Is probably the most important of all completion functions in cased holes. Adequate communication between the wellbore and all desired zones, as well as isolation between zones, is essential to evaluate and to optimize production and recovery from each zone. The objective of perforating a well is to establish communication between the wellbore and the formation by making holes through the casing, cement and into formation in such a manner so as not to inhibit the inflow capacity of the reservoir.
The selection of a perforating technique can be a critical factor in successful testing of a well. It is therefore important to plan early so that the most suitable equipment will be available when required. To optimize perforating efficiency, it is not solely down to the perforating technique but relies extensively on the planning and execution of the well completion which includes selection of the perforated interval, fluid selection, gun selection, applied pressure differential or underbalance, well clean-up, and perforating orientation.
Purpose of Perforating:
- The productivity of a given reservoir in primarily dependent on the near wellbore pressure drop.
- This is governed by drilling damage and perforation parameters.
- Successful stimulation and sand control operations are strongly dependent on perforation parameters.
Although technology is available to insure good perforating in most wells, unsatisfactory perforating tends to be the rule in many areas. The three most prevalent causes for poor perforating probably are:
- A lack of understanding of the requirements for optimum perforating.
- Inadequate control of gun clearance, particularly with through tubing guns.
- The rather widespread practice of awarding perforating jobs on the basis of price, rather than job quality
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Types of Perforators
- Bullet Perforators.
- Jet Perforators.
- Hydraulic Perforators.
- Mechanical Cutters – Knives and Milling Tools
Bullet Perforators
The diameter of the bullet perforators carriage is 3 ¼ in. or more, it used to perforate the inner reproduction and the rocks that resist the pressure less than 6000 pound/square node, but this method cannot be used in perforate the Christmas tree or production pipe because the diameter of the bullet perforators carriage is large but it is used in perforate the wells under drilling fluid pressure or the present of blow out preventer, therefore it is best to use method in low pressure wells.
Bullet guns 3 ¼ in. OD or larger are applicable in formations with compressive strength less than about 6000 psi. It may provide deeper penetration than many jet guns in formations with less than about 2000 psi compressive strength.
Muzzle velocity of bullet guns is about 3300 ft/sec. The bullet losses velocity and energy when the gun clearance exceeds 0.5 in., the clearance at which most comparative tests have been made. At zero gun clearance penetration increase about 15% over 0.5 in. clearance, along with a deburring effect. Loss in penetration with one inch clearance is about 25% of the penetration at 0.5 in. clearance, and at 2 in. clearance the loss is 30%.
Deburring of bullet holes is not dependent on decentralization if the bullet carries a deburring device. This device is more effective in deburring than using zero either selectivity or simultaneously.
Jet Perforating
This method is unique in having small measurement jet perforators container which made the use of it in the perforate of production pipe and chrisms trees, there is two sizes of jet perforators 8/21 node and 11/16 node.
An electrically fired detonator starts a chain reaction which successively detonates the primacord, the high velocity booster in the charge, and finally, the main explosive. High pressure generated by the explosive causes the metal in the charge liner to flow, separating the inner and outer layers of the liner. Continued pressure buildup on the liner causes a needle like high speed jet of fine particles to spew from the cone at speed of about 20000 ft/sec at its tip with a pressure at this point estimated to be 5 million psi. The outer shell of the liner collapses to form a slower moving metal stream, with a velocity between 1500 and 3000 ft/sec.
Water or dampness in the gun, primacard or charge may cause malfunction or low order detonation. High temperature aging of explosive in primacord or charge may reduce charge effectiveness or cause low order detonation.
Conventional jet perforating retrievable hollow carrier steel guns normally provide adequate penetration without damaging the casing.
Through tubing jet perforation including capsule type guns, swing jet guns, wire and strip carrier guns, and thin wall or expendable hollow carrier guns are available. The primary advantage is being able to run and retrieve the guns through the tubing and to shoot with a pressure differential into the wellbore.
The swing jet provides relatively large hole size and sufficient penetration for most wells. Its major disadvantage is the mechanical manipulation required and the large amount of debris remaining after shooting. Guns with exposed charges such as the capsule gun will swell the casing and may split the casing.
Also it is difficult to obtain proper gun clearance with through tubing guns to provide needed hole size and penetration.
Hydraulic Perforators
This method is used to make a fractured in the casing reproduction and the reproduction formation behind it by using high pressure, high injection speed hydraulic flow. This method is practically used in perforating the wells used in water injection. Action is obtained by jetting sand laden fluid through an orifice against the casing. Penetration is greatly reduced as wellbore pressure is increased from zero to 300 psi. Penetration can be increased appreciably by the addition of nitrogen to fluid stream.
