Types of Completions

Once a well is drilled to the desired depth and a viable reservoir’s presence is established, the well is completed by running a completion string to flow the reservoir fluid safely from the formation to the surface. The produced fluid is then transferred to the plant for further processing. If it's an injector well, an appropriately designed completion string is run to achieve injection objectives.

Typically, well-completion strings have several components, such as tubing, packers, nipples, downhole valves, sand exclusion systems, downhole safety valves, wellheads, Xmas Tree, etc. Different completion string configurations are used depending on the type of completion and the well's objectives. The completion might use perforating guns to create a communication channel from the reservoir to the well.

A completion string primarily has two parts: Lower Completion and Upper Completion. Lower completion connects the reservoir to the well, whereas upper completion links the lower completion with the surface facilities. Depending on the reservoir type, properties, and objectives, the well can be either an open hole, a cased hole, or a composite completion.

Open hole completion: It is also called Barefoot Completion. In this type of completion, the casing is run and cemented just above the reservoir to avoid cross-flow or communication between the reservoir and any other formation above it. The Open hole completion also called barefoot completion, is usually used in wells that target reservoirs with a limited chance of producing sand or any other unwanted fluid. Given this, wells downdip of the gas cap and up-dip of aquifers in consolidated reservoir rock might be good candidates.

After cementing the casing, the reservoir section is drilled with non-damaging drilling fluid and is left open. The drilling fluid can be weighted with calcium carbonate instead of barite to drill the reservoir section and minimize formation damage. Acid treatment may be done in the open hole before running upper completion to reduce the formation damage and improve wellbore skin. Open-hole completions may or may not have a tubing string. Open-hole completions are also used in horizontal wells where the rock is competent, and the complete section lies within the same layer. Pros and Cons of Open Hole Completions

Cased Hole Completion: Unlike open hole completion, cased hole completion involves running and cementing a production casing or liner across the production zone. The connection between the reservoir and the well for producing formation fluid is made by perforating the production casing. It is the most common type of completion as it reasonably controls fluid flow. Depending on the reservoir rock type, properties, and consolidation, the gravel pack solution can also avoid excessive sand production while producing oil & gas.

The intermediate casing is set at the top of the reservoir section. After cementing the intermediate casing, the reservoir section is drilled. Required cores are cut, and open-hole logs are run to evaluate and identify the formation layers that must be targeted for testing or production. The section is then cased and cemented with a production liner. In specific designs, a full string may also be run. After carrying out the required well integrity test for the liner top packer, perforation guns are run to perforate required zones as under-balanced or over-balanced perforation. Since the cement tends to damage the formation, wellbore stimulation is carried out before running the completion string. Pros and Cons of Cased Hole Completions

Composite Completion: Composite completion combines open and cased hole completion and is used in particular situations. In composite completion, the primary intent is to have an open hole completion, but the reservoir conditions may require combining it with a cased hole completion. One of the scenarios could be a gas cap above the targeted oil reservoir. Setting the casing above the reservoir and completing the well as an open hole would make gas from the gas cap above the oil layer flow with oil production. Setting casing just above the oil zone to completely isolate the gas zone is also challenging. In this scenario, the decision may be to set the casing well into the oil zone and cement it to isolate any annulus communication with the gas zone. The remaining reservoir section will be drilled with the non-damaging drilling fluid to complete the well as an open hole completion. Later, the casing across the top part of the oil reservoir can only be perforated to produce from the entire oil zone, isolating the gas cap.