HOW TO GET DEEP PENETRATION IN

PERFORATING JOBS TO ENHANCE

PRODUCTION

- Andika Rachman Hakim

- Elisabeth Leonora Leleury

- Faiz Mamduh

- M. Pratikto

- Reva Astra Dipta

A. INTRODUCTION OF PERFORATING JOB

A perforation in the context of oil wells refers to a hole punched in the casing or liner of an oil

well to connect it to the reservoir. In cased hole completions, the well will be drilled down past

the section of the formation desired for production and will have casing or a liner run in

separating the formation from the well bore. The final stage of the completion will involve

running in perforating guns, a string of shaped charges, down to the desired depth and firing

them to perforate the casing or liner. A typical perforating gun can carry many dozens of

charges.

Commonly, perforation guns are run on E-line as it is traditional to use electrical signals from the

surface to fire the guns. In more highly deviated wells, coiled tubing may be used. Newer

technologies allow the guns to be run on slickline. No communication with the surface is

possible with slickline. Instead, a mechanism on the gun arms the charges upon reaching a

certain temperature and pressure. A timer will then fire them following a set interval.

The benefit of this strategy is greater deal of control of the well. Casing the bottom of the hole

allows the well to be completed without having to worry about reservoir fluids. It also allows

precise selection of where in the formation production will be and to be able to seal off

perforations, which are no longer useful or counterproductive, through cementing or straddling.

The disadvantage is that perforating can lead to "skin damage", where debris from the

perforations can hinder productivity of the well. In order to mitigate this, perforating is commonly

done underbalanced (lower pressure in the well bore than in the formation) as the higher well

bore pressure will cause a surge of fluids into the well at the point of perforating, hopefully

carrying the debris with it. Other methods of stimulation such as acidising and proppant

fracturing are often required to overcome this damage and bring the well up to its full potential.

Casing and perforating as a method of completion is common place nowadays, though in some

unconsolidated formations, prone to production of sand ( BP Harding as an example), open hole

completions, using only sandscreens, may be the preferred choice.

Oil Well Perforation may be basically classified in two types. There are over-balanced

Perforating and under-balanced Perforating. Overbalanced perforating is normally carried out

with the help of Perforating Guns or Hollow carriers. In Over -balanced Perforation the weight of

the Well-bore Column is more than the Reservoir Pressure, thus it normally, ensures that the

Well does not start flowing oil or Gas immediately after Perforation. However, it may have the

effect of damaging the formation due to forced entry of well-bore fluid (mud) into the reservoir.

Under balance perforating is perforation with smaller wellbore pressure than formation pressure

condition. When the entering hole is formed, reservoir fluid will flow directly. Under balance

perforating will be used in following condition.

high flow capacity formations where perforating may be a choke on flow.

natural completions in thinner zones with high reservoir pressures

where later operations will be underbalanced

competent sandstones (some exceptions; cavities for instance)

Figure 1. Under balance Perforating

B. PARAMETERS OF PERFORATING GUN

Jet Quality

Manufacturing of shaped charges plays a key role in actual penetration performance because

the reduction in jet penetration caused by poor jet quality can be significant, even if the

decrease in manufacturing quality is minor. Manufacturing must be virtually flawless to ensure

that all charges achieve near-perfect performance. Lax or inconsistent manufacturing

procedures invariably result in poor penetration performance. Besides manufacturing process,

design, storage and handling facilities also play important role in maintaining quality of the

charge.

Gun size

The greater size of gun will have ability to contain more charges (explosive device). Charge is

an explosive device utilizing a cavity-effect explosive reaction to generate a high-pressure, high-

velocity jet that creates a perforation tunnel. The shape of the explosive material and powdered

metal lining determine the shape of the jet and performance characteristics of the charge. The

extremely high pressure and velocity of the jet cause materials, such as steel, cement and rock

formations, to flow plastically around the jet path, thereby creating the perforation tunnel.

Having more explosive devices means having higher explosive power and it results in further

penetration depth and greater perforation size.

