Perforation in Oil Wells: How a Well Connects to the Reservoir

Part 3 of 10 in the series From Reservoir to Surface: Oil and Gas Production for Beginners. Previous part: Types of Well Completion.

Perforation in an oil well is the process of shooting holes through the steel casing and cement sheath into the reservoir rock, so that oil and gas can flow into the wellbore. It is done with a perforating gun loaded with shaped explosive charges. Perforating is needed in every cemented and perforated completion, which is the most common completion type.

Why Perforation Is Needed

When casing or a liner is cemented across the reservoir, it seals the rock completely. That seal is useful because it isolates zones from each other. But to produce, you must open a controlled path from the reservoir to the well. Perforations create those paths at exactly the depths you choose.

How a Shaped Charge Works

Early perforating guns fired bullets. Today almost all guns use shaped charges, also called jet perforators, a technology developed from armour-piercing weapons in World War II. Each charge has four parts:

• A charge case that holds everything together.
• A main explosive, usually a powerful, fast-reacting type such as RDX.
• A primer that starts the detonation, connected to a detonating cord.
• A conical metal liner that focuses the energy into a jet.

When the charge fires, the detonation collapses the liner into a thin, very fast jet of metal. According to the Heriot-Watt Production Technology course, the jet travels at around 20,000 ft/s and hits the casing with a pressure of about 5 million psi. The whole event takes only a few hundred microseconds.

The jet does not burn or melt its way through. It punches through steel, cement and rock by pure impact pressure, pushing material aside and creating a perforation tunnel.

The Crushed Zone

The rock around the tunnel is crushed and compacted. This crushed zone is roughly half an inch thick, and its permeability can be much lower than the original rock. The same course estimates it at about 20% of the original permeability. This damage adds to the well’s skin factor, so cleaning it is important.

Key Perforation Design Parameters

Shot density

Shot density is the number of shots per foot (spf). It ranges from less than 1 spf up to about 16 spf. Higher densities improve inflow, especially in reservoirs with low vertical permeability or a risk of sand collapse.

Phasing

Phasing is the angle between neighbouring shots around the gun. Common options are:

• 0° (in-line): all shots in one direction. Useful for small guns placed against the casing wall.
• 60°, 90° or 120°: shots spread around the well, giving flow closer to radial.
• 180°: shots in two opposite directions, sometimes aligned with the expected fracture direction.

Penetration and hole size

Deep penetration helps the tunnel reach beyond the damaged zone around the wellbore. A large entrance hole matters more for gravel-packed wells and fracturing. Penetration falls in harder rock and when the gun is far from the casing wall (large gun clearance).

Types of Perforating Guns

Wireline casing guns

Large guns run on electric wireline inside the casing, usually before the tubing is installed. They give deep penetration and large holes. Because the well is perforated with a heavy fluid in the hole (overbalance), debris can be pushed into the perforations.

Through-tubing guns

Small guns run on wireline through the production tubing, with sizes from about 1 3/8 in to 3 1/2 in. Penetration is lower, but the well can be perforated in underbalance, with wellbore pressure below reservoir pressure. The inflow surge immediately cleans debris from the tunnels.

Tubing-conveyed perforating (TCP)

Large guns are run on the bottom of the tubing or drill pipe and fired by a dropped bar, pressure or an electric signal. TCP combines large charges with underbalanced perforating, and very long intervals can be shot in one run. It usually costs more than wireline perforating.

Retrievable, semi-expendable and expendable guns

Hollow carrier guns are retrieved after firing and leave little debris. Expendable guns break into pieces that stay in the well. Semi-expendable guns mount expendable charges on a carrier strip that is pulled out.

Underbalance vs Overbalance Perforating

• Overbalance: wellbore pressure is higher than reservoir pressure. It is safer to control, but debris and fluid enter the perforations.
• Underbalance: wellbore pressure is lower. The sudden inflow cleans the tunnels and usually gives better productivity. Too much underbalance in weak rock can cause sand production.

Safety During Perforating

Perforating uses explosives, so strict rules apply. Radio transmitters and welding may be stopped near an electric gun to avoid stray currents. Depth must be checked carefully, using a collar locator or gamma ray log, so the right interval is shot. As with any operation that opens the well to the reservoir, well control must be in place.

Frequently Asked Questions

What does spf mean in perforating?

Shots per foot, the number of perforations created along each foot of the interval.

Which perforating method gives the best productivity?

It depends on the well, but underbalanced perforating with good penetration usually gives cleaner tunnels and lower skin. TCP and through-tubing guns both allow underbalance.

Can perforations be closed later?

Yes. Unwanted perforations, for example in a zone producing too much water, can be sealed with a cement squeeze or mechanical plugs.

Are open hole wells perforated?

No. Open hole and uncemented screen completions do not need perforating because the rock is already exposed.

Conclusion

Perforation opens a cased well to the reservoir with shaped charges that punch tunnels through steel, cement and rock. Shot density, phasing, penetration, gun type and pressure balance all affect how well the well will produce.

Next in this series: Part 4, Downhole Completion Tools.