Oil and gas exploration and production (E&P) companies must perform one very important phase after drilling a well to its total depth. They need to prepare the well for production of oil or gas—in other words, to go through the well completion stage of hydrocarbon extraction.

This article will answer the following questions:

What are well completions?
What is the purpose of well completions?
What are the types of well completions?
What are the types of completion components?
Why is well completion a critical stage in oil and gas production?

What is Well Completion?

Well completion is the term used to describe the assembly and installation of a downhole pipe and related equipment so that oil and gas can be efficiently and safely extracted from a well. Well completions refer to everything that takes place in and around an crude oil or gas well between the end of the drilling process and the moment when a well is put into production.

Well completion is the process which connects the producer/driller to the reservoir. Engineers at oilfield services giant Schlumberger say that the well completion stage begins with the drilling into the reservoir because if a company doesn’t do a proper job when drilling into the reservoir, it doesn’t matter how good the well completion is.
What is the Purpose of Well Completions?

After drilling an oil or gas well, the company evaluates the presence of hydrocarbons in place. If it determines that commercially viable volumes of oil and/or gas can be extracted cost-efficiently, it moves to prepare the well for the start of production.

Well completion types, methods, and modes vary, depending on the type of reservoir, the design of the well, or the geology in the area where a well is drilled. The choice, design, and installation of pipes and equipment have significant impact on the productivity of an oil and gas well, therefore, engineers must ensure that the well completion is as safe and efficient as it can be.

Whatever well completion strategy a company chooses, the key driver behind every strategy or well completion mode is to ensure the recovery of the maximum possible volumes of oil at a reasonable cost.

Oil and gas well completions should have the following outcomes in order to be deemed successful:

Well completions should connect the reservoir to the production tubing so that oil and gas can flow through the pipe to the surface, or fluids can be injected into the reservoir.
Well completions should isolate the oil and gas reservoirs to protect the producing zones from non-producing zones in order to avoid interference with the producing reservoir.
Well completions must protect the reservoir’s integrity and reduce damage to the formation.
Well completions should help reduce the resistance to oil and gas flow.
Well completions must ensure that the completion resists corrosion and creep and does not collapse into the reservoir or the wellbore.
Well completions should also ensure a means to detect and assess changes in the reservoir conditions and hydrocarbon flow rate by well tests.
Well completions must get the wells ready for the subsequent stages of oil or gas extraction.

Ultimately, the way in which engineers decide to complete a well will impact the rate of oil and gas production and will directly influence the time over which oil and gas will be pumped from the well.
What Are the Types of Well Completions?

The type of well completions that engineers choose depends on many variables, including the targeted volumes of oil and gas to be produced, the type of fluids that will be pumped, temperatures at the surface and at the downhole, the depth of the production zone, the rate of production, the expected pressure, the location of the well, the surrounding landscape and environment, and the costs and expected returns on investment.

Basing their well completion design on the above-mentioned variables, engineers can opt for a type of well completion that can vary from the most basic and cheapest open-hole completion to complex completions with many horizontal wellbores from a main wellbore, each of which will include a completion.
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Depending on the place where the completion is made, oil and gas well completions are divided into lower completion or downhole completion, and upper completion. The lower completion connects the oil and gas formation with the wellbore. The upper completion is the link between the lower completion and the surface.

At the reservoir (lower) level, there are two main types of well completions—the open-hole completion in which there is no casing placed across the reservoir, and cased completions, or liner completions in which casing or liner are run and cemented across the reservoir zone.

If the well is completed in an open-hole completion manner, the drilling company does not need to perforate the well to get it to produce hydrocarbons. If the driller places a casing, it will need to perforate the well to allow it to connect with the reservoir rock.
Open-hole completions

Open-hole completion is the simplest and cheapest type of oil and gas completion. Open-hole completion, also referred to as barefoot completion, is a completion that does not have casing or liner across the reservoir formation. In open-hole completions, the production casing is set just above the reservoir’s pay zone, the one that contains economically producible oil and/or gas. The bottom of the pay zone is left uncased. This allows the hydrocarbons to flow directly into the wellbore. The open-hole completions allow for maximum exposure to the pay zone.

Although open-hole completions are the least complex and cheapest type of completions, they have some drawbacks which limit their use. Open-hole completions make control over excessive water or gas difficult because water and gas zones cannot be plugged off. Moreover, the sandface—the physical interface between the formation and the wellbore—is not supported in open-hole completions and could collapse.

