Perforation

Petroleum Production Engineering

Perforation - Definition

• “Holes shot in the casing or liner, cement and producing formation by bullets or shaped-explosive charges to allow oil and/or gas from a producing zone to flow into the well.”

• Also: PERF or perfs

Perforation - Terms

• Perforation Density:– The perforations are commonly described in shots

per foot known as perforation density.• Phasing:– The angular separation between shots.– Zero phasing has each shot going in the same

direction, whereas 180 o phasing has alternating shots going in opposite directions.

Perforation – Terms (Cont’d…)

• Size• Length• Shape– The shaped explosives can be shaped to give

either maximum perforation or maximum diameter of perforation.

Perforating

• In cased hole completions, the final stage of the completion involves 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.

Perforation - Purposes

• Creating a channel between the pay zone and the wellbore.

• Cause oil and gas to flow to the wellbore easily.

• Future stimulation: for example Hydraulic fracturing

Types of perforation

• Oil Well Perforation may be basically classified in two types1. Over-balanced Perforating , and;2. Under-balanced Perforating

Over-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-Balanced Perforating

• 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.

Perforating Guns

• There are four most common used guns– Casing gun– Expendable gun– Retrievable gun– High-shot density gun

Perforating Guns - Casing gun

• Create holes in a casing string• Casing guns are typically 3” to 5” in diameter

and carry up to six perforating charges per foot.

• Allow perforation through production casing using larger diameter gun assembly.

Perforation Guns – Expendable Gun

• A perforating gun assembly that disintegrates upon firing, creating finer debris.

• It is used where wellbore restrictions allow only limited access, as in through-tubing applications.

• It is relatively light and simple in design with phased expendable guns.

Perforating Guns – Retrievable Guns

• It is retrievable from the wellbore after firing.• It generates minimal debris• Minimum distortion of the gun body to help

ensure easy retrieval.

Perforating Guns – High Shot density Gun

• A perforating gun has more than four shots per foot.

• Improves the phasing, or distribution of perforations, around the wellbore.

• Enables improved distribution of the perforations around the circumference of the casing or liner.

Running Perforating Guns• E-line as it is traditional to use electrical signals from the surface to fire

the guns.

• Coiled tubing (CT) may be used In more highly deviated wells.

• Slickline: Newer technologies allow the guns to be run on slickline. Modern slickline technology embeds fiber optic lines that can transmit two-way data on real-time temperature, pressure and seismic responses along the length of the slickline. This information allows very precise operations of various down hole tools, including perforation guns. 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.

Perforation - Disadvantage

• 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.

• 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.

Where perforations are not done?

• Open hole completion– Casing and perforating as a method of completion

is common place nowadays, though in some unconsolidated formations, prone to production of sand, open hole completions, using only sandscreens, may be the preferred choice.