Perforation
In the majority of completions, once the reservoir has been drilled, production casing or a liner is run into the well and cemented in place. To provide the communication path between the reservoir and the wellbore, it will be necessary to produce holes through the wall of the casing, the cement sheath and penetrate into the formation. This is accomplished by a technique called perforating.
The basic operation requires that a series of explosive charges are lowered into the well either on an electric conductor wireline cable, or on tubing or drillstring, and when the charges are located at the required depth, they are detonated to produce a series of perforations through the wall of the casing and the cement sheath.
Initially, the type of charges used in perforating guns were bullets, but with the development of armour penetrating explosives during World War II, shaped charges or jet perforators are now almost exclusively used.
SHAPED CHARGE CHARACTERISTICS AND PERFORMANCE
Principles of Shaped Charges
The basic shaped charge consists of: (1) A conical metallic liner
(2) A primer explosive charge (3) The main explosive charge (4) A charge case or container
The main explosive charge is usually a desensitised RDX (Cyclonite) type of
explosive which besides being extremely powerful in terms of the energy released per unit weight of explosive, also reacts very quickly. In fact, once the main charge is detonated the process is completed after only 100 - 300 μseconds. This fast reaction time is of importance in that it concentrates the detonation energy of the exploding charge to a very limited target area and also excludes any thermal effects.
The main explosive is contained within a charge container which can be manufactured as either a metal or a disintegrateable case e.g. ceramic which will be shattered during the explosion. Whilst a metal case would assist in containing and directing the force of the explosion to a certain target area, the target area would be diffuse as it would depend upon the diameter of the exit area for the explosion from the charge case and its distance from the target area.
To understand more clearly how the use of a metallic liner can influence the
penetration in this way, it is necessary to consider in more detail the actual mechanics of the explosion. The detonation is actuated from surface by either electrical current in the case of a wireline conveyed gun or by mechanical, hydraulic or electrical means if the gun is conveyed on tubing. First, the primer charge is detonated and this in turn fires the main charge.
On detonation of the main charge, a detonation wave is produced which moves from the apex of the charge container at a speed estimated to be 30,000 ft/sec. This explosive detonation wave exerts pressures of up to 2-4 x 106 psi against the liner which then starts to deform. The material of the liner on the outside flows towards the centre of the cone to form a jet of fluidised material, whilst the material of the cone initially in contact with the explosive charge, collapses inwards towards the central axis of the cone, to produce a slug or tail of fluidised material.
The jet leaving the charge has a velocity of the order of 20,000 ft/sec. and has an impact pressure on the casing of 5 x 106psi. Under such high impact pressures, the casing material that it contacts, becomes plastic and moves away from the impact of the jet. The material in the formation will be compacted and moved back into the formation ahead of the jet as the tunnel is created through the casing and cement sheath into the formation. The whole process takes place almost instantaneously and since no thermal effects take place, no fusion or burning occurs. The penetration is due solely to the extremely high impact force exerted on the target by the jet. The jet, whilst it is being created over an interval of a few microseconds starts to extend and move away from the charge. However, the slug material, although it comprises the bulk of the mass of the liner will lag behind the jet and in fact plays no real purpose in creating the perforation. On the contrary, the material of the slug will follow the jet into the perforation where, due to its mass, it will be deposited, thus giving rise to plugging of the perforation. The presence of the slug is therefore
detrimental to the subsequent flow performance of the perforation. One approach to eliminate the slug has been to create a bi-metallic liner system, where the inside surface of the cone which will produce the jet is composed of copper whilst that on the outside is a metal, such as zinc, which will readily vapourise during the explosion.
It is therefore clear that the penetration process is controlled to a large extent by the characteristics of a jet moving at very high velocity which impacts on the target material. The area of the target affected will be directly proportional to the diameter of the jet produced. Since the depth of penetration is directly influenced by the speed of the jet, it is evident that it is of paramount importance to maintain the jet at a minimum diameter.
However, the properties and extent of the crushed zone will depend upon a number of factors including:
(1) Size of perforation charge
(2) Casing wall thickness and strength (3) Cement sheath thickness and strength
(4) Grain composition, size and shape of the formation rock (5) Stress conditions in the near wellbore region
(6) Proximity of nearest perforations in the same vertical plane.
