Perforating Gun Types

• Explosives are categorized according:

– To the ease with which they are detonated– The speed with which they react– The combustion pressure that they generate

• Low explosives – low combustion pressures – react at velocities in the 500- to 1500-m/s range

Explosives used in Perforating

– Gunpowder is the most common example – reaction rate depends on the grain size,

temperature, degree of confinement, and packing density

• High explosives – 3000 to 9000 m/s and generate high pressures– subdivided into primary high explosives (or initiator

explosives) and secondary high explosives– Primary high explosives

• generally high-density compounds of metals and nitrogen• detonate when subjected to a heat source • purpose of primary explosives is to initiate the more

powerful and less sensitive secondary high explosives

– Secondary high Explosives

• highest reaction rate (7000 to 9000 m/s) and generate the highest pressures

• most secondary explosives are so insensitive to detonation that they can be melted, cast, and machined

• These explosives are generally organic compounds of nitrogen and oxygen - when a detonator initiates the breaking of the molecules' atomic bonds, the atoms of nitrogen lock together with much stronger bonds, releasing tremendous amounts of energy

– Typical secondary explosives include:

• RDX • HMX • PS (picryl sulfone) • HNS (hexanitrostilbene) • Composition B (60% RDX, 40% trinitrotoluene) • Amoniumnitrate (fertilizer, also used in seismic

operations)

• When a perforating gun is fired:– an electrical current sets off the primary explosive in the

blasting cap, which in turn sets off an RDX base or booster charge within the cap.

– This in turn detonates the Primacord (5 g of RDX per foot) used to carry the detonation to the individual charges.

– As the pressure wave reaches the shaped charge, it detonates fine-grained RDX primer in the tunnel at the base of the charge casing.

– Detonation of this primer in turn detonates the main body of RDX (generally with 1% wax binder) and the wave achieves full speed and pressure as it bears on the metallic liner.

– This pressure collapses the liner, forming the "jet" of high-speed material that strikes the target at high pressure and does the penetrating.

Perforating Gun Types

• Perforating guns are categorized:– retrievable steel hollow carrier guns– semiexpendable wire or strip carrier guns– fully expendable guns

• With the retrievable hollow carrier gun, the charges are positioned within a steel cylinder.

• Within the carrier, each charge is surrounded by air at surface pressure, and is aligned with a threaded port plug or a thinner portion of the carrier wall (scallop gun).

• Upon detonation, the jet pierces the plug, providing a positive indication of firing when the gun is retrieved.

• The carrier cylinder may expand slightly due to the explosive force, but most of the debris is recovered within the gun.

Steel hollow carrier guns: (a) threaded port plugs; (b) scallop gun).

Casing Guns

• Both retrievable and expendable guns are used for conventional casing perforation operations.

• Retrievable hollow carrier guns normally range from 3 1/8 in. (7.9 cm) up to 5 in. (12.7 cm).

• Since larger guns generally carry larger charges, and larger charges generally achieve larger, longer perforations, the largest casing gun that can be run is ordinarily used.

• Any shot density can be employed to minimize geometrical effects, with 4 shots per foot being a very common design.

• Perforating gun length is limited by the total perforating assembly length that must pass through the pressure control equipment at the surface.

Thru tubing gun

• Through-tubing guns have smaller diameters than casing guns to allow passage through tubing or small-diameter casing strings.

• The retrievable guns are slightly smaller than their expendable counterparts to permit them to be pulled back.

• The length of retrievable through-tubing guns is generally limited by the height of the pressure control riser — practically speaking, about 20 ft (6.1 m).

• The hollow steel carrier retrievable versions are preferred for deeper, higher-pressure wells.

• Because the perforating debris is minimized and retained within the carrier, retrievable guns are also preferred in situations where the well is to be flowed immediately after perforating, prior to gun retrieval.

• In such situations, semiexpendable carriers may become stuck as gun debris settles back to the bottom after a short flow period.

• In addition, if the well is perforated underbalanced and through tubing, the pressure differential must not be so great that it causes the carrier and/ or cable to be blown up the tubing after firing.