Figure 2. Gun

Perforation Density (SPF-shots per foot)

Perforation density is the number of perforations per linear foot. It describes the configuration of

perforating guns or the placement of perforations, and is often abbreviated to spf (shots per

foot). An example would be an 8 spf casing gun. Because most of formations are not

homogenous, by having higher SPF will ensure all formations perforated and increase the

possibility of successful perforation in permeable formation. The purpose of perforation

determines the perforation density. If the long term plan states formation will be stimulated by

fracturing or acidizing, the perforation density should be set at low density. On the other hand, if

formation will not be stimulated, the perforation density should be set at high density to ensure

fluid from reservoir can be produced without high pressure loss and to ensure communication/

channeling between reservoir and the wellbore. By having high perforation density, it can assure

the perforation penetrates not only solid part of the formation, but also the porous part that can

flow fluid of reservoir.

Phasing

Phasing is the radial distribution of successive perforating charges around the gun axis. Having

more directions increases the possibility of perforation in permeable formation. Perforating gun

assemblies are commonly available in 0-, 180-, 120-, 90- and 60-degree phasing. The 0-degree

phasing is generally used only in small outside-diameter guns, while 60, 90 and 120 degree

phase guns are generally larger but it provides more efficient flow characteristics near the

wellbore. The 0-degree phasing means all charges face same direction and standoff can be

minimized by utilizing magnet to increase penetration depth.

Figure 3. Perforation Geometry

Standoff

Standoff is the distance of perforation. It means the space between the shaped charge and the

internal surface of the perforating gun body. Standoff affects depth of penetration and size of

perforation, having closer standoff increases depth of the penetration and size of perforation.

The standoff is generally sufficient to allow the shaped charge jet to form before exiting the gun

body

.

Figure 4. Effect of standoff to penetration depth

The effect of gun-to-casing clearance on perforation penetration is more pronounced on deep

penetrating charges than on big hole charges. The preferred practice is to run HSD deep

penetrating gun systems eccentered, which creates maximum-penetration perforations where

the gun and casing come into contact (i.e., where there is near-zero clearance). Nevertheless,

gun-to casing clearance generally does not significantly affect the penetration of deep

penetrating charges until the clearance exceeds approximately 30% of the gun diameter. This

effect is shown qualitatively.

Figure 5. Qualitative effect of clearance on penetrative depth of a deep penetrating charge

Figure 6 illustrates the relationship between the entrance hole and gun-to-casing clearance for a

big hole charge. Optimum charge performance occurs with the gun positioned in the center of

the wellbore, corresponding to point C on the figure. At this position, the entrance hole is

largest, the total AOF is greatest, and the holes have consistent diameters. If the gun is

eccentered (i.e., lying against the side of the casing), however, some charges will be shot at

near-zero clearance (point A) and some will be shot at near-maximum clearance (point E).

Thus, without gun positioning, entrance hole sizes may be anywhere along the curve from

points A to E, depending on the random location of the gun in the wellbore. The average

entrance hole size can be improved substantially by standing the gun off the casing as

illustrated by point B (minimum clearance with standoff) and point D (maximum clearance with

standoff). With the gun in this position, the range of entrance hole sizes is between only points B

and D on the curve. Big hole charges provide optimum entrance hole diameter when the gun is

positioned at the center of the casing. The hole in the gun is actually smaller than the hole in the

casing.

Figure 6. Typical effect of clearance on entrance hole size for a big hole charge.

C. THE TYPE OF PERFORATING GUN

Perforating gun is divided into several groups:

Casing gun

Expendable gun

Tubing Conveyed Perforating gun

(a) Casing gun

This type of gun creates holes in a casing string. Casing guns are typically 3-to 5-inches in

diameter and carry up to four perforating charges per foot. Allow perforation through production

casing using larger diameter gun assembly. Gun is carried in a casing to the perforation zone

using wire line. Perforation with this type of gun cannot use under balance method. This type of

gun also categorized as a retrievable gun

Figure 7. Casing gun

(b) Expendable gun

A perforating gun assembly that disintegrates upon firing creates finer debris. It is used where

wellbore restrictions allow only limited access, as in through-tubing applications (Gun is brought

down inside the well tubing). It is relatively light and simple in design with phased expendable

guns.

Figure 8. Expendable gun

(c) Tube Conveyed Perforating gun (TCP)

Gun is brought down inside the tubing. The advantages of this type are, the gun could have a

bigger diameter; greater phasing and shooting density, under balance perforating method can

be used so that the well can be produced as the perforation has been done.