Factors Influencing Charge Performance
The physical performance of a shaped charge is normally gauged from a number of characteristics:-
(1) Penetration length (2) Perforation diameter (3) Perforation hole volume
(4) Burr height on the inside of the casing around the perforation entrance hole.
(a) Gun size/explosive charge size
The size of the perforating gun will dictate the maximum explosive load which can be accommodated in the charges.
In general terms, both the penetration and the diameter of the entrance hole will increase as the gun diameter and hence the size of explosive charge also increases. (b) Wellbore fluid pressure, temperature and density
It would be expected that if the fluid in the wellbore were very dense, then it could reduce the jet velocity and impair its physical performance. However, in reality the
thickness of the fluid film through which the jet moves is normally small. If however the charge size is small or the gun clearance is large, penetration could be reduced. Similarly well pressure has shown no observable effect on charge performance. This is particularly important given the frequent use of reduced hydrostatic pressure in the wellbore whilst perforating in an underbalanced mode. The effect on flow performance is much more important. However, elevated well temperatures can lead to
significant degradation of the charges with consequent poor performance. However, the effect is only serious in deep hot wells where the gun contact time is large. In such situations, the protection of the explosive charges by their inclusion in a hollow gun carrier is advisable.
(c) Gun clearance
Since all perforating guns have a diameter which is substantially less than the casing inside diameter it follows that the gun cannot be expected to be centralised. If the charges are loaded on a design which calls for them to fire at different angular phasings then each charge will face a varying gap between the gun outside diameter and the inside diameter of the casing. This gap is known as “gun clearance”.
The effect of gun clearance upon penetration and entrance hole size is shown in Fig below for a simulated perforating configuration. It can be seen that maximum entrance hole size is achieved with a gun clearance of 1/2 inch (normally provided as a defined stand
off in the gun) but in general both penetration and entrance hole size decrease with increasing clearance.
The effect will be most serious when a very small diameter gun is used as is the case with wireline conveyed through tubing guns where the gun size is selected to pass through the completion string. In such cases it might be preferable to place all the charges to fire in-line and align the gun in the casing using a positioning device, to provide minimum gun clearance.
(d) Compressive strength of formation rock
It would seem logical for the compressive strength of the rock to have a large effect on the physical performance of jet perforators. Although the effect is not clearly quantified, the perforation obtained is inversely proportional to rock compressive strength as shown in below Fig. It will be necessary to extrapolate test firing results obtained from a standard test material to specific reservoir rocks.
(e) Strength of casing and radial support of cement sheath
If the casing to be perforated is constructed from high grade tensile steel, it will absorb more energy whilst being perforated and hence reduce the overall length of the
perforation. In reality, the effect is relatively small.
However, as the number of perforations shot into a casing increases, the structural integrity of the casing is reduced and the possibility of splitting the casing cannot be discounted. This will be a very serious consideration where the cement sheath is incomplete, as perforating a casing, behind which no cement exists, could give rise to casing rupture.
Perforation Charge Arrangement
The arrangement provides for variation in the number of shots to be fired per unit interval, i.e. the shot density and the direction in which all, or individual, charges will be shot, i.e. the shot phasing.
The number of shots installed in a perforating gun varies from low shot density, e.g. less than 1 shot/ft, to higher shot densities of up to 16 shots/ft. The lower shot densities are normally adequate for production in reservoirs of moderate to high productivity or are selected for specific injection operations where flow control is required. The higher shot densities will provide improved inflow performance in all reservoirs but may only be significantly beneficial in reservoirs with a low vertical permeability or where severe local drawdown might give rise to formation sand collapse.
The orientation of perforations defined as the angular phasing can be:
(a) 0or in-line firing which can provide the minimum clearance for all perforations if the gun is positioned to fire on the low side of the hole.
(b) 45to 90phasing which provides the nearest approximation to radial flow. (c) 180phasing in either of the two planar directions.
(d) 120phasing either with all 3 shots firing at 120to each other or omitting 1 charge such that the 2 shots fire at +60and -60angular phase.
The phase orientations are depicted in below Figure. All perforation flow patterns are utilised. 90phasing which provides the best radial depletion can be very effective when conducted with high shot densities. However, the selection of phasing will depend not only on shot densities but gun size, gun clearance, formation isotropy or anisotropy with respect to permeability. It is clear that for each shot density a number of options regarding phasing can exist, for example, 4 shots/ft can normally be fired at 0, 90or 180phasing.
