Types of Well Completion Explained

Part 2 of 10 in the series From Reservoir to Surface: Oil and Gas Production for Beginners. Previous part: The Oil and Gas Production Process.

Well completion is the set of operations and equipment that turns a drilled hole into a well that can produce oil or gas safely, or inject fluids. It decides how the reservoir connects to the wellbore and how fluids travel up to the surface. Choosing the right type of completion affects production rate, well safety, maintenance cost and how well the reservoir can be managed over its life.

What a Completion Must Achieve

Whatever its design, a completion should:

• Give the best possible production (or injection) performance.
• Keep the well safe, with barriers that can shut it in during an emergency.
• Stay reliable for as long as possible without costly repairs (workovers).
• Keep the total cost per barrel produced as low as possible.
• Meet special needs of the reservoir, such as controlling sand.

Completion design is a team effort between drilling engineers, reservoir engineers and production engineers. It is also planned early, because the completion affects hole size and casing design.

Types of Well Completion by Bottom Hole Technique

The first decision is how the reservoir section itself is completed. There are three main options.

1. Open hole (barefoot) completion

The reservoir section is left uncased after drilling. Casing is set and cemented just above the pay zone, and fluids flow straight from the rock into the open hole.

• Advantages: low cost, quick, and the whole reservoir face is open to flow.
• Disadvantages: almost no control over which zone produces. If water or gas breaks through, it is hard to shut off. The hole can collapse or produce sand in weak rock.
• Used in: strong, consolidated formations, naturally fractured reservoirs, some horizontal and multilateral wells, and low-cost developments.

2. Screen or pre-slotted liner completion

A slotted pipe or a wire-wrapped screen is hung across the open reservoir section without cement. Its main job is to hold back sand while letting fluids pass. It also keeps the hole open in high-angle wells.

• Advantages: cheaper than full casing, cementing and perforating, and gives basic sand control.
• Disadvantages: still no zonal isolation, and screens can plug with fine particles, which reduces production.

3. Cemented and perforated casing or liner

Casing (run to surface) or a liner (hung from the previous casing) is cemented across the reservoir. The well is then perforated with explosive charges to create flow paths through the steel and cement into the rock.

• Advantages: you choose exactly which intervals to open, you can isolate water or gas zones later, and you can treat zones separately.
• Disadvantages: higher cost and more rig time. It depends on a good primary cement job; poor cement allows fluid to move behind the pipe.

Because of the control it offers, the cemented and perforated completion is the most widely used method when good reservoir management is needed. Learn more about the cementing step in Introduction to Casing and Cementing.

Types of Completion by Flow Path

The second decision is how fluids travel from the bottom of the well to the surface.

Tubingless (casing flow)

Fluids flow up the casing itself. It is simple and cheap, but only suits very high-rate wells. The casing is exposed to corrosion and erosion, and the well is hard to kill safely for repairs.

Tubing and annulus flow

Production tubing is installed and fluids flow up both the tubing and the space around it (the annulus). This allows high rates and lets operators circulate kill fluid deep in the well.

Tubing flow without a packer

Fluids flow only up the tubing, but the annulus is left open at the bottom. In wells producing below the bubble point, gas collects in the annulus and is released in slugs. This unstable flow is called annulus heading.

Tubing flow with a packer

A packer seals the annulus just above the reservoir, so fluids can only enter the tubing. This protects the casing and gives stable flow. It is by far the most common configuration.

Single, Dual and Multiple Zone Completions

• Single completion: one tubing string produces one zone. Simple and reliable.
• Dual completion: two tubing strings, or one string plus the annulus, produce two zones separately. Packers isolate the zones.
• Commingled completion: several zones flow together into one string. It is simpler, but measuring each zone’s contribution is harder.
• Intelligent completion: downhole valves and gauges let engineers control and monitor each zone from surface.

Other Ways to Classify Completions

• By location: land, platform or subsea. Subsea wells have the tree on the seabed and are expensive to enter, so reliability matters even more.
• By lift method: natural flow completions versus completions designed for a pump or gas lift.
• By sand control: completions with gravel packs, frac packs or standalone screens for weak sandstones.

How Engineers Choose a Completion

The choice depends on:

• Rock strength and the risk of sand production.
• The number of zones and whether they need separate control.
• Expected water and gas breakthrough.
• Fluid type, including corrosive H₂S or CO₂.
• Expected rate and the tubing size needed. Tubing that is too small causes high pressure losses.
• Formation damage risk. Damage near the wellbore, measured by the skin factor, can greatly reduce production.
• Cost, location and future workover needs.

Frequently Asked Questions

What is the most common type of well completion?

The cemented and perforated casing or liner, with production tubing and a packer, is the most widely used because it gives good zonal control and well safety.

What is a barefoot completion?

Another name for an open hole completion, where the reservoir section has no casing or liner.

What is the difference between a liner and casing?

Casing runs from the bottom of its section all the way to the wellhead. A liner hangs from the bottom of the previous casing string, so it uses less steel.

Is completion part of drilling?

It usually follows drilling and often uses the same rig, but it is a separate phase with its own design and equipment.

Conclusion

Well completions are classified mainly by how the reservoir is completed (open hole, screen or liner, or cemented and perforated) and by the flow path to surface (casing flow, tubing flow, with or without a packer). Most modern wells use a cemented and perforated liner with tubing and a packer because it combines safety and control.

Next in this series: Part 3, Perforation in Oil Wells.