Perforating equipment

The most important consideration in selecting a perforator is choosing a gun system that matches the requirements dictated by the completion.

Guns/carriers

In shaped-charge perforators, there are two basic carriers:

• Retrievable hollow carrier
• Expendable or semiexpendable carrier

Thick-walled guns

On the larger diameter, thick-walled guns, there is much less distortion than on the small, thin-walled through-tubing guns. In wells in which clearances between the gun and tubulars are critical, the amount of distortion of the gun should be determined from the service company before the gun is used. Gun body swell ranges from approximately 10% diameter growth in small, 1 11∕16 -in. guns shot in low pressure wells to less than 1% diameter growth in larger guns and those shot at high pressure. Fig. 1 shows a gun swell after firing in a low-pressure test. Gun bowing is often noted in small guns of 2 1/8 in. diameter or less, whereas larger guns, because of the increased resistance to bending with increasing diameter, show no evidence of bowing.

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  Fig. 1—Gun swell after firing in a low-pressure test.

Hollow-carrier guns

Hollow-carrier guns can be run either on wireline or on tubing. They may carry large charges, which normally minimize casing damage. The carrier contains most of the debris from the charge and the alignment system. Hollow-carrier guns are tubes that contain the shaped charges. The guns may be of a small size, able to pass through tubing and restrictions and place initial perforations or add perforations, or of larger sizes that are run through casing, conveyed by either work strings or the production tubing. Both reusable and single-use guns are offered, although higher pressure and more expensive wells typically use the single-use guns to minimize leaks and problems. Single-use guns are designed as expendables because the shaped charge perforates through the gun body. There is usually a “scallop” spot milled in the outside of the hollow-carrier tube at the charge location. The scallop contains the exit burr from the charge firing, which prevents scoring of polished bores if the gun is moved after firing and may minimize gun swelling. The scallop also may minimize the metal thickness penetrated, although this affects the perforation charge performance less than 10%. Keeping the charge exit point within the scallop becomes critical when through-tubing guns are used in which polished bores must be traversed with the gun after firing or when tubing clearances are critical. Hollow-carrier guns, depending on their diameter and design, may be loaded with 1 to 27 shots/ft and have all the commonly used phase angles as well as specialty phasings.

Hollow-carrier performance

Two factors that affect the charge performance in hollow-carrier perforators are standoff and gun clearance. Standoff is the distance between the base of the charge and the inside of the port plug or scallop and is a fixed part of the gun/charge system design. Gun clearance is the distance from the outside of the port plug or scallop to the wall of the casing. The gun clearance distance for a 4-in. hollow carrier, 90° phased gun in 7-in., 23 lbm/ft, N-80 casing can be anywhere from 0 to 2.3 in., depending on the gun position. Unless centralizers are used on the gun, one edge of the gun will contact the casing wall, and maximum clearance will occur at 180° to the wall contact. For this reason, small guns are decentralized purposely by magnets, and the charges are all aligned to fire in the direction of the magnetic positioning (0° phasing). Larger guns with smaller clearance distances use charges phased around the gun. Typically, the maximum gun diameter selected should permit washing over the gun with washpipe in the given casing size.

Hollow-carrier distortion

There is some distortion (swelling) in the body of almost all hollow-carrier guns after firing. The amount of the distortion is a function of the size of the gun and the type and size of the charge used. The following are all factors in the gun distortion:

• Gun diameter
• Gun wall thickness
• Charge size
• Shot density
• Shot phasing
• Well pressure

Through-tubing guns

The smaller through-tubing guns should be run through a lubricator and typically are limited to approximately 40 ft in length, less for larger, heavier guns. The advantages of through-tubing guns are low cost, the ability to perforate underbalanced, and the ability to maintain positive well control. The disadvantages of through-tubing guns are limited penetration, small entry hole, and the production limitation of 0° phasing.

Expendable guns

Expendable guns have charges that are exposed to well fluids and pressures. The expendable guns are popular for through-tubing applications. They are more vulnerable to damage, but without the bulk of the gun body, larger charges can be run through any given small or buckled tubing restriction. The expendable and semiexpendable carriers normally can use a larger charge for a given tubing or casing size than the hollow-carrier guns because only the skin of the capsule around each charge separates it from the walls of the casing. With expendable guns, there is also more flexibility because some bending can be achieved. The expendable guns are popular for through-tubing applications. The charges are lined together by a common strip, wire/cable, or a linked body design. The expendable guns force the casing to endure a much higher explosive load during firing because the recoil is not contained in a sacrificial shell as in a hollow-carrier gun. Casing splits are sometimes seen with a downhole television camera after perforating with expendable guns in cased holes with poor cement or low-strength casing. Expendable guns are used because their perforating performance is significantly better than hollow-carrier guns in the smaller diameters. When the gun is fired, some or all the linking materials, as well as the charge capsule remnants, are left in the hole. Problems with these guns have centered on:

• Misfires from damage to the detonating cord
• Tubing and surface line plugging from debris
• Carrier strip disintegration or severe bending after firing

Detonator systems

Once on depth, charges are fired by an initiator or detonator. Detonator systems have been redesigned in recent years to improve safety and to prevent several perforating problems that occur from leaks, pressure problems, and temperature effects. Any wireline-conveyed, hollow-carrier gun should have a detonator system that will not allow the charges to fire if the gun is completely or partially filled with water. If a water-filled hollow-carrier gun is fired, the outer body shell may rupture and result in a fishing or milling job. Specialized detonators have methods of preventing wet (fluid-filled) gun firing, as well as offering a number of other safety benefits ranging from resisting stray currents, such as static and radio energy, to pressure switches that prevent accidental surface firing or resafe the gun when a live gun is pulled from a well. The standard explosives detonator (also called a blasting cap) is a mainstay of the construction industry but is not well suited to the petroleum industry. Several accidental discharges of perforating guns have been linked directly to stray currents or poor electrical panel operational procedures. The resistor detonator incorporates resistors that reduce the possibility of discharge from low-power electrical signals. More modern detonators are available, including:

• Flying foil
• Programmable chips
• Other units that are radio safe and allow for extra safety

Conveyance systems

The conveyance system for the perforating gun may be:

• Electric line
• Tubing
• Coiled tubing
• Pumpdown
• Slickline

The choice of conveyance depends on:

• Length of the interval to be perforated
• Size and weight of guns to be run
• Geometry and inclination of the wellbore
• Desire to accomplish other actions such as underbalanced or overbalanced perforating, gravel packing, fracturing, etc.

Well control requirements are also a consideration because live-well perforating requires a lubricator or advanced snubbing techniques. There is a significant difference in cost between the conveyance systems. Wireline generally is the lower cost system in wells in which only a few gun runs are necessary to complete the perforating design.

In wells with deviations of less than 50° to 60° and short pay zones, electric line conveyance is the primary conveyance process. Electric line is quickly rigged up with a minimum of equipment, and the short guns fit the standard lubricator lengths. Running a lubricator allows the wells to be perforated live, without the need for expensive and potentially damaging completion fluids. Modifications to lubricator and pressure-control equipment also allow coiled tubing and some snubbing operations to run and retrieve perforating guns. When a well is perforated with a wireline gun with the differential pressure into the well, the flowing fluid tries to move the cable up the hole because of the lift effect produced by fluid drag and the effect of differential pressure on the area of the gun or cable. In normal operations, this drag is minimal and probably will not be noticed unless the well produces several thousand barrels per day.

The magnitude of the drag on the cable depends on the flow. Following perforating, the liquid column used to control the amount of underbalance pressure is lightened by gas production from the formation. The liquid in the tubing also starts to flow upward because of fluid influx from the formation. As more gas enters the casing, there is a period of time in which slugs of water are rapidly lifted by the gas. The velocity increases as the slugs rise because of the expansion of the gas. After all the liquid has been produced from the tubing, the gas flow can be described as quasisteady state. The maximum lift on the cable occurs during the flow of water and gas slugs when the liquid slug velocities are high. After firing underbalanced perforations with a wireline gun, the gun, if possible, should be lowered beneath the perforated zone to minimize the lift force on the gun body. If it is necessary to flow the well as the gun is run or pulled through the tubing, sinker bars will be needed on the gun, and the well should be choked back. Very close clearances between the gun and tubing will result in very high lift forces if the well is flowing.

Because of the need for depth control during perforating, electrical responses from logging tools to confirm depth are the best method. The logging cable may be standard electric line or electric line inside coiled tubing. Alternate conveyance methods such as tubing conveyed, nonelectrical coiled tubing, pumpdown, or slickline also may be used, but a separate method of confirming depth, usually relogging to the set gun or a mechanical option, is required.

Through-tubing, hollow-carrier guns are attractive because they can be run through the production tubing and packer and require only a service truck-based unit. Generally, the phasing for the smaller through-tubing guns ranges from 0° to a staggered pattern of 15° to 45° either side of the 0° plane (low side of the hole). Complete circumference phasing rarely is used in small through-tubing guns because increasing clearance from the gun to the casing wall substantially reduces performance of small charges. In 3½-in. and larger outside diameter (OD) tubing, through-tubing hollow-carrier guns with larger charges can be used with 180° phasing to provide adequate penetration.

A major drawback to tubing-conveyed perforating is that there is no way of knowing, except by pulling the guns, how many charges were fired. A signal charge device that either fires a small explosive charge or trips a hammer device a few seconds after the primer cord detonation reaches the bottom of the gun can be used in conjunction with a sensitive sound-recording device to determine that the detonation cord was ignited to the bottom of the gun. Although the detonation of the signal charge will not tell how many charges were fired, it does signify that the primer cord has burned past all the charges. Because the major mechanical problems of tubing-conveyed perforating systems have been in two areas, failure to initiate the guns at the firing head and failure to initiate the next gun at the gun junctions, the use of a bottom-shot detector is very advantageous. The reports of early use of this system indicate it has been very successful on land-based wells but has problems on offshore wells because of the high noise levels associated with platforms.