PERFORATING GUN SYSTEMS
Several key features classify perforating operations including:
(1) Whether the gun will be run on wireline or be conveyed on tubing or a drill string.
(2) Whether the pressure in the wellbore at the time of perforating will be less than reservoir pressure, i.e. an underbalanced pressure condition, or be greater
than reservoir pressure, i.e. an overbalanced condition.
(3) The extent to which the gun, the charges or charge carrier will be retrieved from the wellbore after perforating.
(4) Whether perforating will be conducted prior to, or after, mechanical completion of the well.
The majority of wells are perforated using wireline conveyed guns. However, there are two alternative approaches:
(1) The guns can be lowered into the cased wellbore prior to installation of the production tubing. In such cases, the guns are referred to as casing guns and both during and after the operation it is necessary to maintain a wellbore pressure which exceeds the reservoir pressure unless surface pressure control equipment is being used. Thus the approach utilises overbalanced pressure conditions.
(2) Alternatively, after installing the completion tubing, a gun can be selected to be run down the inside of the tubing, out of the tailpipe and perforate the casing. The guns in this case are referred to as through tubing guns.
Alternatively, the perforating guns can be tubing conveyed either on the end of the completion string, coiled tubing, or at the end of a drill pipe test string which will be retrieved after well clean up prior to running the completion string.
Perforating guns also vary according to the extent to which they are expendable, and are classed as follows:
(a) Retrievable or hollow carrier guns are designed such that the individual charges are fitted to a carrying strip and then connected to a primer or detonating cord. The carrier strip with charges is then inserted into a steel carrier tube which is sealed prior to running downhole on wireline. The advantages and
disadvantages of this type of perforating gun are listed in Table below.
(b) Expendable perforating guns are designed such that the gun will self-destruct on detonation and thus only the connectors and depth correlation equipment will be retrieved from the well. Such guns comprise a number of charges which are essentially strung together, i.e. no rigid carrier strip or tube is used. The charges are also designed such that the charge case or container will fragment / disintegrate on detonation. The material used for fabrication of the charge case must be friable, e.g. ceramic or aluminium, but must offer a reasonable
degree of robustness to protect the charges during handling operations.
(c) Semi expendable perforating guns are designed to offer the advantages of gun durability and robustness which exist with the hollow carrier and the charge disintegration of fully expendable guns. The guns are designed such that the charges which are expendable are mounted on a carrier strip for running into the wellbore.
Gun Type Advantages Disadvantages
Retrievable hollow carrier gun Robust and less liable to be damaged during running in charges protected from: (a) well fluids
(b) well pressure (c) well temperature Fast running speeds into the well bore.
No debris
Gun length is limited by height availability for handling or lubricator size (normally 60' max). Weight of hollow carrier can be significant. Large intervals to perforated will require multiple runs.
Fully Expendable guns Flexible and thus can be run in longer lengths (up to 200 ft per run). Most economical in terms of both gun cost and time
Debris left in wellbore. Components immersed in well fluids. Pressure and temperature may limit the use of certain
required. charges. The gun is not rigid nor durable and maylimit running in speeds or tension to be pulled if gun becomes stuck. Semi-expandable Debris limited to crushed
charge cases. More economical than hollow carrier guns.
Rigid carrier strip constrains gun length as for hollow carrier guns. Charges may suffer pressure and temperature limitations since they are immersed in well fluids.
Wireline Conveyed Casing Guns
These guns are largely constrained by two factors:
(1) The gun diameter must be less than the casing inside diameter. This allows a large diameter gun to be used and hence large charges.
(2) The length of gun is defined by either the weight which can safely be suspended by the wireline or by the length of lubricator into which the gun will be retrieved after perforating in underbalanced conditions.
Thus, guns are normally in the range of 3 3/8" to 5" diameter and have the following
advantages:
(1) The gun diameter can allow for the use of fairly large explosive loads in the shaped charges.
(2)The gun diameter which can be run into the well must allow for a minimum clearance of 1/2". However if the gun size is fairly large there will be reduced
standoff or clearance for the charges, and such guns can take full advantage of 90shot phasing to provide improved flow performance.
Casing guns are available in all 3 classifications of retrievability. In the majority of operations where the interval to be perforated can be accomplished
with a limited number of guns, then a retrievable hollow carrier gun or a semi expendable gun will be used. These guns can be used with a pressure control system at surface which would comprise a lubricator and wireline BOP system mounted on the Xmas tree.