Figure 9. Tube Conveyed Perforating gun

Besides type of gun, there are several types of bullets in perforation. The charge could affect

the size of perforation diameter and the depth of penetration. For fracturing perforation large

diameter of bullet is used to produce larger diameter of perforation hole (to allow greater

propping agents to enter the hole). For deep penetration smaller diameter of bullet is used. The

types of bullet which is used in perforation are:

1. Steelflo

This type of bullet is used to obtain deep formation penetration. The size of bullet vary

between ¼” – 9/16”

2. burrfree

Less penetration is obtained than steelflo. This type eliminates the burr in the perforation

hole. The size of bullet vary between ¼” – 9/16”

3. mushroom

This type of bullet produce large perforating hole. Hole diameter is larger than bullet

diameter. The size of bullet vary between 15/32” – 9/16”

Figure 10. Bullets type, sizes and typical perforation characteristics

D. THE EFFECTS OF DEEP PENETRATION TO PRODUCTION

There are several factors that must be considered in designing the optimum perforating

operation for optimum perforation performance. These factors are inter-related and the

penetration depth is a critical factor, depending on the type of completion. Penetration depth is

strongly influenced by other factors. Generally, deeper penetration depth may increased the

perforated well productivity but decreased the perforation stability and difficult to clean. A

theoretical approach is still necessary to quantify the productivity perforated completions as a

function of all known influencing parameters.

Penetration depth is included as one of the parameter in geometrical variable. The productivity

of a perforated completion is a function of three groups of variables:

geometry of the perforations around the perforated casing

formation damage

reservoir characteristics

Maximizing productivity—or minimizing the equivalent skin effect and the pressure decrease it

causes—requires optimizing the influence of these three groups of parameters.

Figure 11 shows the typical geometry of a perforated completion.

Figure 11. Typical geometry of a perforated completion

The impact of the shaped charge jet creates a zone of reduced permeability (called the crushed

zone) around the perforation tunnel; removal of this zone results in “clean” perforations. Partial

penetration and well deviation are other important geometrical parameters, but there are few

opportunities for optimizing these two parameters.

Figures 12 and 13 illustrate the effects of the geometrical parameters on the productivity ratio in

an ideal isotropic formation with no crushed zone and no formation damage. Figure 10 shows

the importance of shot density associated with the phasing. At 0° phasing, shot density seems

to play a minimum role, but only because the phasing is not optimized. At 90° phasing, for

example, the productivity ratios are higher than for 0° phasing, and shot density is an important

factor. Figure 13 confirms that the perforation diameter has a relatively minor effect on

productivity in comparison with the effect of the other parameters.

Figure 12. Effects of shot density and phasing on the productivity ratio

Figure 13. Relative importance of perforation entrance hole diameter on the productivity ratio

The productivity of the perforated well will be increased with increasing penetration depth,

depending on the damaged zone, completion type, shot density, phasing and gun clearance.

Penetration depth becomes important to productivity because the greater penetration depth

allows more fluid to flow into the perforation and wellbore. When formation damage from the

drilling operation exists, it is critical that the perforation penetrates beyond the formation

damaged zone radius in order to get the maximum well productivity. If the damaged zone is

severe, and penetration depth is less than the damaged zone radius, the perforated well

productivity is significantly reduced until the penetration depth extends up to 4 – 50% of the

damaged zone, where the flow rate starts to increase significantly. In addition, that deep

penetration can cause the perforated well productivity to increase especially when the

perforation spacing is large or when the phasing is small. As mentioned earlier, penetration

depth effects on the well productivity also depend on the completion type, that is, for natural and

stimulation completion, it is necessary to perforate with deeper penetration depths, but for sand

control completion, it is necessary to perforate with larger perforation diameters in order to

maximize the well productivity. Deeper penetration is also required to increase the perforated

well productivity in cross bedded formation.

E. HOW TO OBTAIN PENETRATION

Penetration depth is more critical and important to the perforated well productivity than other

parameters. There are many factors affecting penetration depth:

Formation rock characteristic

Penetration depth is very affected by compressive strength of the formation rock.

Penetration depth will decrease as the compressive strength increase

Standoff

Standoff or clearance is the minimum distance between gun and casing. Standoff affects

penetration depth and the entrance hole. Jet perforator is designed to penetrate

formation at the same depth as the clearance set up at 0 – 1.5 inch.

