Perforating is a service provided by specialist companies, however, it is common for Production Operations to be involved in the operation to facilitate well production after perforation in completed wells, or as the responsible party during a recompletion activity. Aspects of well completion design addresses a number of the issues associated with the selection of the specific technique, i.e. wireline run, through tubing, tubing conveyed (TCP),This article provides a brief summary of issues needing consideration during the perforating operation.

1 Perforating through tubing

Perforating through tubing is an old technique. The use of a seal around the cable at surface allows the gun to be run into, and pulled out of the hole with pressure at the surface. This allows the well to be perforated without the presence of excess hydrostatic pressure from heavy drilling or workover fluids in the borehole. The length of guns that can be run together is limited by the height of the surface lubricator required to accommodate the guns, positioning device and collar locator, and the weights that must be run to counter the well pressure.

Three main types of through tubing gun are in use:

• ·those using a fully retrievable carrier;
• ·those using a retrievable carrier strip to support charges that are contained in expendable cases;
• ·those in which all parts of the gun assembly remain as debris in the well.

The most powerful guns are the expendable and semi expendable capsule types. Their advantages must be weighed against the problems that can result from the presence of large quantities of debris in the well and the possibility of casing damage that can result from the more powerful charges and the absence of a carrier to contain the explosion. Hollow carrier guns can be used to perforate longer intervals on each run than capsule guns since the weight of the carrier itself reduces the need for additional weights. In spite of this the maximum length that can be run at one time is limited by the surface pressure equipment to about 10 m.

1.1 Retrievable hollow carrier guns

For through-tubing applications hollow carrier guns are available in sizes from 30 mm (13/16") to 73 mm (27/8"), with the size to be used determined by the minimum internal diameter of the completion string. The maximum shot density with these guns is 19 shots per meter (6 shots per foot). The guns can be configured to give almost any perforation phasing. The reduction in geometrical skin due to phasing is likely to be more than offset by the increased skin due to the poor penetration of the small charges when fired across the wellbore. The guns are therefore normally run with zero phasing, with the gun mechanically or magnetically positioned to minimise the standoff between the gun and the casing. Hollow carrier guns minimise casing damage as the carrier contains the force of the explosion and the high velocity fragments of the shaped charge cases. A cross section of a hollow carrier gun is shown in Fig. 2377.

1.2 Expendable and semi-expendable guns

Expendable and semi-expendable gun performance is generally superior to hollow carrier guns of a similar size. These guns are made up of individual shaped charges, each of which is contained in a separate pressure vessel, supported by wires or carrier strips. When the gun is fired, the pressure vessels, which may be ceramic, aluminium, or steel, are reduced to small fragments which remain in the well. The supporting wires or strips are recovered with semi-expendable guns, but with expendable capsule guns these too are destroyed and remain in the well. The high density debris should fall to the bottom of the well but may be carried up the well by the initial surge flow. In highly deviated wells friction may prevent the debris descending into the well, resulting in steel or other fragmented material being carried to the surface when the well is produced. Damage to chokes, sub-surface safety valves and surface production equipment may result unless provision is made for this debris. The debris will also include some low density polymer material, much of which will return to surface with flow. Fragments of material from the pressure vessel and shaped charge case are driven by the force of the explosion and impact on the casing causing significant damage, which may lead to accelerated corrosion. The size of this type of gun is limited due to the excessive casing damage which would result from large sizes.

1.3 Pivot guns

The Pivot Gun has a performance comparable to casing guns and can pass restrictions with diameters as small as 45 mm (17/8") by using large charges that are assembled in the gun parallel to its axis. When the gun is on depth, the charges are rotated 90° and then fired.

The increased performance of this fully expendable 43 mm (111/16") gun makes it applicable for certain through tubing applications where the inside diameter of the production tubing is considerably less than that of the production casing and where effective shot perforation is an important design criterion.

2. Tubing conveyed perforating (TCP)

Tubing Conveyed Perforating (TCP) allows the power and low debris characteristics of casing guns to be used in conjunction with high wellbore underbalance pressures. Highly deviated or horizontal wells in which conventional wireline techniques cannot be used can be readily perforated with TCP guns, which can be pushed along the hole to the required depth.

