Burn Over

A burn over is a fishing operation in which a specialized milling tool called a burn shoe or burn mill is run over the outside diameter of a stuck downhole tool (the fish) to cut away its exterior, creating a profile that a releasing or retrieval tool can then engage. The term "burn" refers to the early oilfield practice of using a ring-shaped cutting tool literally burned or torched around the outside of a stuck fish in shallow wells. In modern practice, the burn shoe is a hollow cylindrical mill with tungsten carbide or natural diamond cutting faces that grinds away the outer steel of the fish as the string is rotated, allowing a new engagement point to be cut into the fish exterior or interior so that retrieval can proceed.

Key Takeaways

A burn over is chosen when conventional releasing mechanisms have failed and the geometry of the fish prevents a standard overshot or spear from engaging. The burn shoe creates new geometry on the fish by removing its outer surface, exposing an interior collet groove, packer element, or slip detail that a retrieval tool can then latch onto. In cases where the fish cannot be retrieved, a burn over can also mill a window in the fish exterior to establish a circulation path through the stuck assembly.
The burn shoe must be sized precisely to the outside diameter of the fish. It should have minimal clearance from the fish (2 to 4 millimetres) so the carbide teeth engage the fish exterior uniformly around the circumference rather than passing over it. An oversized burn shoe will not cut and an undersized one will not pass over the fish.
Weight on bit and rotary speed during a burn over are controlled carefully to avoid damaging the casing or open hole. A burn shoe pressed too hard against a fish in open hole can deviate off the fish and start cutting the formation, which complicates the wellbore geometry and can cause the string to stick above the fish. Weight must be maintained within tight bounds, typically 5 to 15 kilonewtons for a 4-1/2 inch burn shoe.
Progress is measured by the depth advancement of the burn shoe per hour of milling. Steel mills at approximately 0.3 to 1.5 metres per hour depending on the hardness of the fish, the shoe design, and weight and RPM. If the mill is not advancing, the fish may be harder material than expected, the shoe may have lost its carbide cutting elements, or the shoe may not be properly engaged on the fish.
A burn over may be run in combination with jarring. The jar is used to work the string and assist in releasing the fish after the burn over has created the necessary engagement profile. Burning over 0.5 to 1.0 metres of fish exterior may be sufficient to expose the engagement point, after which jarring can complete the retrieval.

What Is a Burn Over and When Is It Needed?

Picture a jar lid that is stuck. You can try to grip it with your hand (the equivalent of a standard overshot or spear). You can try to break the seal by tapping around the edge (the equivalent of jarring). If neither works, you might resort to puncturing the lid with an awl to release the vacuum (the equivalent of opening a circulation path by milling). A burn over is the most invasive version of this: you grind the outside of the lid until you can reach a different mechanical feature that you can grab.

In the wellbore, a burn over is needed when a permanent downhole tool (a packer, a bridge plug, a liner hanger, or a stuck valve) cannot be released by its normal mechanism, and the exterior geometry of the tool does not allow any available overshot to engage. The reason might be that the tool is set in the wellbore at an angle, that scale or cement has built up around the exterior, or that the engagement neck of the tool has been damaged during previous retrieval attempts. The burn shoe gives the fishing team the ability to create a new engagement point where none currently exists.

In Alberta and British Columbia, burn overs are most commonly required on set packers that failed to release after mechanical shear or hydraulic release attempts. A hydraulic hold-down packer set in a horizontal Montney wellbore at 3,500 metres has several tonnes of holding force from the formation pressing the slips into the casing wall. If the shear pin fails to release cleanly or if the hydraulic release mechanism corrodes, the packer cannot be unset without milling. The burn over gives the team a chance to retrieve the packer body rather than having to mill it entirely to junk and remove the junk with a magnet and junk basket.

Fast Facts

The original "burning over" technique, used in the early US and Canadian oil patch before World War II, literally used an acetylene or oxygen torch to cut a ring around the outside of a stuck fish in wells shallow enough to access with hand tools or light service equipment. The cut ring allowed the fish exterior to be peeled back, exposing a catching point for a hook or chain that could then be used to pull the fish. The transition to tungsten carbide milling shoes occurred as well depths exceeded 300 to 400 metres in the 1940s and 1950s, where torch cutting was no longer feasible. The term "burn over" remained in the vocabulary even as the tool changed completely, a common occurrence in oilfield terminology where historical practice names outlive the practice itself.

Burn Over Tool Design and Selection

Modern burn shoes are available in inside diameter sizes matched to standard casing and tubing outside diameters across the range of common downhole tool sizes: 2-3/8 inch through 9-5/8 inch casing. The shoe body is a hollow cylinder of alloy steel, typically H₂S-resistant if sour service is expected. The cutting face is dressed with milled tungsten carbide (TC) matrix or with natural diamond or polycrystalline diamond compact (PDC) cutters depending on the expected hardness of the fish material.