In general terms, the penetration obtained with casing guns is high, due to the larger charge sizes which can be used in combination with the reduced gun standoff.
However, the larger the gun size, the higher is the entrance hole size and the CFE (Core Flow Efficiency) obtained.
principal advantage of these guns is that the entrance hole is substantially larger than with guns with a diameter in the range 3 1/8" - 4".
Casing guns can normally provide perforations which are larger in diameter and deeper than those obtained with through tubing wireline guns. Their size provides an opportunity to use not only larger explosive charges but also higher shot densities. The main disadvantage is that this type of gun is most frequently used prior to mechanical completion and since it will thus provide communication between the formation and the wellbore it is necessary to either isolate the perforations or retain a fluid in the wellbore which exerts a bottom hole pressure greater than the reservoir’s pressure.
The use of such guns therefore is a compromise between better charge performance and ultimate reservoir performance. Compared with through tubing guns, casing guns offer substantially better charge performance.
Wireline Conveyed Through Tubing Guns
Wireline conveyed through tubing guns will obviously be constrained in diameter, and consequently charge size, by the smallest inside diameter in the production tubing string. In low flowrate wells where very small tubing sizes are necessary, this
technique will thus not be feasible and in such cases wireline conveyed casing guns are used. However, through tubing guns are available in sizes down to 1 3/8".
Since the gun can be run in after the well is mechanically complete and the equipment pressure tested, the well can be perforated under drawdown to simulate the reverse pressure firing mode. In such cases the drawdown can be controlled by a combination of fluid density and surface pressure.
Through tubing guns are available in a range of diameters from 1 3/8" to 3 1/2", as
retrievable, semi-expendable and fully expendable perforating guns.
Most commonly, however, it is the retrievable and semi-expendable guns which are used. In general terms, the through tubing guns give reasonable performance, with the semi-expendable charges giving slightly better performance for comparable charge size in terms of penetration and flow efficiency. In comparison with casing gun charges, the through tubing gun charges offer substantially reduced penetration and in terms of flow efficiency they at least match the performance of casing guns. The real benefit of through tubing guns is their ability to perforate under drawdown conditions. The use of a drawdown prevents fluid inflow from the wellbore into the perforation and also serves to flush out material lodged within the perforation tunnel or surrounding matrix. It must be stressed however that due to the restriction on gun length caused by gun retrieval only the first gun is detonated in under balance. In general terms to remove damage from the tunnel itself, the higher the differential pressure the more effective will be the clean up process. However the use of excessive drawdowns can lead to:
(1) Collapse of the formation around the perforation tunnel.
(2) Mobilisation of fine particulates within the formation which can cause destabilisation of the formation or blockage of the pore space.
In most cases the drawdown must be held within reasonable limits although for the lower permeability rocks higher drawdowns can be used because:
(1) Higher drawdown will be required to create sufficiently high flowrates. (2) If the reduced rock permeability is due to grain size and not just to pore space infill, then the compressive strength of the rock will be higher.
Tubing Conveyed Perforating Guns
As the name implies, tubing conveyed perforating, or TCP, involves the assembly of a perforating gun on the end of drill pipe string, production tubing or coiled tubing and its lowering and positioning in the wellbore prior to detonation. The technique has increased rapidly in both application and development during the 1970s and is now widely employed. After detonation, the gun can either be pulled from the well or detached to drop into the wellbore sump below the perforated interval.
Deployment Options
Tubing conveyed perforating can either be employed by:
(1) Running the guns with a conventional drill stem test assembly. After clean up, the well would have to be killed prior to retrieving
the test string, normally with the spent gun. Subsequently, the well would be completed if required.
(2) The gun could be run attached to the base of the completion string tailpipe below the packer. The string would be run into the hole, landed off, the packer set and the gun detonated under drawdown. Normally, the guns would be detached and dropped into the sump.
(3) Running and retrieving the gun on coiled tubing using a CT deployment system.
Firing Options
When the gun is in position, it can be detonated by one of a number of optional techniques:
(1) Mechanical firing
A bar or go-devil can be dropped down the tubing onto a plunger which contacts a blasting cap on top of the gun. An inferior method is to incorporate the blasting cap on the bar which is dropped downhole. This method can be unreliable if debris is allowed to accumulate on top of the firing head. This potential problem can be alleviated by running a ported debris barrier which will allow circulation above the firing head prior to detonation. Apart from surface pressure, there is no reliable indication of detonation and this can lead to uncertainties regarding safety if the string has to be pulled.
