Perforation Fundamentals – Basic Knowledge about Perforation Used in Oil and Gas Industry

Perforation is a special operation to crease an efficient communication path between a wellbore and a reservoir by creating tunnels. The effective paths allow reservoir fluid to flow into the well with minimum pressure loss (less skin as much as possible).

Perforation (Courtesy of Schlumberger)

The process of perforation involves lowering a perforating gun into a wellbore to a planned depth and energizing the gun to be safely fired. When perforating a well, shape charges are fired and then energy from the explosion will create tunnels through casing, and cement and then into a reservoir. Length and diameter of perforation hole are dependent on the objectives which will be discussed later. Figure 1 shows the cross sectional of perforation.

Figure 1 – Perforation Cross Section 

Several factors influencing perforation performance are as follows;

·         Rock properties – compressive strength, fracture pressure

·         Mineral content of the rock metric

·         Tectonic stress and overburden pressure of the reservoir

·         Reservoir pressure and temperature

·         Reservoir fluid

·         Completion fluid

·         Wellbore configurations such as size and grade of casing, wellbore deviation and orientation

Types of Perforation Systems

Three perforation systems which are used in oil and gas industry are casing guns, tubing conveyed guns and through tubing guns.

Casing Gun System (Wireline Convey)

The casing gun system is the oldest perforation technique and it involves running perforation in order to perforate a well before running a completion. Wellbore conditions can be either overbalanced or underbalanced when perforating. Additionally, a wellbore should be neutralized before running the completion because it will minimize formation damage.

Some advantages of a casing gun are listed below;

·         Perforation guns can be run with wireline or electric line in order to get an accurate depth control

·         Larger diameter guns can be utilized.

·         Effective well control

·         Operations are mechanically simple and reliable

Some disadvantages of casing gun are listed below;

·         Takes rig time for perforation

·         Requires rig up equipment on the rig floor

Tubing Conveyed Perforation System (TCP)

For this system, the perforation gun is attached and run with a completion string. This system requires drilling an additional hole called a “sump” in order to accommodate a perforated gun to be dropped and left in the well after a gun is fired.

Some advantages of tubing conveyed technique are listed below;

·         Long reservoir interval can be perforated by one run

·         Larger explosion charges than through tubing system

·         Perforation can be done within the underbalanced condition so formation damage can be minimized.

·         Significant reduction in rig time

Disadvantage of tubing conveyed technique are listed below;

·         Take a long time before perforation charges will be fired

Through Tubing System

 Through tubing perforation allows perforation to be performed with the existing completion string. This system has limitations on size of charge and perforation gun because the guns must be small enough to run into a completion string. Typically, the gun size is smaller than 2-1/8.”

Advantages of through tubing system are as follows;

·         Perforate a well with a completion string

·         Rapid deployment and retrieval by using wireline or electric line units

·         Minimizes loss of production

·         Accurate depth control

·         Reduced cost because no completion retrieval is required

Disadvantages of through tubing system are as follows;

·         Smaller diameter of perforation charges can be used when compared to other perforation system. Therefore, deep of penetration is shallower than others.

·         Length of perforation in one run is limited by surface equipment.