Tubing conveyed guns can be run using almost any tubular string, such as drill pipe, production tubing, or coiled tubing. The full benefit of their capabilities can best be achieved by running the guns as part of a completion string, avoiding the need for additional trips into the well. This also allows the Christmas tree and production equipment to be assembled and tested prior to perforating. Perforating can be conducted under any desired conditions of underbalance, and the well can immediately be produced to clean up the perforations. The gun may be fired mechanically, electrically, or hydraulically, or by a combination of these methods. The TCP guns cannot be removed from the well until the completion is retrieved (unless conveyed on coiled tubing or pulled with a snubbing unit), and may remain in the well until a workover is required. The consequences of a misfire are considerably more serious than with wireline guns as the completion or drillstring must be removed from the well if the gun is to be re-run. The reliability of the equipment used for tubing conveyed perforating is therefore of great importance.

The maximum length of gun that can be run is virtually unlimited, depending on the operating parameters of the running string. The maximum gun diameter that can be used is limited only by the inside diameter of the production casing. The increase in gun diameter compared with through tubing guns permits the use of more powerful charges at higher shot densities, improving inflow performance.

3. Perforation cleaning

The results of perforating a well are strongly influenced by the conditions in the well at the time the perforations are made, and the degree of cleaning that can subsequently be achieved.

The formation in the vicinity of the borehole is likely to be damaged by the conventional drilling process, and the perforations themselves are susceptible to damage by invasion of contaminants from the wellbore. Perforating with an overbalanced, mud filled wellbore is likely to increase the degree of damage in the formation in the immediate vicinity of the perforations, causing a further reduction in permeability. The perforation tunnel itself will contain pulverised rock, explosion products, and metallic liner debris which will remain in the tunnel if the wellbore pressure is higher than the formation pressure.

Perforation clean up is a result of both transient and steady state flow. To ensure proper clean up the post shot flow can be extended (indefinitely) by running TCP guns as an integral part of the completion string. However, when "shoot and pull" techniques are employed (e.g. coiled tubing run guns) for underbalance perforating it is important to ensure that adequate flow velocities (and produced volumes) are attained in order to produce debris into the tubing above the flow control valve, otherwise losses will result in perforations becoming plugged by gun and formation debris.

All techniques for lowering the wellbore pressure involve filling the well with a fluid or combination of fluids whose hydrostatic head is lower by a specified amount than the reservoir pore pressure. The means by which this is achieved will depend on the well and completion design, which may allow the use of a sliding side door or side pocket mandrel to provide a means of circulating a low density fluid or gas. For the high drawdown pressures, required for low permeability formations, the well may need to be partially or completely gas filled. Commonly a coiled tubing unit is used to replace part of the tubing contents with a low density fluid such as nitrogen.

If the well is being perforated for the first time, the underbalance can be established by running the tubing into the well empty or partially empty with a plug or valve preventing the entry of well fluid. However, this will preclude internal pressure testing of the completion assembly and may restrict the choice of other completion accessories.

When considering through tubing perforating techniques, drawdown can also be achieved by perforating so called 'trigger' intervals. This principally applies to gas wells, and involves firstly perforating (through tubing) a trigger interval to allow the well to be evacuated to gas. This enables subsequent perforation runs to be carried out underbalance in a gas filled wellbore. This method can also be used to perforate zones exhibiting significant permeability contrasts. Low permeability (trigger) intervals are perforated first to ensure effective clean-up, followed by the more prolific higher permeability zones.

Underbalanced perforating implies that the hydrostatic head of the well fluid prior to perforating is insufficient to control the flow of fluid from the formation into the well, and should therefore only be conducted with surface pressure control equipment, even if the well is not expected to produce fluid to surface. The underbalanced perforation of reservoirs containing H2S may require additional precautions to ensure that the completion is adequately internally pressure tested prior to perforating.