(2) Hydromechanical
In this technique, the annulus can be pressured up and the pressure routed through a bypass valve above the packer, onto a series of shear pins on the firing head. Once a differential pressure is exerted sufficient to shear the pins, the firing pin is driven down against the detonator. This technique would be more reliable than the mechanical method, especially in deviated wellbores.
(3) Wireline firing
In this system, a special wet connect is run on wireline after the guns are positioned. This wet connect attaches to the firing head which can then be fired by passing an electrical current down the cable from surface. The main advantage of this technique is that, besides surface pressure being created on successful firing, there are also electrical indications at the surface.
TCP Gun Disposal
After detonation the gun can be dropped. However, if it is decided to flow the well either before detaching the gun, or if it is intended to retrieve the gun, a vent assembly or perforated joint must be provided below the packer where fluid can enter the flow string.
Advantages / Disadvantages of TCP
A major advantage of TCP is that as with using casing guns, the gun size and,
accordingly, that density and charge size can be quite large. Given the size of the guns, it would be expected that the charges would offer deep penetration with large entrance hole size and good flow efficiency. The principal advantages of TCP are that it can offer the ability to use large charges with high shot density (up to 16 shots/ft) and perforate under substantial drawdown if required. It therefore combines some of the advantages of casing and through tubing guns.
(1) The ability to use high shot densities and to create large entrance hole sizes can lead to reduced hydraulic erosion in the formation around the perforations. This allows higher flowrates to be realised without formation breakdown.
(2) The perforating operation can be completed in one run even for long intervals. Intervals in excess of 1000 m have been shot in one run with TCP.
(3) Unlike wireline operations, even if the interval is fairly large, the ability to shoot the interval with one gun means that all the perforations are created
simultaneously, which benefits well clean up and productivity.
(4) As with the through tubing techniques the well is not perforated until the well is completed and it is safe to allow well fluids to enter the wellbore.
(5) With wireline conveyed guns, whilst perforating under drawdown, there is a danger that the guns will be damaged or blown up the wellbore if too high a pressure drawdown is used. With TCP, the durability of the system allows high differential pressures, e.g. >2500 psi, to be used.
(6) If the hydromechanical firing option is used the technique is feasible, even in highly deviated wells, as there is no dependence on wireline nor on a go devil detonation system.
(7) If either the mechanical or hydromechanical firing system is used there is no necessity for radio silence.
The disadvantages of TCP are:
(1) If a misfire occurs, then gun retrieval will require a round trip which is both time consuming and costly. There is also a safety concern as to why the gun has
not detonated. Detonation whilst the string is being retrieved, although rare, has occurred.
(2) If the gun is not detached but is to remain opposite the perforated interval, it will prevent production logging or through tubing wireline below the tailpipe.
(3) Since the running procedure for the guns is prolonged, the charges may be exposed to well temperatures for an extended period and this may lead to degradation.
(4) In general, the costs of TCP are higher than conventional wireline perforating. The cost differential will decrease as the length of interval to be perforated increases. Further, if the gun is to be dropped into the sump the cost of drilling the additional sump length must be considered.
(5) Surface observation of the degree to which the charges have fired is not possible unless the gun is retrieved.
Hydraulic Fracturing
Propped Hydraulic Fracturing consists of pumping a viscous fluid at a sufficiently high pressure into the completion interval so that a two winged, hydraulic fracture is formed. This fracture is then filled with a high conductivity, proppant which holds the fracture open (maintains a high conductivity path to the wellbore) after the treatment is finished. The propped fracture can have a width between 5mm and 35mm
and a length of 100m or more, depending on the design technique employed and the size of the treatment.
Propped hydraulic fracturing is aimed at raising the well productivity by increasing the effective wellbore radius for wells completed in low permeability carbonate or clastic formations. The radial well inflow equation:
shows that the well production rate (Q) can be increased by:
(i) increasing the formation flow capacity (k.h) {the fracture may increase the effective formation height (h) or connect with a formation zone with a higher permeability (k)};
(ii) bypassing flow effects that increase the skin (s) e.g. near wellbore formation damage; (iii) increasing the wellbore radius (rw) to an effective wellbore radius (r'w) where