New perforating methods recently have centered on the use of casing-conveyed perforating. In these methods, the perforating gun is attached to the outside of the casing string, and the guns are deployed during the initial running of the casing string. After the string is cemented in place, the guns may be fired by a signal, from either the surface or inside the casing itself, opening the well to production at initial time or at a later time when a zone is ready to be brought on. This type of perforating could be very beneficial when sequential stimulations of stacked pay zones are planned.

Getting on depth

No matter how good the perforating system, it is useless if the perforations are not made in the best pay zone. Typical methods of depth control include gamma ray tie-in and correlation to the original openhole gamma ray system. Until the development of sturdy gamma ray logs that could stand the shock of firing, the primary depth control method was to match openhole gamma ray to cased-hole gamma ray strip log and then tie into the collar locator log. When this method was executed properly, the depth control was accurate to within half the length of the collar. Unfortunately, a miscount would result in shooting the gun one joint off depth, which is a complete miss for many zones. With gamma ray logs that run with the gun, the process is simplified and more reliable.

The second piece of the depth-control puzzle is the distance from the gamma ray detector to the top shot of the perforating gun. A record of all the measurements of the gun should be available before the run, and depths should be worked out in advance.

Wireline measurments

Wireline measurements, even if corrected for stretch, may still be in error. The wheels in the depth-measurement device on logging trucks are calibrated for new cable. Cable wear, cable stretch, and wear of the measurement wheels can all cause inaccuracy. Magnetic marks or depth flags on the cable are helpful but can be thrown off by cable stretch. To account for creep in the wireline and to accurately zero in on the depth, the collar locator should be raised very slowly into the collar above the pay and stopped when the signal for the peak (collar location) is only half formed, which indicates that the tool is exactly in the center of the collar. To find the spot where the tool is centered on the collar and remains without changing may take several very slow passes. Once located, the wireline depth of the collar above the pay can be correlated to the openhole gamma ray log. If the casing (or the tubing in a tubing-conveyed operation) is run with a short joint or pup joint near the pay, it will be much easier to correlate tool depth on repeat runs.

Openhole and cased-hole

Openhole and cased-hole gamma ray logs rarely agree exactly on depth because of differences in cable and chart paper. The depth correlation is to be made to the openhole log. If two sections are to be perforated and a single shift will not align the cased-hole log to the openhole log, each section should be aligned independently to the openhole log.

Improving depth control is relatively easy if a short pup joint of casing is run near the top of the pay during the initial completion. Recognition of the short joint by the collar locator log is easy and relatively foolproof. Other methods of depth-control assistance are radioactive tags in the threads of one casing coupling joint near the pay. The most common depth-control problem with perforations is shooting them one joint off. The well’s plug-back depth (or float collar) also may be “tagged up” with the bottom of the gun in some wells to check depth. If the float collar has been drilled out, it also can be used as a short joint for identification.

Perforating fluid

The ideal fluid for perforating operations is a solids-free fluid that will not cause byproducts when exposed to the formation. Acceptable fluids may include:

• 5 to 10% HCl
• 10% acetic acid
• 2% (or more) KCl water
• 2% NH4Cl water
• Clean brines
• Filtered diesel

If a dirty fluid is used, there is a distinct possibility that formation damage will occur because of particle plugging at the surface of the perforation tunnels. Even when a higher pressure differential toward the wellbore is used, clean fluids are still recommended to avoid flow of particles into the perforations in the event of a mechanical breakdown, when formation pressure of productivity is less than expected, or when the well has to be shut in before all the wellbore fluids have been produced.

Occasionally, high-solids-content fluids must be used during perforating, either for well control or because of other restrictions. High particulate fluids such as drilling mud usually are designed to form a mud cake on the face of a permeable formation. If drilling mud is used as a perforating fluid and the pressure differential (either by design or by accident) is toward the formation from the wellbore, a drilling mud cake will form in the perforations that may be difficult to remove unless the formation can be produced at a high drawdown for a long period.

Lighter fluid columns such as oil or diesel may be used as perforating fluids if the full column is diesel or oil, but 6.8 lbm/gal diesel cannot be kept spotted below 9 to l0 lbm/gal brine water. Produced oil and diesel also should be filtered before use. Filtration requirements may vary with the task, but typically a 2- to 5-μm filter with a beta rating of 1,000 is adequate for most applications.

Limited penetration charges

Tubing puncher charges are used when a hole is needed in tubing for circulation or flow, but damage must be avoided to downhole equipment outside the target pipe. The tubing puncher charge is designed to expend all its energy penetrating the wall without forming additional penetration.

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See also

Perforating

Perforating methods

Perforating design

Pipe cutoff methods

PEH:Perforating