[Editor's note: A version of this story appears in the May 2020 edition of E&P. Subscribe to the magazine here.]
Operators are on a never-ending quest to improve drilling and completions efficiency while reducing expenses and risks. In deepwater perforating, for example, it is common practice to drill a long rathole (up to 200 ft) after reaching total depth to accommodate the remnants of detonated perforating guns. Once the perforating operation is complete, the gun carriers are dropped into the rathole. Perforating guns are typically housed in a hollow steel carrier tube, which protects the charges and detonation system but also adds material and weight to the overall perforating string.
To save the costs and time associated with rathole drilling costs in deepwater wells, an operator worked with DynaEnergetics to develop an engineered perforating system. The new system operates without the hollow steel carrier and is designed to fragment or break into small pieces upon detonation of the perforating charges. Only the firing head and top connector remain for retrieval— the rest of the string falls to the bottom of the well into the rathole where it remains.
The fragmenting gun produces a debris pile that is only 15% of the length of the gun—vastly reducing the amount of drilled rathole required to accommodate gun debris while saving associated costs, time and well complexity. As an added benefit, the design permits larger charges to be used to deliver better penetration over traditional guns of similar diameter.
Early tests and field trials
The initial downhole test of the fragmenting system was completed in an abandoned well in West Texas in early 2017, which validated engineering concepts, demonstrated functionality of the system and helped refine operational procedures.
The first string was used in the Middle East region, where 14 m of guns were shot and fragmented in 7-in. casing. After firing the guns, multiple magnets were run to fish the debris for inspection. The debris was retrieved, and the remnant pieces proved that the gun fragmented as designed. Soon after, 30 m of the fragmenting gun were deployed on five separate runs through 3½-in. tubing in an onshore well using slickline. The job was a success, resulting in higher than expected well production.
Thru-tubing and remedial applications
While initially developed for offshore applications, the new fragmenting gun found new life in key onshore applications. Wells with production tubing can require compromises in charge load to run perforating guns. Pulling tubing prior to perforating can be cost-prohibitive, and running small-diameter perforating guns with tubing in place may not deliver enough charge-load for adequate perforation penetration. Even with smaller guns, the possibility of gun swell upon detonation raises the risk of safe gun retrieval. As a result, operators have traditionally made trade-offs—running technology that maximizes the charge size and penetration depth while lowering the risk of sticking guns from swelling carriers.
With the new fragmenting gun, the hollow steel carrier is removed, eliminating the risk of gun swell and other potential damage from charge detonation. Larger charges can be deployed successfully through the production tubing below a packer and out into the larger diameter casing below.
As an example, a typical U.S. land well may use 27/8- in. diameter production tubing that limits the options for traditional hollow steel carrier gun systems. These situations call for a small-diameter 19/16-in. perforating gun, restricting charge load to only 6 g. For the larger inside diameter casing that is below the tubing, this small charge load may be inadequate for good perforation penetration depth.
By running the fragmenting gun, larger loads, such as the 15-g deep-penetrating (DP) charge, can be deployed successfully through the production tubing for perforating the larger diameter casing. This approach can deliver outstanding perforating results without pulling production tubing, saving costs and downtime.
Reliable perforating in unconsolidated sands
In perforating guns that utilize hollow steel carriers, a volume of air at atmospheric pressure is trapped inside. Known as free-gun volume, the air inside the perforating gun is exposed to the extreme surrounding downhole environment upon detonation, creating a surge of pressure into the guns. In unconsolidated sands, the pressure surge also can bring in sand from the formation, increasing the risk of sticking the perforating guns downhole and raising the probability of a costly fishing job. The absence of the hollow steel carrier on the fragmenting gun eliminates the pressure surge caused by free-gun volume, reducing the risk of stuck guns caused by an influx of sand.
Improving perforating tunnel creation
The lack of free-gun volume caused by the absence of the hollow steel carrier also creates a phenomenon called dynamic overbalance. When charges are detonated downhole, an overbalance pressure surge occurs that can improve the creation of perforating tunnels. When coupled with high-shot density and high-charge weight, the lack of free-gun volume can be used to achieve deep tunnels, minimize the risk of rock failure and lower the resulting influx of sand and debris.
Absence of hollow carrier increases charge reliability and performance.
The fragmenting gun is equipped with larger charges and higher shot densities than conventional perforating systems with similar outside diameters. Because the exposed charges and the fragmenting process eliminates concerns from gun swell or carrier rupture, the likelihood of successful gun retrievability is improved.
The fragmenting gun can be used with multiple shaped-charge types (DP, good-hole, big-hole and DP energetic liner [e.g., DPEX]) in a wide range of applications and downhole conditions. For example, a 2¼-in. fragmenting gun is typically equipped with 15-g charges at a density of six shots per foot, a charge size normally reserved for traditional 27/8-in. guns with hollow steel carriers. The 4⅝-in. system utilizes different size charges: 25-g DP or DPEX, or 30-g big-hole charge. The larger system is available in three different shot densities of 12, 15 and 18 shots per foot.
Initially developed to reduce rathole drilling in high-cost deepwater wells, the fragmenting gun’s applications have been expanded. Engineering the gun system to operate reliably without a hollow steel carrier tube allows the gun to fragment as designed, but it also alters the detonation dynamics, improving fracture initiation and tunnel creation. Without the hollow steel carrier, the gun can easily pass through production tubing and deliver a preferred charge load typically found in larger diameter guns.
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