Mechanical cutters – knives and Milling Tools
They have been used to open slots or windows to provide communication between the wellbore and the formation. Milling a window in the casing, under reaming and gravel packing are the standard procedures for sand control in some areas.
Perforating Fluid:
- Salt water or oil.
- Acetic acid.
- Nitrogen gas.
Selection of Perforated Interval:
The perforated interval in the production formation is chosen according to the following factors:
- Electrical, sound, radiation tentacle where this tentacle shows the limits of the oil formation.
- The formation test results while well drilling.
- The core separated from the core.
- Geological information.
When choosing the formation interval must consider the following:
- The distance of the lower end of the perforated interval from water and oil tangency level to avoid the problem of product water with oil because of the water coning phenomenon.
- Choosing formations with high permeability and porosity and avoiding perforating shale zone.
- Avoiding perforating casing connection.
- Choosing formations where it should be completed with less possible tries of setting down the bullet carriage.
- It is preferred to do the perforation process inside the well in the middle of a fluid that does not do any harm to the formation.
Factors Affecting Gun Perforating Results
- Perforation Plugging.
- Cleanout of Plugged Perforations.
- Effect of Pressure Differential.
- Effect of Clean Fluids.
- Effect of Compressive Strength.
- Perforation Density.
- Cost-Perforating price.
- Pressure and Temperature Limitations.
- Well Control.
- Casing and Cement Damage.
- Need for Control of Gun Clearance.
- Depth Measurements.
- Oriented Perforating.
- Penetration vs. Hole Size.
Plugging of perforators with charge liner residue slugs can be very severe. Through the use of sintered metal or powered metal cones in charges, large residue slugs have been eliminated in a number of the premium priced charge. If perforations are made in mud, the perforations are filled with crushed formation rock, mud solids, and charge debris. Perforations made in high weight muds, mixed from high density solids are more difficult to clean out. A large number of plugged perforations remain and may result in failure to drain specified formations such as sand-shale sequences.
Cleanout of plugged perforations – for unconsolidated sands, backsurge tools and perforation washers have been successfully used to clean out perforations in many areas. If perforations in sandstone wells cannot be cleaned out with perforation washers or backsurge tools, the next approach should usually be to break down each perforation or fracture with clean water or oil using ball sealers. This procedure results in silt, fines, or mud being driven into formation fractures.
Acidizing sandstone wells will not usually clean out all mud plugged perforations unless each perforation is isolated and fractured and mud is driven into the formation fracture.
Limestone or dolomite wells are frequently perforated in HCl or Acetic acid with a slight differential into formation, particularly if the well is to be fractured.
When perforating in mud with a pressure differential into the formation, perforations are filled with mud solids, charge debris, and formation particles. Mud plugs are difficult to remove, often resulting in some permanent perforation plugging and reduced well productivity.
If a particular perforator provides adequate hole size and penetration under given well conditions, well productivities will be maximized by perforating in clean oil or saltwater with pressure differential into the wellbore during perforating and well clean up period.
Penetration and hole size of jet perforators are reduced as compressive strength of casing, cement, and formation rock is increased. Penetration of bullets is severely decreased with increases in strength of the casing, cement, and formation.
Shot density usually depends on required production rate, formation permeability, and length of perforated interval. For high volume oil or gas wells, perforation density should permit the desired flow with reasonable pressure drawdown. For sand consolidation, four shots of large diameter deep perforations is usually preferred. High strength casing collars may be damaged by multiple perforations in the collars.
Perforating prices vary from area to area, however, reduced perforation density usually results in lower job costs.
Pressure and temperature ratings are available on all perforators. Bottom hole may impose limitations on some exposed charge guns. As a general rule, high temperature charges should not be employed even in wells in the 300 – 340 °F range.
Low pressure oil wells can be perforated with oil or water in the casing with little surface control beyond a wiper type of packing gland.
Hollow carrier jet guns absorb unused energy from jet charges. This prevents casing splitting and virtually eliminates cement cracking. Little casing damage occurs with conventional bullet guns. Jet guns with exposed charges, such as strip or capsule type guns, can cause deformation, splitting, and rupture of the casing, and appreciable cracking of cement.
Clearance control can be achieved through spring type deflectors, magnets, and others methods.
The accepted method of ensuring accurate perforation depth control is to run a collar locator with the perforator and to make measurements from casing collars which have been previously located with respect to the formations using radioactive logs.
Oriented perforating in multiple required for through tubing perforating in multiple completions where adjacent tubing strings. Mechanical, radioactive, and electromagnetic gun orientation devices are available. When using oriented through tubing perforators in multiple completions, a thin wall hollow carrier gun should always perforators can cause collapse of an adjacent tubing string.
In the design of any shaped charge, greater penetration can be achieved by sacrificing hole size.