Completions can be divided into three categories:
- Open hole completions
- Liner completions
- Perforated casing completions
The completion type should be selected based on the reservoir management plan. For example, the size, weight, and grade of the completion will determine the ultimate use of a wellbore.
1. Open Hole Completions
This is the first type of completion that was used in the petroleum industry during the early days. The casing is run as a hole drill till the formation starts to produce gas/oil. At this point, the drilling stopped and was well-produced as an open hole completion. The casing is only run before the producing interval. This type of completion is not widely used these days. However, some examples are found in horizontal wells in the Austin Chalk and in the Devonian shales in Appalachia.
- There is lack of control in an open hole completion. One cannot control the flow of fluids from the reservoir into the wellbore nor the injection profile in an open hole completion.
- Sloughing zones can cave into the wellbore and restrict flow to the surface.
2. Liner completions
Like open hole completions, the casing is set prior to drill production intervals in liner completions. The advantage of a liner completion is that the non-damaging fluid can be used to drill the production interval. There are several types of liner completions that include:
- Slotted liner
- Screen and liner
- Cemented liner
2.1. Slotted liner
The slotted liner completion has the same advantages and disadvantages as open hole completions. However, the slotted liner is hung at the bottom of the casing in the open hole interval to minimize the sloughing of the formation into the wellbore.
2.2. Screen and liner
A screen and liner completion are similar to the slotted liner completion in that a screen and liner are set in the open hole section of the wellbore. However, the gravel is sometimes placed behind the screen. Its pros and cons are the same as for slotted liner completions. The screen and liner completion is used in unconsolidated formations to prevent the movement of formation materials into the wellbore.
2.3. Cemented liner
The cemented liner completion is used when the intermediate casing is set in a well prior to reaching total depth. The cemented liner completion is beneficial because the particular intervals behind the liner can be selectively perforated and testing after drilling. Moreover, this selection allows the control of the production and injection of fluids in those intervals. However, a good cement job for the liner is the major challenge of this type. If a good cement job is obtained, then a cemented liner completion is very similar to a perforated casing completion.
3. Perforated casing completions
This is the most common completion type in the industry. In this type of completion, the well can be drilled and logged to the total depth prior to the running and cementing production casing. The estimation of economic values can be done by obtaining cores and logs of the potential producing interval prior to the completion of the well. The cement job is relatively easy in this completion than a liner completion. The perforated casing completion is quite adaptable to multiple completions and alternate completions.