Perforation(1)

     Overview

             Perforating is a critical part of any well completion process.
             The perforating process generates holes -perforation tunnels- in 
             steel casing surrounding cement and the formation.

                       In the past, perforation was regarded simply as holes in steel casing made .

                       By different methods. But perforation is not just a simple 
                           hole drilling process.Perforated completions play a crucial 
                       role in economic oil and gas production.

                       Long term well productivity and efficient hydrocarbon recovery.

2.2. History of Perforation in Brief

       1.      Prior to the early 1930's, casing could be perforated in place by 
         mechanical perforators. These tools consisted of either a single 
        blade or wheel-type knife which could be opened at the desired 
        level to cut vertical slots in the casing.

        2. Bullet perforating equipment was developed in the early
        1930's and has been in continuous and widespread use since that time.

            -The major drawbacks with this   method were that the bullet 
               remained in the perforation tunnel, penetration was not very good,
               and some casings could not be perforated effectively.

         3.       After World War II the Monroe, or shaped – charge,
          principle was adapted to oil well work, and the resulting 
           practice is now commonly referred to as jet perforating.

               -The principle of the shaped charge was developed during 
                World War II fo  armor piercing shells used in bazookas
                to destroy tanks. This new technology allowed the oil 
                producers to have some control over the perforating design 
               (penetration and entry hole size) to optimize productivity.

  2.3. Gun systems

2.3.1. Overview

In order to allow oil and gas to flow into the well, conduits
 need to be made into the formation. To do this, a gun is 
positioned across the producing formation and is detonated
 to create perforations through the casing and cement. 
The guns used for this purpose are known as perforating guns.

2.3.2. Perforating guns are divided into two primary categories:

·        Capsule guns

·        Carrier guns

2.3.3. The perforating gun performance is affected by the

·        Gun size

·        Clearance

·        Entrance hole diameter

·        Shot density

·        Gun phasing

·        Perforating length

·        Temperature rating

After firing the gun and while retrieving, unwanted solids enter 
into the wellbore or formation through perforating tunnels. 
These are called the perforating debris. Perforating debris 
can create problems in highly deviated or horizontal wellbores
 and can also create problems with the completion hardware.

Sources of debris are not only gun system, but also from 
the casing, cement and formation. 

Gun hardware contributing to debris are:

·        Gun body

·        Shaped charge liner slug and jet

·        Shaped charge case

·        Shaped charge retaining system (that holds the charge inside the gun).

2.3.4.1. Shaped charge liner

Perforating debris sources can be controlled if properly engineered.
 Shape charge liner used in deep penetrating charges is made of
 powder metal, which eliminates the carrot and slug associated with 
liner penetration into the formation during charge detonation.

Big hole charges us solid liners in order to produce large 
hole into the casing. However pf4621 power flow liners, 
produce big holes and yet leave no slugs into perforating tunnels,
 this new technology charge can replace the ultrapack charges.

Attempts are made to contain the debris in the gun, 
collect it after perforating or minimize the quantity expelled. 
To address this problem of controlling the debris,

 two methods are used. These are:

·        Zinc casing method

·        Patented packing method 

Additional techniques that contribute to reduced perforating 
debris include powder metal liners and non-plastic charge retention systems.
These recent innovations help in limiting problems arising from
perforation debris.

2.3.4.1.1. SHAPED CHARGE THEORY

The ultimate goal of perforating is to provide adequate productivity. 
Test laboratories evolved over the years to provide means of predicting 
and improving well performance. Today, the performance of the charges
 is determined according to the procedures outlined in the API RP 43 
 (standard procedure for evaluation of well perforators) fifth edition, 
published in 1991. From Figure B1 it can be seen that the penetrating
 power of a cylinder of explosive is greatly increased by a cavity at 
the end opposite to the detonator. Furthermore, placing a thin metallic
 liner in the cavity increases penetration. A typical shaped charge
 consists of four main components: a case, a high order explosive powder,
 primer and a liner, as shown in Figure shown

 The case simply holds all the components together.

- The explosive (RDX, HMX and HNS) is a complex mixture
 designed to allow packing and shipping in the case.

