Perforating methods

Bullet gun, abrasive, water jets, and shaped charges are perforating methods used to initiate a hole from the wellbore through the casing and any cement sheath into the producing zone.

Bullet gun perforating

Projectiles from these guns (bullets) must penetrate the following:

• Casing
• Cement
• Formation

Bullet speed exiting the barrel is usually approximately 900 m/s (3000 ft/sec). Penetration is easiest in low alloy, thinner walled pipe [H-40, to K-55, and L-80 American Petroleum Institute (API) casing series pipe grades]. Penetration in higher strength casing alloy pipe and harder formations is more difficult in most cases and not feasible in others. When successful, the bullet creates a very round entrance hole but may often create a hole with sharp internal burrs. Fig. 1 shows a bullet-perforated casing from a surface test.

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  Fig. 1—Bullet-perforated casing from a surface test.

Tunnel length creation with a bullet gun drops sharply with increasing formation strength. Penetration extremes of 15 in. –/+ in soft chalks to 2 to 3 in. in dolomites are common. In contrast to shaped-charge perforating, however, bullets often shatter the rock rather than smoothly push back and compact the rock in their path. The shattering can be a definite advantage when the cracking improves the permeability next to the perforation.

Bullet penetration is primarily a function of the density and strength of the target in its path, as well as gun performance factors. The energy from the bullet is proportional to its mass, the amount of propellant, and the performance of the seal between the bullet and the barrel. Early performance-measuring tests with bullet guns showed a direct correlation between the penetration in a target and the use of new gun barrels. Performance dropped sharply with barrel enlargement and/or wear.

Entrance hole roundness and the pronounced shattering around the perforation tunnel help improve stimulation through bullet-perforated completions. Perforation ball sealers seal quickly and efficiently on bullet perforations. This is partly because of the additional brittle cracking of the formation (increasing permeability), the ability of the ball sealer to create a seal on the perforation, and the reduced number of bullet perforators normally used in a well. The reduced number of bullet perforators used in a well is an indication that a large number of perforations spreads out the flow entering the formation, resulting in a lower flow rate into a given perforation and less tendency to attract and seat a ball sealer.

Advantages of bullet perforating are:

• High permeability connection to the immediate reservoir
• Very controlled hole size and shape

Disadvantages are:

• Shallow penetration
• Ineffectiveness in hard formations and high alloy or heavy pipe
• Leaving a solid mass of steel in the perforation tunnel
• Low density perforating

Abrasive perforating methods

Abrasive perforating methods use high-volume flow of abrasive-laden fluid to erode through the target pipe or cut it off when the nozzle or tubing string is rotated. Abrasive impingement of hard particles such as sand on steel can cut through 0.25 to 0.3 in. of casing in a matter of minutes. Perforations in the casing or even 15 × 1.2 cm (6 in. × 0.5 in.) slots can be formed within 10 to 20 minutes per slot (hole). Abrasive methods often use a shaped nozzle that focuses the stream on the steel surface. The nozzle helps preserve energy, shorten cutting time, and decrease the effect of clearance distance, but the nozzle wears with use. Clearance distance between the nozzle and the target is important but not as critical as in nonsolids jet cutting.

Perforation depths formed by abrasives are typically short because the returning fluid and solids interfere with the ability of high-pressure fluids to access deeper targets. Depths of 2.54 to 23 cm (1 to more than 8 in.) have been measured in tests performed with backpressure. Abrasive perforating or cutting in surface targets often produces quicker cutting and may achieve deeper perforation depth, but these tests are not a valid representation of tool performance in a well. Adding backpressure on any type of a jetting tool rapidly diminishes its performance because of the reduction of pressure and flow velocity across the nozzle and collapse of bubbles and cavitation that may occur exiting the nozzle on a low-pressure test. Required equipment includes a rig with tubing large enough for the required rate with minimal friction drop. A fixed nozzle for perforating or a rotating nozzle for abrasive cutoff is the main bottomhole assembly.

The type of abrasive varies with the job, but sand is the most common material for perforating and pipe cutoff. Other abrasives can be used, such as calcium carbonate, soda glass, and other mineral and synthetic materials.

There are some differences in the cutting efficiency of materials. The quickest cutters are harder, more angular materials. Bauxite is the single most erosive material in the abrasive process but is used rarely because of its density and cost.

Liquid selection is less important and usually is dictated by the damage potential to the formation. Because some fluid is lost to the formation in any jetting job, the damage aspect of the carrier fluid should be investigated. The ability of the fluid to lift solids usually is limited to lifting the sand abrasive. The steel removed in the process is too fine to cause significant plugging problems.