4.  Operational considerations

4.1 Transport of perforating equipment

The transport of explosives is governed by a large number of national and international regulations, to which certain exemptions have been obtained by the perforating companies. These exemptions apply only to limited quantities of explosives contained in their original packing material. The regulations apply to the transport of the explosives from the perforating company base to the wellsite, and to the storage of the explosives on the well site, both of which may be the responsibility of the operating company. The rules governing the transport of explosives to the wellsite by air apply to both helicopter and fixed wing transportation, and in many cases preclude the carriage of passengers on an aircraft carrying explosives. Explosives should be transported in their original packing, which is designed to minimise the risk of an explosion occurring in the event of a fire.

4.2 Storage and handling of explosives

It is extremely important to transport and store explosives correctly. Primary explosives must never be allowed in close proximity to secondary explosives until immediately before they are used. Primary explosive devices invariably contain a quantity of explosive sufficiently small that if unintentional detonation occurs the resulting explosion can be contained by the packaging used for storage and transport. In the event of primary explosive devices being exposed to high temperature or a fire, detonation will occur. If secondary explosive devices are stored or transported with primary explosives the detonation of the primary explosive will cause the secondary explosives to detonate, with possibly catastrophic results.

4.3 Elimination of stray voltages

The detonators used in wireline perforating equipment consist of a filament which is heated by an electrical current of 0.2-0.4 ampere, igniting a chemical mix which produces sufficient heat to cause high order detonation of a primary explosive pellet. The two wires connected to the filament each contain a 25 ohm resistor requiring a minimum of 10 volts to fire the detonator. The resistors are contained in the housing of standard detonators, but must be attached externally to high temperature detonators in the field. A potential difference of sufficient magnitude to fire a detonator may be caused by electrical welding, radio transmitters, cathodic protection, wiring faults and many other sources, and could cause the unintentional initiation of a detonator unless special precautions are taken.

To reduce the risk of unintentional detonation, operations involving standard electrically initiated detonators should not be carried out within:

• ·50 m: of public roads, waterways and railways;
• ·150 m: of high voltage lines or the overhead wires of tramways or railways;
• ·2500 m: from fixed transmitting installations whilst they are in operation.

If operations are anticipated in a location close to a fixed transmitter, overhead power cables, or electric tramways or railways the service company should be advised well in advance to enable the appropriate precautions to be taken.

Prior to starting any operations involving electrically initiated detonators the wellsite supervisor and service company engineer must ensure that:

• ·all electrical welding has been stopped;
• ·electrical measurement and control apparatus on the well or flowline have been switched off;
• ·electrical trace heating systems have been switched off;
• ·cathodic protection systems have been switched off;
• ·helicopter operations have been scheduled to take account of the periodic need for radio silence;
• ·all motorised equipment operating in the area used for work with explosives should be shut down unless required in conjunction with the work being carried out.

The service company engineer must ensure that the casing, derrick or crane, and logging unit are grounded to each other and any potential differences eliminated. After the grounding connections have been made the voltages between the casing, derrick or crane, and truck must be measured with a meter modified for this purpose. If it is not possible to reduce the residual voltage to below 250 mV the immediate superiors of both the wellsite supervisor and the service company engineer must be informed. Further work may not be carried out until permission from both of these persons has been obtained.

When requested by the service company engineer all electrical circuits and systems not required for the safe conduct of operations should be disabled by removal of the fuses or blocking the operating switches. Signs must be posted to indicate when this has been done; any circuit disabled in this way must not be switched on without permission from the service company engineer.

The circuits to be disabled will depend on the location type:

• ·on land locations all generators and electrical systems within the area where work with explosives is taking place;
• ·on offshore installations any generator not required during the operations with explosives.

The service company engineer will notify the company supervisor and the radio operator prior to the start of operations of the time at which radio silence will be required. At this time all radio transmitters within a radius of 500 m, including those on standby or supply boats, must be switched off and all portable transmitting equipment collected. The service company engineer must confirm with the radio operator that radio silence has been established prior to commencing any operation involving electrically initiated explosive devices. Radio silence will remain in force until notification has been given by the service engineer and company supervisor to the radio operator that it is no longer required.