Wellbore geometry

Wellbore geometry and casing thickness will affect penetration depth. Perforation at

“wash out” hole will reduce depth of penetration. The reducing of penetration depth will

also occur when we shot at casing with higher grade (high thickness)

Gun Size

The purpose of a gun is to create a hole in the casing, cement and to extend the hole

into the formation rock to allow efficient fluid flow between the formation and wellbore.

Penetration depth will increase as the gun size increase, because of the space for the

charge

Charge

Charge can be considered as the key element to the perforation and they have several

important pre-requisites, such as the design, manufacturing, and qualities. The ability of

shaped charged to produce a deep penetration depends on its ability to concentrate its

energy along a single axis, that is to produce a single round hole.

The penetration depth will increase as the charge load increases. The average

penetration depth of the smaller charge is roughly 65% of the larger charges. Charges

with a sharp liner angle may produce a larger penetration depth. Charges with an

imperfect surface can produce less penetration depth depending upon the nature of the

surface.

Technique of perforation

Perforation is divided into two parts, which are underbalance and overbalance

perforation. Overbalance perforation causes left debris in perforated zone, while

underbalance gives a better result for penetration

A good penetration can be obtained by designing a good perforation. Since the parameter of

formation target cannot be changed, an engineered gun can be used for a good perforation,

such as:

Place the gun at proper place

A gun has optimum standoff value will result in a maximum penetration depth. If we use

0o phase shot, then a magnet can be used to minimize the standoff, but if we use > 0o

phase shot, then it’s better to place the gun in the middle of the casing.

Figure 14. Clearance vs Penetration

Clean the borehole before conducting perforation

The more obstacles encountered before the jet comes to the target zone are, the

penetration depth will decrease. A clean hole can minimize the bad effect for the

perforating process.

Use proper gun size

Use casing gun rather than through tubing gun, because it can contain more charge in

casing gun. It is also important factor to consider about the burst effect after perforation

conducted. Not all large-sized guns can produce a lot of money, because most large-

sized guns will stuck in the wellbore after perforation conduct.

Use a good quality charge

By shooting with the proper alignment, the charge can produce greater penetration

depth. On the other hand, misalignment may produce a penetration depth of 20% to

50% less than properly aligned charges. Misalignment of the charges can be caused by

improper installation, faulty alignment equipment. The charge will produced a deep

penetration hole if its energy is concentrated along a single axis. If the charges element

are asymmetric, there may be multiple penetration axis and double jetting, resulting in a

decrease of about 50% in penetration depth.

Use deep penetrated charge rather than big hole charge to get a deep penetration.

Table 1. High Shot Density Perforating Systems Performance and Mechanical Data

Summary

Figure 15. Penetration depth comparison of perforation charges

Use underbalanced perforation

Underbalance gives a good result because the debris will be transported immediately as

the shot conducted. In overbalance perforation, the debris will stay at the end of the

penetrated hole, and that makes the hole is less penetrated than underbalanced

perforation’s hole.

Figure 16. Perforation techniques

F. CONCLUSION

Perforating gun is divided into several groups: casing gun, expendable gun and Tubing

Conveyed Perforating (TCP).

Besides type of gun, there are several types of bullets in perforation. The charge could

affect the size of perforation diameter and the depth of penetration.

Types of bullet which is used in perforation are: steelflo, burrfree and mushroom.

There 5 parameters of perforation: jet quality, perforation density, standoff, phasing and

gun size.

The productivity of the perforated well will be increased with increasing penetration

depth, depending on the damaged zone, completion type, shot density, phasing and gun

clearance.

Penetration depth becomes important to productivity because the greater penetration

depth allows more fluid to flow into the perforation and wellbore.

When formation damage from the drilling operation exists, it is critical that the perforation

penetrates beyond the formation damaged zone radius in order to get the maximum well

productivity.

Factors affect penetration depth: formation rock characteristic, standoff, wellbore

geometry, gun size, charge and technique of perforation.

To obtain deep penetration: place the gun at proper place, clean the borehole before

conducting perforation, use proper gun size, use a good quality charge and use

underbalanced perforation.

G. RECOMMENDATION

Follow all of the given ways of how to obtain deep penetration.

Use underbalanced perforation.

Use high-tech charges e.g. triple jet system.

Make sure charges for perforation are in excellent condition and use high DP shot.