Expendable Gun | Oil Authority

An expendable gun in completion engineering is a perforating gun assembly designed to disintegrate upon detonation — the carrier components (the gun body, end caps, and structural elements that hold the shaped charges in position) are manufactured from materials (typically soft aluminum alloy, compressed powder metal, or brittle ceramic composites) that break into small fragments when the explosive train fires, rather than remaining intact as they do in conventional retrievable carrier guns — leaving only a small retrievable subassembly (typically the firing head and a short stub of connection hardware) while the fragmented gun body, now in pieces small enough to pass through the tight wellbore restrictions encountered in through-tubing applications, falls to the bottom of the well or is swept out with circulation; expendable guns are the preferred perforating system for wells where the wellbore restriction between the production tubing inner diameter and the casing perforations is too small to allow retrieval of a conventional gun assembly through the tubing, which would require either temporary tubing removal or use of an inherently smaller conventional gun that would sacrifice charge performance.

Key Takeaways

Through-tubing perforating constraints that drive expendable gun selection arise from the geometric relationship between the completion string and the casing: production tubing (typically 2-3/8 to 3-1/2 inch outer diameter) is set inside production casing (typically 5-1/2 to 7 inch inner diameter), leaving an annular restriction at the tubing hanger and packer that limits the outer diameter of any tool that must pass through the tubing to below the tubing drift inner diameter (approximately 1.90 to 2.99 inches for common tubing sizes); a retrievable gun assembly that fits through this restriction to reach the perforating zone below the packer would be too small to carry perforating charges capable of creating adequate entrance holes in the casing and formation penetration depths needed for production or injection; the expendable gun resolves this by allowing a gun body slightly larger than the tubing drift diameter (since it will not be retrieved through the tubing) while ensuring the disintegrated fragments are small enough to fall to the bottom without bridging or require only minimal circulation to remove.
Expendable gun materials must simultaneously provide structural integrity to hold charges in precise alignment during deployment and firing, thermal stability to survive wellbore temperatures (up to 150°C or higher in some HPHT wells) without premature degradation, adequate fragmentation on firing to produce only small debris that does not create a fish in the wellbore, and chemical inertness to wellbore fluids; aluminum alloys are the most common expendable gun carrier material because they are lightweight, easily machined, structurally adequate for the deployment loads encountered in through-tubing applications, and fragment reliably into small pieces on detonation; magnesium alloys have also been used because their lower density further reduces running weight and their rapid corrosion in brine creates a self-disposing carrier that dissolves before completion of the workover program even if fragmentation is incomplete; ceramic and glass carrier materials have been used for wells where metallic debris is particularly problematic, as the ceramic fragments are inert and can be dissolved by acid treatments used in subsequent stimulation operations.
Charge performance in expendable guns is limited by the smaller carrier size compared to conventional retrievable guns of the same deployment configuration — a through-tubing expendable gun operating inside 5-1/2 inch casing through 3-1/2 inch production tubing (gun outer diameter limited to approximately 2.6 to 3.0 inches) carries smaller shaped charges than a conventional 3-3/8 to 4-inch retrievable gun that would be run in the same casing without through-tubing restrictions; smaller charges produce smaller entrance holes and shorter perforation tunnels, reducing the productivity per perforation compared to large-bore conventional guns; however, the ability to perforate new zones in a producing well without pulling tubing has significant economic value — avoiding a workover string pull saves the cost of the workover rig time (typically $50,000 to $500,000 per day for offshore wells), so the productivity sacrifice of the smaller expendable gun charges is routinely justified by the operational cost savings.
Deployment systems for expendable guns include slickline (a single, smooth, non-electric wireline used for mechanical operations), electric wireline (a multi-conductor cable allowing electrical firing signals), coiled tubing (a continuous flexible tubing that can be pumped or run into live wells with tubing in place), and tubing-conveyed operations (the gun string is run on the production tubing itself and fired by either wireline or drop-bar mechanical firing mechanisms); wireline through-tubing deployment is the most common for simple perforating operations in producing wells, allowing the gun to be positioned opposite the target interval using the casing collar locator (CCL) log, fired electrically from the surface, and confirmed detonated by tension change on the wireline before the firing head is retrieved, leaving the disintegrated gun body in the wellbore.
Post-detonation wellbore cleanup after expendable gun firing requires verifying that the gun fragments have not bridged across the wellbore in a configuration that would restrict production flow or prevent future tool passage — a gauge run (passing a calibration tool of the planned completion inner diameter through the perforated interval) confirms that the fragments are lying on the low side of the wellbore or have fallen to below the producing perforations, and if bridging is detected, a bullheading operation (pumping fluid down the tubing at sufficient rate to circulate the fragments out) or a mechanical jetting tool (to break up any bridge) is used to restore the wellbore to an adequate clear bore; fragment accumulation at the bottom of the perforated interval is generally acceptable and does not significantly affect the production from the perforations, though extensive fragment accumulation in a horizontal well may require periodic bailer operations to remove the debris from the low side of the lateral.

