Through-Tubing Gun | Oil Authority

A through-tubing gun (TTG) is a perforating gun assembly designed with an outer diameter small enough to be run through the bore of the existing production tubing string into the casing below, allowing new perforations to be shot in the casing and formation without the cost and time required to first pull the production tubing from the wellbore; through-tubing guns are used in workovers, recompletions, and production enhancement operations where additional perforations are needed in an already-completed well (to add a new producing interval, to restore production from a perforated interval where perforations have become plugged by scale, sand, or asphaltene, or to perforate below the current completion for a deeper zone evaluation), and the ability to run the gun through the tubing rather than pulling the string and running a conventional-sized casing gun saves the workover rig-up cost and time that would otherwise be required to pull, inspect, and re-run the tubing string; the outer diameter limitation imposed by the tubing bore (typically 1.5-2.5 inches for the gun body in a 2-3/8 to 3-1/2 inch production tubing string) constrains the charge type, charge size, and shot density achievable with a through-tubing gun relative to a full-bore casing gun of conventional size (typically 3-1/2 to 5 inch gun OD in the same casing), resulting in shorter penetrations, lower shot density, and reduced perforation efficiency compared to a conventional gun but avoiding the workover cost that would otherwise be required to run the conventional gun.

Key Takeaways

The through-tubing gun size selection — gun OD relative to the tubing ID and casing ID — is a critical engineering decision that governs both the operability of the gun run and the perforation performance achieved: the gun OD must be small enough to pass through the minimum ID restriction in the tubing string (which may be the tubing connector ID, the landing nipple ID, the gas lift mandrel ID, or any downhole safety valve ID — whichever is the tightest restriction encountered during the run); for a 2-7/8 inch tubing string (nominal OD) with a 2.44 inch minimum tubing connector ID, a through-tubing gun OD of 1.9-2.0 inches is typically the maximum that can be reliably run and retrieved through all tubing hardware; in a 5-1/2 inch casing (4.892 inch ID), a 1.9 inch gun runs in a casing annulus with approximately 1.5 inches of radial clearance, which positions the shaped charges significantly off-center from the casing wall and reduces the penetration efficiency compared to a casing-centered gun; the through-tubing gun operates in the eccentered (off-center) position in the casing because there is no mechanism to centralize it in the casing annulus, and the off-center position results in shallower penetration on the far side of the casing (where the gun-to-formation distance is maximized) and reduced penetration consistency compared to a centralized casing gun; perforating performance specifications for through-tubing guns (tested in API RP 19B test sections) reflect the off-center API test configuration and provide the penetration and hole diameter data used to predict productivity of the perforated completion in the specific gun-casing size combination.
Wireline-conveyed through-tubing guns offer the fastest and lowest-cost deployment but are limited in the depth and well deviation that can be practically achieved: the wireline unit runs the gun on conductor cable or braided line through the lubricator and wellhead into the tubing, and the gun's own weight and the lubricator hydraulic pressure control the descent through the tubing; in vertical or low-deviation wells (less than 45 degrees inclination), gravity provides adequate conveyance force and the gun can be run to any depth accessible through the tubing bore; in deviated or horizontal wells (greater than 60 degrees inclination), the wireline cable and gun string cannot gravity-convey to the desired depth because the horizontal run friction exceeds the downward component of the gun string weight, requiring coiled tubing or tractor conveyance for deep horizontal access; for high-deviation applications, coiled tubing-conveyed through-tubing guns (with the gun string attached to the end of the coiled tubing and pumped into the horizontal section with coiled tubing) extend the reach to 5,000-8,000 feet in horizontal laterals, and electric line tractor-conveyed guns extend further with the powered tractor pulling the gun through the tubing against friction; the choice between wireline, coiled tubing, and tractor conveyance for a specific through-tubing gun job depends on the well deviation profile, the target depth, the gun string length (which affects the conveyance force required), and the cost of each conveyance method for the specific well location and operating environment.
Differential underbalance perforating with through-tubing guns requires careful pressure management because the tubing provides both the flow path for underbalance fluid and the conveyance conduit for the gun: conventional underbalance perforating with a casing gun is achieved by setting the wellbore pressure (in the tubing-casing annulus or the open wellbore) below the formation pore pressure before firing the gun, so that formation fluid immediately flows into the wellbore and cleans the perforation tunnels of crushed zone debris after the shaped charges fire; with a through-tubing gun, the underbalance pressure must be established in the casing-tubing annulus (the space between the outside of the production tubing and the casing wall, below the production packer if a packer is set), because the formation is perforated in the casing rather than through an open wellbore; establishing underbalance in the casing-tubing annulus while maintaining wellbore control through the production tubing is more complex than open-wellbore underbalance perforating, and requires either a tubing-conveyed perforating (TCP) system where the guns hang below the production packer in the casing annulus (which is a conventional TCP job, not a through-tubing job), or an annular underbalance established by temporarily displacing the annulus fluid with a lighter fluid (gas or foam) before the perforating run, with the risk that this displacement operation itself requires a workover rig if the tubing cannot be produced up through the tubing without contaminating the underbalance fluid.
Gun retrieval from deep or deviated wells through the tubing after perforating is a primary operational risk in through-tubing gun operations, because a gun that cannot be retrieved blocks the production tubing and may require a workover to pull the tubing to fish the stuck gun: gun retrieval problems arise when the gun swells or distorts during detonation (detonation damage can change the gun OD by 0.05-0.15 inches for guns detonated in confined casing space without clearance for expansion), when gun components are mechanically damaged by wellbore debris or by contact with the tubing wall during retrieval, when the wireline becomes tangled or kinked at a deviation change in the wellbore, or when charge loading hardware that was supposed to drop off the gun before retrieval remains attached and increases the effective OD above the tubing bore; these risks are mitigated by selecting guns with adequate clearance between the nominal OD and the minimum tubing restriction (at least 0.1-0.15 inch), by using drop-off carrier designs that shed hardware after firing, by running the gun slowly through the tubing on the retrieval stroke to detect obstruction before applying full line tension, and by including a contingency plan (a washover or overshot tool on a backup coiled tubing unit) for each through-tubing gun job that could encounter retrieval difficulties; stuck gun fishing in a vertical well through the tubing (without pulling the tubing) is possible but technically demanding; in a horizontal well with the gun stuck in the lateral, the fishing operation may require a full workover to retrieve the obstructed completion string.
Through-tubing gun perforation performance is quantitatively inferior to equivalent-size casing guns run on tubing-conveyed perforating (TCP) assemblies, but the through-tubing approach is cost-effective when the completion already in the well does not need to be replaced and the incremental production from the new perforations justifies the intervention cost without requiring the full workover cost of tubing pull and replacement: the productivity ratio (actual well productivity index versus ideal open-hole productivity index) of a through-tubing gun completion depends on the penetration depth (which is limited by the charge size that fits in the small gun OD), the perforation density (limited by the minimum spacing of charges in the gun OD), and the damage to the crushed zone around each perforation tunnel (which is larger proportionally in through-tubing guns where the charge is smaller and less effective at breaking through the damaged zone); for tight formations (less than 1 millidarcy) where perforation penetration into the undamaged formation beyond the crushed zone is most important for well productivity, through-tubing guns may be inadequate and a conventional casing gun run after a workover may be required to achieve acceptable productivity; for high-permeability formations (greater than 50 millidarcy) where perforation skin is relatively small compared to the formation's natural deliverability, through-tubing guns provide adequate productivity at far lower cost than a full TCP completion.

