Completion is a process of having a well ready for production or injection after it has been drilled to the required depth. Equipment used in well completion ranges from being a simple packer to sophisticated intelligent systems that would ensure the optimum production with little intervention from production engineers. Components of the completion equipment include production packers (various types), liner systems, subsurface flow control equipment (sleeves, nipples, chokes, valves plugs), subsurface safety valves (SSSV) and screens when required. SSSV is the most critical component and subject to extensive testing and inspection. Components of well completion equipment are very specialized and in some cases designed and manufactured to meet specific well requirements.
Key reasons for well completion are:
- Prepare the bottom of the well
- Connect the reservoir section, so the hydrocarbons can be produced
- Isolate the production zone from other areas
- Reservoir integrity
- Access for well intervention and stimulation
Well completion process involves cleaning the well, running production tubing, perforating and stimulating, if required. There are many types of completion systems and methodologies, each geared towards specific reservoir types and conditions. Selection criteria are driven by the following factors and achieving the right balance between costs and anticipated performance is vital.
- Down-hole pressure & temperature
- Operating modes (Natural Flow vs. Artificial Lift)
- Flow rates
- Type of reservoir
- Metallurgy of equipment
- Number of zones
- Well intervention costs ( how expensive it is to re-enter the well, if required)
Completion is broadly divided into following categories:
Open hole completions (barefoot completion) is a completion method whereby casing or liner is set above the production zone and produced fluids flow directly to the open wellbore. It is mainly used in reservoirs, which are well known to engineers, and have less risk. Future remedial works or well treatment can be challenging.
Liner Completion is a type of completion whereby casing is set through the production zone, as in open-hole completion, but a liner package is installed in the production section of the well. Advantages of this method include better sand control, clean out and do not require perforation, in most of the cases.
There are different types of liners used, namely:
- Perforated Liner significantly reduces well completion costs
- Screen & Liner used mainly in formations with soft walls, it is required to prevent any risk of walls caving into the wells and restrict flow of fluids. Installed screens stop the particles entering the wellbore.
- Gravel Packs (special coarse sand) is used to allow hydrocarbons to flow, but leave the sediments out. It works as a simple filtration mechanism.
- Cemented Liner used when the liner is cemented throughout the pay zone or horizontal section of the well. It provides a better isolation control, but higher risk of damage to the reservoir.
Cased Hole completion - is one of the most frequent method used in the industry, but costly. Production casing is run to the required depth and set in the pay zone. Perforating guns are used to perforate the selected areas. Cased hole completions could be single string and dual string (parallel tubing), big bore / monobore.
Multiple completions is a completion methodology which allows completing several intervals in the wellbore, be it vertical or laterals. It also allows producing / closing the areas of the reservoir as may be required. While single completion are mainly used in shallow formations and onshore, where drilling costs are lower, multiple completion predominantly used in a more costly environment. The key advantage of the multiple completion is that production of multiple reservoirs can be achieved, via one wellbore. The major drawback of the multiple completions is the work-over, being very complex and costly.
Slim-hole completion is a completion approach when small diameter wells are drilled due to economic reasons (small potential and well life), technical challenges (too many wells drilled in the same field). A small diameter pipe is set as casing in the wellbore. The same pipe is then used as production tubing when a well starts producing. If the well is planned to be under artificial lift, slim-hole completions are not used.
There is a large variation of completion equipment types, even within the same family of application