Advantages of abrasive-perforating methods include the ability to make perforations with maximum flow area and with minimum damage to the formation or to the integrity of the steel pipe. The perforations are shallow in most cases, limited by the backwash of returning fluids, but are notably undamaged in most tests. The best applications have been in heavy oil completions in which large inflow area in the pipe is a necessity and pipe cutoff is advantageous. Disadvantages of these methods are the time needed to create each perforation, the amount of equipment required (coiled tubing or small tubing), the need to kill or control the well while creating the perforation, and solids cleanup.

Water jets

Although water jets with pressure impact on the order of 20,000 psi are used as steel cutters in surface applications, they usually are not effective downhole when the backpressure is more than approximately 1,500 psi. The use of these tools is limited sharply by friction pressure drop in the small diameter, long-tubing strings used to supply fluid to the point of cutting. Water jets have been used to create perforation tunnels in openhole completions. A special adaptation of the water jet used a hydraulic punch to open a “door” in the casing through which a flexible water-jet lance was fed to extend a long perforation into the rock. With few exceptions, water-jet perforating is a special application.

Shaped charges

The shaped charge or “jet” perforator uses a small amount of high explosive and a carefully shaped case and liner to create a focused pressure punch that is highly effective in piercing steel, cement, and rock. The jet is formed through a highly critical, but usually reliable, sequence of events. The sequence begins with the firing of the initiator or detonator cap, which ignites the detonation cord at high energy, followed by the initiation of the charges. The entire sequence of the explosive event must be carried out in high order. Failure to achieve or maintain high-order firing at any point in the explosive sequence will cause all subsequent explosive to initiate low order and burn with very slow energy release.

Fig. 2 shows the components of a shaped-charge perforator. Fig. 3 contains an X-ray of a 20-g, steel-cased charge that shows the detail assembly necessary for these charges. The charge case holds the explosive powder and focuses the firing explosive event. The primer area usually holds a small amount of slightly destabilized, secondary high explosive. The primer initiates the main explosive in the charge. As the explosive front moves through the charge, it strikes the apex of the liner deforming the liner and fluidizing part of its mass into a focused jet that punches a hole through the material in its path. Fig. 4 shows a jet formation from a shaped charge. As the jet forms, it stretches out with the jet tip approaching speeds of 6100 m/s (21,000 ft/sec), and the tail of the jet traveling at approximately 3,000 m/s (11,000 ft/sec). For illustration, several unusual targets have been used to capture jet performance with high-speed cameras. In one of the most unusual experiments, the path of a jet through both sides of a crystal wine goblet was captured on ultra-high-speed film and shows full jet development before the goblet shattered. In effect, the hole is placed before the target “knows” that it has been hit.

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  Fig. 2—Components of a shaped-charge perforator.

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  Fig. 3—X-ray of a 20-g charge.

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  Fig. 4—Jet formation from a shaped charged.

Penetration of a shaped-charge jet through a target proceeds with the jet pushing aside everything in its path. The effect is similar to driving a nail through a block of wood. The wood around the nail hole is compacted tightly. Permeability in porous rock is reduced frequently in the compacted zone. There is almost no heat transfer during the jet penetration, although some target heating usually is seen from the post-explosion byproduct gases. Almost any target, including paper, can be perforated with a shaped charge. A classic example of penetration and compaction is the penetration of a jet through a thick telephone book. The area around the perforation tunnel in the paper is highly compacted to a radius of approximately 0.4 in. (1 cm). Straightening out the uncharred paper in the crushed zone revealed that very little of the paper was lost during perforating. Because fluids must flow through this crush zone, understanding how and why it forms and how to remove or bypass it is of primary importance in completion engineering.

With shaped charges, the perforation penetration usually is thought to be proportional to the weight of the charge. Although the charge size has an effect on the performance, the shape of the liner, the internal standoff in the gun, and the overall design are also important. In a through-tubing application in which the carriers are small, the charge size will vary from 2 to approximately 8 g. The smallest charges are used in 1 9∕16 - and 1 11∕16 -in. hollow carriers and the larger sizes are used in expendable strips. In hollow-carrier casing guns with diameters of 3 1/8 in. or larger, charge weights of more than 12 g are common (typically 22 to 37 g for 5-in.-diameter guns). Normally, the largest charges are used in the large expendable guns and casing guns in which the charges are more than 50 g. Openhole perforating guns that are designed to reach beyond mud damage in an openhole completion may use charges of 90 g or more.

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See also

Perforating

Perforating equipment

Perforating design

Pipe cutoff methods

PEH:Perforating