Fast Facts

Expendable perforating guns were developed in the late 1960s and early 1970s as through-tubing well intervention became increasingly common in maturing oil fields where reperforating new zones or deepening perforations in existing wells without costly workover string operations was economically attractive. Early expendable guns used cast iron and soft steel gun bodies that broke on detonation, but these materials created larger, heavier fragments that were more likely to bridge in the wellbore than the aluminum alloy and powder metal designs developed subsequently. The introduction of the oil industry's first systematic API performance testing program for expendable guns (part of API RP 19B) established minimum fragmentation standards — requiring that fragments be below a specified maximum dimension to prevent wellbore bridging — that drove the adoption of purpose-engineered aluminum alloy gun body designs that fragment reliably to API-specified maximum dimensions across the full range of downhole temperature and pressure conditions encountered in through-tubing applications.

What Is an Expendable Gun?

Well intervention — going back into a producing well to perforate a new zone, deepen the existing perforations, or establish communication with a bypassed interval — is a routine part of field management in mature oil and gas producing fields. In wells with the production tubing still in place, the path to the perforating target runs through the tubing string, whose inner diameter is often too small to allow retrieval of a conventional perforating gun assembly that is large enough to carry effective shaped charges.

The expendable gun solves this constraint by changing the question from "how do we retrieve the gun through the tubing?" to "how do we make the gun harmlessly disappear after firing?" By manufacturing the gun carrier from materials that disintegrate on detonation, the expendable gun design eliminates the retrieval requirement entirely. The charges fire, the casing and formation are perforated, and the gun body breaks into small fragments that fall to the bottom of the well or are swept away by production. The only piece retrieved is the firing head above the tubing restriction — a simple tubular connector that passes through the tubing bore without difficulty.

The cost of this elegance is smaller charge performance compared to a full-bore conventional gun, since the expendable gun's outer diameter is constrained by the requirement to pass through the tubing. For the typical through-tubing workover, this trade-off is almost always economically favorable: avoiding the rig time, cost, and production deferment of pulling the completion string to accommodate a conventional gun is worth a modest reduction in perforation quality per shot.

Expendable Gun Design and Performance Optimization

Phase angle and shot density selection for through-tubing expendable guns follows the same principles as conventional guns but is constrained by the smaller carrier size — the number of shots per foot (shot density) that can be accommodated in a smaller-diameter carrier is lower than in a full-bore conventional gun, and the phase angle (the rotational offset between successive charges) must be designed within the structural constraints of the disintegrating carrier; typical expendable through-tubing guns fire 4 to 6 shots per foot at 60-to-90 degree phase angles, compared to the 4 to 12 shots per foot at 60 to 120 degree angles achievable in large conventional guns; the lower shot density and smaller individual charges require careful hydraulic design of the perforation program to ensure that the reduced perforation flow area is adequate for the required injection or production rates, using the perforation friction equation to verify that expected rates can be achieved without excessive pressure drop across the perforations.

High explosive selection for expendable gun charges must account for the higher temperature exposure that may occur if the gun is held at temperature for extended periods before firing — conventional retrievable guns are typically deployed and fired relatively quickly, while expendable guns used in through-tubing squeeze cementing or slow workover operations may be held at temperature for hours before the firing signal is sent; HMX explosive (high-melting explosive, stable to approximately 150°C for short exposure and to approximately 175°C for brief exposure) is standard for most expendable gun applications, with HNS (hexanitrostilbene) or PYX (used for temperatures above 175°C) specified for HPHT wells where the temperature exposure before firing exceeds HMX stability limits.

Expendable Gun Across International Jurisdictions

Canada (AER / WCSB): WCSB through-tubing perforating operations use expendable guns for reperforating existing producers in the oil sands thermal recovery pilots (CSS and SAGD wells where new perforation intervals are added during steam operations), for deepening perforations in Mannville formation producers, and for cased-hole formation evaluation (setting packers and perforating test intervals) in wells where tubing cannot be economically pulled; AER's well completion reporting requirements include perforating gun type (expendable versus conventional) and the post-perforation cleanup procedure as components of the well intervention record submitted for regulated well operations; Canadian perforating service providers including SLB Canada, Halliburton Canada, and BJ Energy Solutions maintain inventories of expendable gun systems in standard through-tubing sizes (1-1/2 to 2-5/8 inch outer diameter) for rapid deployment on WCSB workover programs.

United States (API / BSEE): API RP 19B provides the standardized performance testing framework for expendable guns including fragmentation tests that verify gun body disintegration into pieces below specified maximum dimensions under simulated downhole conditions; BSEE GoM offshore regulations require that perforating programs be documented in the well intervention report, with through-tubing operations (including expendable gun programs) subject to the same blowout prevention requirements as initial well completion perforating; the mature GoM shelf fields (Eugene Island, West Cameron, Ship Shoal, South Timbalier) with thousands of wells in various stages of depletion use expendable through-tubing guns extensively for zone reperforating and infill shooting in older completions without pulling the production tubing, a practice that would cost two to five times the expendable gun program cost on offshore platforms with high daily rates.