Fast Facts

The through-tubing perforating concept developed in the 1960s and 1970s as the petroleum industry began managing large inventories of completed wells where additional perforations or recompletions were needed but the cost of pulling production tubing for each intervention was economically prohibitive in low-productivity wells. The original through-tubing guns were conventional perforating charges loaded into a small gun body that barely fit through the tubing — a mechanical engineering compromise between the minimum gun size needed to carry effective shaped charges and the maximum gun size permitted by the tubing bore. The development of specialized high-energy, small-diameter shaped charges (using more energetic explosive compounds and optimized charge geometry) in the 1980s and 1990s significantly improved the penetration efficiency of through-tubing guns, narrowing though not eliminating the performance gap relative to full-bore conventional casing guns and expanding the range of well conditions where through-tubing perforating provides acceptable productivity.

What Is a Through-Tubing Gun?

A through-tubing gun is a perforating gun small enough to fit inside the production tubing already in the well, so it can be run through the tubing bore and out the bottom into the casing below — adding new perforations without pulling the tubing. Pulling production tubing is expensive: a workover rig, a day or more of rig time, the cost to pull, inspect, and re-run the string, and deferred production while the well is shut in. For a well where only new perforations are needed — to add a zone, restore plugged shots, or evaluate a deeper interval — a through-tubing gun eliminates all of that cost. The tradeoff is performance: a gun that fits through 2.5 inches of tubing bore is a smaller gun than one run on tubing-conveyed perforating in the open casing, and smaller guns carry smaller charges that penetrate less deeply. For high-permeability formations where penetration depth matters less than for tight reservoirs, the performance tradeoff is acceptable and the cost savings are large. For tight formations where deep penetration past the crushed zone is critical for productivity, a full workover with a conventional gun may be the better economic choice despite the higher upfront cost. The through-tubing gun is the right tool when the well economics support the simpler intervention over the complete workover — and knowing when that is the case requires quantifying the perforation performance difference and comparing it to the workover cost differential.