Casing Deformation Considerations

• Casing deformation or damage is caused by the extremely high pressures generated by the exploding charges.

• The degree of damage depends on (1) the gun characteristics and (2) the well conditions.

• Retrievable hollow carrier guns (of any size) do not produce casing damage

• This is because the steel carrier absorbs the energy of the explosion, as evidenced by carrier swelling.

• Expendable or semiexpendable guns can cause some degree of casing damage depending on the amount of explosive casing thickness and grade wellbore hydrostatic pressure degree of casing support (cement sheath

thickness and strength)

Perforation Productivity Considerations

• In a perforated completion there are basically four geometrical parameters that affect productivity.shot density (SPF or number of flowing perforations

per unit length); perforation depth (penetration into formation);gun phasing (angular displacement between

adjacent perforations); perforation diameter (within the formation).

• Together these parameters combine to create a distinctive flow geometry that creates a "skin" caused by the convergence of flow into the perforations.

• This skin is independent of permeability and can either enhance or impair performance relative to that of an openhole completion.

• (Drilling and perforating damage--generally k1>k2>k3>k4) shows that there are permeability reductions due to drilling damage and the crushing of the formation due to the perforating process.

• The total "skin" represents the combined effect of all these conditions (geometry, drilling damage, perforating damage).

Perforating for Gravel Packing

• In an ideal gravel-packed completion, the perforation tunnel is filled with a clean, evenly sorted, high-permeability sand or "gravel."

• The linear flow of formation fluids through the packed tunnel results in a substantial pressure drop.

• If the perforation diameter can be increased, the pressure drop is reduced, permitting greater flow.

• Gravel-pack completions are typically shot with large hole diameters (0.6 to 0.8 in. [15 to 20 mm]) and high densities (8 to 12 SPF [26 to 39 SPM])

• In a typical 7- to 9 5/8-in. (178- to 244-mm) casing gravel-packed completion, either a 5-in. (127-mm) or 6-in (l52-mm) gun is used.

• The importance of maximizing perforation diameter when perforating for gravel packing rearranges the priority listing established for perforating a "natural completion."

• In this case, the order becomes (1) shot density, (2) perf diameter, (3) gun phasing, and (4) penetration.

• As mentioned earlier, minimizing clearance is essential to maximizing penetration.

• In deviated holes, the effect of decentralized perforating (where the gun lies on the low side of the casing) on phased guns is to decrease the average perforation diameter.

• The alternative in these cases is to perforate using an oriented gun, which directs the perforations against the low side

Perforating for Fracturing

• Perforations are a key consideration in the design and execution of hydraulic fracture treatments.

• Bell et al. (1995) identify the following perforation parameters as particularly significant: casing entrance hole size effective shot density gun phasing character of perforations in the formation

• Casing entrance hole size: The friction pressure loss across perforations may be expressed as

pf=0.2369 qt2 o/n2d4Cd

2 where • pf=friction pressure in psi

• Cd=discharge coefficient (dimensionless) • d=casing entrance-hole diameter, inches • n=number of perforations • o=liquid density, lbm/ft3

• qt=total flow rate through perforations, bbl/min.

• At a given flow rate, smaller-diameter perforations give a significantly higher friction pressure loss than do larger diameter perforations.

• A larger frictional pressure loss requires a larger bottomhole pressure to initiate a fracture.

• Therefore, smaller perforations maximize the pressure loss at lower pump rates, minimizing the hydraulic horsepower requirements (and thus the cost).

• Effective shot density: Shot density requirements are based on maintaining reasonably low breakdown, treating and initial shut-in pressures. Typical treatments employ 4-8 shots per foot.

• Gun phasing: Regardless of perforation orientation, the far-field orientation of fractures depends entirely on formation stress charateristics.

• For typical fracture treatments, a phasing of 20o to 60o is recommended, while limited-entry treatments commonly use 0o shaped charge guns for placement of individually located perforations.

• Character of perforations in the formation: Because fractures are generally initiated from the base of perforations, depth of penetration is not as important a parameter as it is in an unfractured completion. An intermediate penetration of about 4 to 6 inches is generally adequate