On remote drilling and production locations the requirements for radio silence and electrical shut down can easily be met, and it is unlikely that other activities are taking place whose priorities seriously conflict with those of perforating safety. However, on large production facilities or where wells are in close proximity to commercial or military radio transmitters or power cables the criteria for perforating safely may be difficult or prohibitively expensive to achieve with conventional electrically initiated detonators. Radio silence is typically required for less than one hour per perforating run, however circumstances such as the gun or cable becoming stuck while near the surface may require several days to resolve, during which time the shut down must be maintained. The costs of deferred production or diversion of a railway line during this period may be considerable. When SAFE or EWB detonators are used, no special precautions to prevent stray voltage are needed.

4.4 Removing a gun from the hole

All procedures applicable to the arming and running of guns apply equally to their removal from the well. This requires that radio silence and electrical shutdowns are observed in the same manner as when the gun is armed and run, even if the gun is believed to have fired normally. This will require the presence of a perforating company engineer on the wellsite if a completion from which a TCP gun is suspended is retrieved from the well, or if a TCP or wireline gun is fished from the well.

When a hollow carrier wireline gun is removed from the well it will contain a large quantity of steel debris from the charge cases. Frequently, particularly when perforating in mud filled holes, the mud and debris will seal the perforation holes in the carrier which, when the gun is removed from the hole, will then remain filled with high pressure fluid. A bent rod should always be used to clear one or more perforation holes in the carrier to ensure that all pressure is released before attempting to disconnect the gun from the cable or pipe on which it was run.

4.5 Operational safety

Operations involving explosives should normally be conducted during daylight hours.

Operations involving explosives should not be conducted if any of the following conditions exist:

·any part of the working area is obscured by fog or smoke;

·an electrical storm or cumulo-nimbus clouds can be seen;

·if strong winds prevent the operations being conducted safely, due either to induced static electrical potentials or mechanical wind loads. Operations should not be conducted with wind strengths greater than force 8.

Should such conditions arise while operations are in progress, explosives should be replaced in their storage or transport containers. Explosives which are already in the well may be fired at the programmed depth, but the gun must not be pulled to surface and removed from the well until conditions have improved.

4.6 Safety guidelines for perforating operations

All locations

• ·Work with explosives should be performed under the "permit to work" system.
• ·Working areas around the Xmas tree and separators should be kept clear. Unobstructed access to these areas should be available at all times.
• ·Adequate kill fluid (150% of production casing capacity) of the correct gradient should be available. A suitable kill pump should be on location.
• ·All testing and kill equipment must be satisfactorily pressure-tested, at a pressure exceeding the maximum that may be expected during the operation.
• ·On new wells the Xmas tree should be pressure tested against a plug in the tubing or hanger nipple. Each valve of the Xmas tree should be individually tested.
• ·Gas explosion meters, a H2S detector and portable breathing apparatus sets should be available. Any gas produced should be tested for the presence of H2S either by the production supervisor or the petroleum engineer on site.
• ·For through tubing operations, before the first gun is run in the hole, and any subsequent runs that are preceded by a period of flow, a wireline dummy run must be made to check that the tubing and casing are free of obstructions. The dummy should have the same OD as the perforating gun to be run, and be at least 1.2 m long; ideally a fired gun of the same type as will be used for perforating should be used.

Immediately before starting perforating operations the nominated supervisor should hold a safety meeting at the site with all the personnel involved which should cover the following subjects:

• -permits required;
• -possible dangers;
• -precautions required for working with explosives;
• -programme to be followed, and reporting and alarm procedures;
• -organisation on site;
• -layout of the location, fire extinguishers and escape routes;
• -surface measurements to be made;
• -personal protection requirements;
• -H2S procedures;
• -emergency kill procedure;
• -expected pressures;
• -a current diagram of the well status;
• -individual responsibilities and reporting procedures on location.

Offshore

• ·Provision should be made for two blow-off lines, with one on each side of the rig or platform.
• ·A "Muster and Abandon Rig Drill should be held prior to commencing perforation operations.
• ·Before commencing perforating operations, the fire-fighting water system should be under pressure.
• ·A standby boat should be present during perforating operations.