For standard carbon steel fish, a TC matrix shoe is sufficient. For harder alloy steels (such as in slick joint or production casing with higher tensile rating), a natural diamond or PDC shoe is preferred. For tools made from nickel alloys or chrome steels (which may be encountered when burning over premium completions equipment), diamond cutting faces are necessary because TC cannot efficiently cut these materials.

The shoe must also be designed for the circulation path. Junk slots cut into the shoe face allow the milled steel (swarf) to be flushed out of the annulus between the shoe and the fish by drilling fluid circulation. Without adequate circulation, swarf packs around the shoe, halting the milling operation. A minimum circulation rate of 200 to 400 litres per minute is typically required during burn over operations in vertical wells; horizontal burn overs require higher rates to move the swarf updip against gravity.

The Fishing Sequence Before a Burn Over

A burn over is not the first option in a fishing sequence. The sequence typically follows escalating invasiveness.

First, attempt mechanical release with jars (straight pull, accelerator jars, or hydraulic jars). A set packer with a shearable release mechanism can sometimes be released by overpulling the string beyond the release force while jarring. If mechanical release is achieved, the fish is retrieved intact and the wellbore is undamaged.

Second, attempt hydraulic release if the tool has a hydraulic release mechanism. Apply pressure in the tubing or annulus as specified by the packer manufacturer. If the release mechanism works, the fish releases and can be pulled.

Third, attempt an overshot or spear if the fish has an accessible fishing neck or bore. An overshot engages the outside of the fish; a spear engages the inside bore. Both tools are retrievable (the operator can pull them back if they do not engage correctly) and leave the fish geometry intact.

Only when all of these approaches have failed is a burn over justified. The burn over modifies the fish irreversibly: once the exterior has been milled, the original overshot and spear engagement geometry may no longer be available. Choosing to burn over is a commitment to a more complex fishing job.

A burn over operation is also called a burn mill job or a swage mill job. The tool is called a burn shoe, burn mill, or milling shoe depending on the service company's terminology. Related terms include fishing (the process of recovering stuck or dropped equipment from the wellbore; burn over is one of the more advanced fishing techniques in the escalating sequence of retrieval attempts), mill (a downhole cutting tool with hard-faced carbide or diamond surfaces used to dress or remove metal obstructions in the wellbore; a burn shoe is a type of mill designed for the specific geometry of burning over a fish), overshot (a fishing tool that engages the outside of a fish by extending a grapple or bowl over it; the preferred tool before committing to a burn over because it leaves the fish geometry intact and is retrievable if it does not engage), packer (a downhole sealing tool that isolates zones in the wellbore; set packers that fail to release are one of the most common subjects of burn over fishing jobs in Alberta completions), and sidetrack (the drilling of a new wellbore section to bypass a stuck or lost fish; the alternative to completing a successful fishing job; a burn over is often the last attempt before the decision to sidetrack is made).

When a Burn Over Saved a Deep Basin Packer Retrieval and Avoided a CAD 3.4 Million Sidetrack

A completion crew had set a retrievable packer in a Falher-C well in the Deep Basin of west-central Alberta at a depth of 3,280 metres during a refracturing operation. After the frac treatment, the crew attempted to unset the packer by pulling up on the string to shear the release pins. The pins did not shear at the expected 180 kilonewtons pull force. The crew increased pull to 240 kilonewtons; the pins still did not release. A third attempt at 280 kilonewtons pulled the string free but without the packer: the overshot had parted from the work string at the release connection, and the packer and overshot assembly were now stuck in the wellbore as a combined fish.

The fish assembly had an outside diameter of 114 millimetres. No available overshot could latch onto it because the overshot body was now part of the fish and had damaged the fishing neck profile in the parting event. A spear could not engage the bore because a ball-actuated release device inside the overshot body was blocking the bore. After two days of unsuccessful jarring, the well service engineer recommended a burn over to mill the top 0.6 metres of the overshot body, exposing a catch groove in the packer top sub that a new overshot could then engage.

A 4-1/2 inch burn shoe with PDC cutters was run on a 7.3 centimetre OD drill string. After 3.1 hours of milling at 45 RPM and 8 kilonewtons weight, the shoe had advanced 0.65 metres into the overshot body, exposing the packer top sub groove. A replacement overshot was run and successfully engaged the groove on the first run. The fish was recovered intact.

Total fishing time was 4.2 days. Cost of the burn over equipment, fishing string, and daily rig time was CAD 290,000. The alternative, directional sidetracking to bypass the fish, had been quoted at CAD 3.4 million including casing and completion costs for the new wellbore. The burn over decision saved CAD 3.1 million.