressive and extrusion resistance, so operators specify metal or composite darts rated for the pump rate and coil diameter rather than soft balls that risk mid-string lodging.
Multistage isolation: Ball-drop and dart-drop completions treat multiple frac stages in sequence by dropping progressively larger diameters, each seating in its matched sliding sleeve to isolate the stage below. A single string can carry a dozen or more graduated seats, letting one coiled tubing or wireline intervention stimulate an entire lateral without individual plug sets.
Dissolvable materials: Magnesium-alloy and composite dissolvable darts degrade in wellbore brine and temperature after the job, eliminating the drillout trip needed to remove conventional seats. On a long Montney lateral this can save one to three days of coiled tubing milling and CAD 150,000 or more in rig, fluid, and disposal cost per well.

Dart Geometry and Sealing Under Aggressive Pump Rates

The working end of a dart is its sealing shoulder, an elastomer or engineered polymer band that swells against the seat under differential pressure. Nose profiles are tapered or ogive shaped to guide the dart into the seat bore and prevent cocking in a horizontal well where gravity offers no help. When a coiled tubing crew pumps a dart at 300 to 600 litres per minute, the dart accelerates through the string and arrives with momentum that must be absorbed by the shoulder without extruding through the seat gap. Undersized clearance improves sealing but risks the dart bridging early in a wrap; oversized clearance leaks and fails to build differential. Field engineers in the Deep Basin therefore match dart diameter to seat inner diameter within tight tolerances, often 0.5 to 1.5 mm, and confirm the dart's extrusion rating exceeds the planned treating pressure.

Ball-Drop Multistage Frac Sequencing

In a ball-drop or dart-drop multistage completion, the liner carries a series of sliding sleeves, each with a seat sized slightly larger than the one deployed downhole of it. The toe stage is opened first, often hydraulically, and treated; then the smallest dart is dropped to seat in the next sleeve up, isolating the treated zone and opening fresh perforations for the following stage. Each successive dart is a step larger, marching the treatment up the lateral. A 20-stage Montney liner might use seats ranging from 40 to 60 mm across. The method avoids running and setting individual bridge plugs, but the retained seats restrict the production bore, which is why dissolvable darts and full-bore drillout have become the preferred cleanup paths on high-rate WCSB gas wells.

Fast Facts

The drop-ball concept predates modern completions by decades: US patents for downhole ball-drop tools stretch back through the 1990s, and the mechanical extrusion darts that replaced many of them were commercialized to solve a specific coiled tubing headache, balls hanging up in the reel wraps and shearing before they ever reached the tool. Engineers found that a rigid, robust dart could be pumped far more aggressively than a ball, so a job that once required cautious low-rate pumping to coax a ball downhole could be cycled in a fraction of the time.

A dart cannot function without a matching sliding sleeve or seat, the tool it lands in to shift a port open or closed. Its most common cousin is the frac plug, an alternative isolation method in plug-and-perf completions where a dart-actuated setting tool places the plug. Darts are workhorses of coiled tubing interventions, where they actuate circulating subs and disconnects, and they enable multistage fracturing by sequentially isolating stages so an entire horizontal lateral can be stimulated in a single continuous pumping operation.

Real-World WCSB Scenario: Dissolvable Darts on a Kaybob Duvernay Lateral

An operator completing a 3,000 metre Duvernay lateral near Kaybob, Alberta, plans a 40-stage ball-drop liner and must choose between conventional composite seats and dissolvable magnesium darts. Conventional seats would leave 40 restrictions in the production bore, demanding a coiled tubing drillout estimated at CAD 220,000 across roughly three days of milling, fluid, and cuttings disposal, plus deferred gas production during the cleanout. The dissolvable option adds about CAD 90,000 in premium dart and seat cost up front but removes the drillout entirely, since the magnesium alloy degrades in the formation brine at bottomhole temperature within days of the frac.

The completions engineer selects dissolvable darts, banking a net saving near CAD 130,000 per well and returning the lateral to full-bore flow days sooner. Across a 12-well pad the deferred-production and rig-time savings compound into seven figures, and the operator standardizes dissolvable ball-drop systems for all subsequent Duvernay programs in the field.