- The primer is a purer mixture of explosive which is more sensitive
to the detonation of the detonating cord.

- The liner is used to form a jet which physically does the perforating.

- The detonating cord, which is initiated by a blasting cap, detonates
 each charge.
The selection of explosive material is based on the well 
temperature and anticipated exposure time at that temperature (Figure B3).
 RDX, HMX and HNS are all explosives used in oil well shaped charge
manufacture. For deep penetrating charges, the liner is made from a mixture
 of powdered metals pressed into the shape of a cone. High precision in
 the pressing operation is required and it must be done in an extremely
 uniform and predictable manner. For Big Hole charges, the liner is
 drawn from a solid sheet of metal into hemispherical, parabolic, 
or more complex shapes. 

For each of the two types of charges, there is a trade-off between 
entrance hole size and penetration. The sequence of events in 
firing is illustrated in Figure B4 from top to bottom. 
The detonator initiates the cord which detonates at a rate 
of approximately 7000 m/s (23,000 ft/sec.) The pressure impulse 
from this detonation initiates the primer in the charge and the
 explosive begins to detonate along the length of the charge. 

The high pressure wave 30x10⁶ kPa, 4,500,000 psi) strikes 
the liner and propels it inward. The liner collapses from 
apex to skirt imparting momentum with a velocity approaching 
2500 m/s (8000 ft/sec). At the point of impact on the axis 
the pressure increases to approximately 50x106 kPa (7,000,000 psi) 
and from this high pressure region, a small amount of material is 
propelled out at velocities in excess of 7000 m/sec (23,000 ft/sec).

As the liner collapses further down the cone, more and more material
 must be propelled by less and less explosive such that the 
impact pressure is substantially less. Thus the tip of this so-called
 jet is travelling 20 times faster than the rear portion and gives the
 elongated shape to the jet. The penetration depth depends on this
stretching action. As the liner walls collapse inward, the resultant 
collision along the axis divides the flow into two parts, as in 
Figure B5. The inner surface of the liner material forms 
the penetrating jet which is squirted out along the charge. 
The outer surface of the liner, which was in contact with the explosive, 
forms a rear jet or slug which moves forward slower than the forward jet.
 In the zone of collision, where division of the material forming the jet
 and slug takes place, there is a neutral point which moves along the axis
 as the liner collapse process continues. The very fast jet impacting 
a casing generates a pressure of approximately 70x106 kPa (10,000,000 psi). 
At this pressure the steel casing flows plastically and the entrance hole is formed.

A similar behavior occurs with formation material as the jet penetrates.
 In addition, crushing and compacting of the formation material around
 the perforation may also occur. The entire process from detonation
 to perforation completion takes from 100 to 300 microseconds.
 The jet material arriving last at the target, making the end of the perforation,
 comes from the skirt or base. As discrete portions of the jet strike at this
 end of the hole, they penetrate, expending their energy in the process. 
Portions of the jet continue the penetration process, until the entire jet is expended.
 The perforation occurs so fast that, essentially, no heat is transferred.
 Indeed, it has been demonstrated that a stack of telephone directories
 can be penetrated without singeing a single page. It follows that no fusing 
of formation material occurs during penetration. However, crushing and 
compacting of formation material is to be expected, and will be reviewed later.

   

2.3.4.1.2. SHAPED CHARGE DESIGN

Liner aspects, such as geometry, angle, material, and distance from base to apex,
 as well as stand off, and explosive density are more important  than the amount
 of explosive (Figure B7a). Only about 20% of the available explosive energy 
goes into the useful jet. It has been proven that properly designed charges
 can out perform poorly designed charges that have twice the explosive load.

 This is important in situations where a higher explosive load causes 
casing damage. Once a charge is designed for entrance hole and 
penetration efficiency, manufacturing quality control and 
consistency become significant in shaped charge performance.
 Perforation efficiency is accomplished with maximum penetration,
 uniform crushed zone, and minimal plugging due to slug debris. 
This is achieved by designing a liner that will provide 
a uniform jet diameter and velocity with little to no deviation from the conical liner axis. For example, it is critical that the liner thickness and density be precise around the cone at any given point away from the apex. Figure B7b is an example of a less desirable jet due to poor quality control