For the oil and gas industry, production of the hydrocarbons in a surveyed reservoir is the main goal of a petroleum expedition.
After drilling operations, a quick deliberation will be made whether a well will be prepared for producing hydrocarbons, having fluids injected in them, or for plugging it in for abandonment.
Engineers will take good care in preparing it so that it could deliver a reservoir’s contents safely to the waiting refinery processes in the surface.
Well completion is a process in which a well is prepared for having fluids run within it safely and securely. Transforming a drilled well to a producing one will take steps such as, casing, cementing, perforating and gravel packing.
There are three kinds of well completion that has been used throughout the years as technologies of production have evolved.
1. Openhole Completion
Also known as a “barefoot” completion, openhole well completion is used in thick reservoir sections with relatively competent rock type where engineers are confident the that the well walls will hold and not collapse into itself and into the reservoir.
It depends whether the rock formation surrounding the reservoir is unconsolidated (loosely packed sand or gravel) or consolidated (metamorphosed or cemented together), and the type of rock that exists above the reservoir.
This means that if a collapsible formation is above the reservoir section of interest, then leaving a well open-hole might not be the best idea.
You must also be aware of the type of fluid to be produced, particularly if there is H2S present in it. Some thought must be put in place to avoid a catastrophic incident.
Openhole completions aren’t used as much as other completion types in modern days. Although in the pre 1940s to 1950s, almost all wells were completed openhole. But was quickly phased out because of how its advantages are outweighed by problems and challenges that arise in using this type of completion.
The production casing is set in the cap rock above or just into the top of the pay zone, while the bottom of the hole is left uncased.
With an open configuration, there will be no perforation expenses since the entire pay zone is open to the wellbore. Deepening the well will be relatively easier to do too. But the reservoir will then be more open to contamination. Well control will be difficult, and in time, increasingly more difficult.
2. Uncemented Liner Completion
To overcome the problems of collapsing sands plugging the production system faced in an openhole completion, the early oil producers placed slotted pipe or screens across the openhole section. This screens act as a downhole sand filter.
Sand can be a problem in production operations, it not only becomes a problem in the wellbore, but it also poses problems on equipment in the surface. It can block flow paths, help weaken wellbore and equipment walls, and be a headache in refining processes.
The use of uncemented liners as a method of sand control remains popular today in some areas.
This type of well completion requires no perforation in its finishing. Sand control becomes easier and is adaptable to other sand control techniques. It also permits easier well deepening.
A slotted pipe is run into the openhole and slots are cut small enough that the produced sands bridge off on the opening rather than passing through.
For very fine sands, the slots cannot be cut small enough so wire-wrapped screens or sintered bronze is used. This technique is an effective sand control method in coarse sands with little or no fine particles.
However, the uncemented liner completion is no longer in common use. Sand movement into the wellbore tends to cause permeability impairment.
Blockages happen when fine sands plug the slots or screens. The liner itself can collapse if the formation starts to break because of the poor support this completion can offer.
3. Perforated Completion
This is the most common type of well completion today.
It involves cementing the production casing (or liner) through the pay zone(s), and subsequently providing a flow path through a formation by perforating holes through the casing and cement.
These perforations are designed to penetrate any damaged area around the wellbore and create a clean conduit within the undamaged formation.
In using various depth control techniques, engineers can decide precisely which sections should be perforated and opened to flow.
This avoids undesired fluids (gas, water) that might contaminate the neighboring pay zones, and weak zones that might produce sand, and unproductive sections or shale barriers.
This selectivity is completely dependent on a good cement job and adequate perforating. Tools used for measuring rely on the cement job.
The selectivity also allows a single wellbore to produce several separate reservoirs without having these fluids mix in the flowlines. This is done by setting isolating packers within an unperforated section of the pipe.
This type of well completion provides safer operations. This is because the maneuvrability of the well’s design and increased well control greatly helps secure a producing well from disaster.
In this type of completion, sensitivity to drilling damage is reduced; it can facilitate selective stimulation of the well; and multizone completions can be possible for this type.
This well completion is generally used unless there is a specific to prefer openhole or uncemented liner completions.
Perforated casing or liner completion, however, faces challenges too. The wellbore diameter through the pay zone is restricted. Perforations are small in diameter it could restrict fluid flow unlike that of openhole completions.
Additional costs due to perforation work are involved; additional cement work and rig time; and formation damage caused by cementing and perforation work are common problems with this completion.
These are the three fundamental completion types in preparing a well for the next processes.