Remedial work and treatments of the flow and of the reservoir are more difficult when an oil or gas well is an open-hole completion. This type of completion would be generally used for reservoirs that are estimated to have little chance of producing unwanted fluids or sand. These completions also typically target hard and consolidated reservoir rocks.
Cased-hole completions

Cased-hole completions refer to the well completion method in which casing or a liner goes through the pay zone and is subsequently cemented across the reservoir zone. ‘Cased-hole’ as a term can apply to any section of the hole, but in well completions, it typically refers to the techniques applied after a casing or liner has been set across the reservoir zone.

Drillers ensure the connection between the wellbore and the reservoir by perforation, in cased and perforated completions. The perforation in the casing allows for precise positioning and isolation of the different zones in the reservoir and stimulation of different zones in a multi-layer reservoir.
Liner completions

A liner is a type of casing in which the top is suspended from the inside and does not go all the way to the surface. That liner is hung from a liner hanger. Because the liner does not extend to the surface, it allows for more flexibility in the completion design of the upper wellbore by increasing, for example, the diameter if the characteristics of the fluids and flow rates call for larger diameters at the upper end of the wellbore. The design of many conventional wells includes a production liner set across the reservoir zone.

The most commonly used types of liner completions in well completions include a:

slotted liner completion,
screen and liner completion, and
cemented liner completion.

A slotted liner refers to a liner in which slots have been prefabricated. A screen is a liner in which holes have been pre-milled. When placed across the reservoir zone, the slotted liner and the screen do not require additional perforations in order to access the oil or gas formation.

A cemented liner completion has many of the advantages of the cased and perforated completion because it also allows for precise selective perforation at particular intervals. In this way, the driller isolates the layers and can control production in the intervals where the cemented liner is perforated, as well as the injection of fluids into those differentiated zones. The cemented liner makes the initial cementing job more difficult, but if the cementing is properly done, then the cemented liner completion has a lot of the characteristics of the perforated casing completion. The cemented liner completion is typically a lower-cost well completion method than the cased and perforated completion.
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Drillers may also use gravel pack completions in order to prevent the movement of sand from the reservoir into the wellbore or the area near the wellbore. Gravel packing is the typical method of installation of equipment or application of techniques to control sand movement to the wellbore. As part of the gravel packing, a steel screen is inserted in the wellbore, while the surrounding space is packed with gravel so that sand cannot pass into the wellbore. The key goal of the gravel pack method is to stabilize the reservoir formation with the least possible impact on the oil or gas well productivity.
Completion Components

Oil and gas well completions require not only a good well design, but also many components and different types of equipment to ensure the safe and efficient flow of oil and gas from the reservoir to the surface.

The primary completion components, the main elements of a completed oil or gas well, ensure that a type of well completion works the way it was designed to. The type of well completion determines the completion components that will be used in an oil or gas well.

One of the most important components is the production tubing, which is the pipe used as the main tubular to produce fluids from the reservoir or to inject fluids into the reservoir. The characteristics of the production tubing will depend on the size and geometry of the wellbore, on the type of fluids in the reservoir, and on the production characteristics.

The tubing, assembled with other completion components, makes up the production string—the primary conduit through which oil is brought to the surface.

Another very important component in almost every well completion is the packer. The packer is a downhole device typically placed just above the production zone in order to isolate the production string from the inside of the casing or liner. The production packer also helps to anchor and secure the bottom of the production tubing string. Production packers protect the casing from bursting under high pressures and against corrosion from fluids. They prevent the movement of fluids between productive zones in multiple reservoir zones.

Depending on their application, the method of setting, and the possibility to be run and retrieved, packers can have different characteristics. They can be

retrievable mechanical packers
permanent packers
rotation-set packers
hydraulic-set production packers
inflatable packers
compression-set packers
tension-set packers

Apart from the production packer, the downhole completion equipment may also include downhole gauges to measure and record the pressure and temperature at the downhole.

The downhole safety valve (DSV) is part of almost every oil and gas well and acts as a kind of failsafe when an emergency occurs or in case of equipment failure on the surface. DSV are typically heavily scrutinized and subject to rigorous requirements at local, regional, and national levels.

Most completions also include landing nipples placed at strategic predetermined intervals along the completion string to allow the placing of various devices for flow control.

At the upper level, a key completion component is the so-called Christmas tree—the assembly of various valves, pressure gauges, spools, and chokes at the wellhead, the surface end of the wellbore. The Christmas tree contains the main and backup equipment to control production and shut in the wellbore, as well as equipment for safe access for well intervention operations.

From the initial engineering well design to the production tubing and the smallest screw, well completions are the essential stage between drilling and the start of oil and gas production.

By Tom Kool for Oilprice.com