The completion will affect the entire well design, especially the casing design. The completion proposed must be considered for all stages of the well’s lifecycle: running the completion, pressure testing, production, stimulation, workover, and abandonment.
Refer to the requirements of the well proposal in regard to what we need to know about the completion.
Preparation for the completion. There may be work required after the production casing or liner are cemented and before the completion is run. This work may just be a bit and scraper run or it may be necessary to install packers, perforate, and gravel pack, etc.
The following preparations may affect the production casing and/or liner string, including the cement:
- Perforated intervals require high-compressive strength cement (2000 psi is recommended) and a competent (360° coverage) sheath for zonal isolation. If it is a gas well, gas-blocking additives may be called for. Where future recompletions on other zones are anticipated, these intervals also need to have carefully tailored cement. Wells with bottom hole static temperatures above approximately 230°F require silica flour in the cement for long-term temperature stability.
- The sump required below the bottom perforation (e.g., to drop guns after perforating) will affect the final TD. Below the sump will be the shoetrack (normally two casing joints plus float equipment) and a pocket below the shoe.
- Fluid gradients, temperatures, and potential surface pressures will dictate the strength of the casing required for any treatments carried out before the completion is run.
- Permanent packers set in the well will work over a range of casing sizes and weights. The correct packer must therefore be used. If the packer is only available to fit a certain casing’s inside diameter (ID), it may affect the choice of casing. Where heavier wall casing (smaller ID) is run higher up, the packer will have to clear through the smaller ID when run and there must be sufficient clearance. Special drift casing could also be used.
- If the completion is to sting into the liner polished bore receptacle (PBR) then the liner must incorporate a PBR and generally will require a polishing mill to be run before the completion. This could be combined with a bit and scraper run.
- It is essential that the liner lap seals. Liner hangers can incorporate integral packers, which are set after cementing; this may save time compared to running a tieback packer and can isolate the formation from well pressures while the cement is still fluid (e.g., when reversing out excess cement).
Other preparatory work that may affect the well design apart from the casings includes:
- Completion fluid characteristics may be dictated by the type of perforations, reservoir physical characteristics, and reservoir fluids chemistry.
Running the completion. The following circumstances may affect the well design:
- Tubing accessories outside diameters (ODs) (such as SSSV nipples, side pocket mandrels, packers, etc.) may dictate the possible range of casing IDs. In some cases a tapered casing string is required; for instance if a 7 in completion is run in 95 /8 in casing, the SSSV nipple may be too large for the 95 /8 in casing ID. It may be possible to run 103 /4 in casing higher up, swaged down to 95 /8 in below the SSSV depth. Of course this introduces further complications for running and cementing.
If a dual completion is run, the sizes of tubings, collars, and accessories must be carefully checked to ensure that sufficient clearance exists inside the production casings. Remember that the strings will move relative to each other during running as the telescopic joints take up the differences in joints run. Thus, tubing accessories may move opposite collars on the other string.
In high-angle wells, the maximum practical deviation for cable or wireline tools is about 60°. This may necessitate alternative strategies such as using coiled tubing or, if possible, pump down tools or setting nipples higher up in the well.
The type of completion will also dictate what kind of wellhead system to use and how it is to be configured.
Pressure testing the completion.
- Fluid gradients, temperatures, and potential surface pressures will dictate the minimum strength of the casing required during pressure testing. Tubing and packer leaks must also be considered in terms of where the pressures may be exerted and whether in collapse or burst. Temperature correction factors (TCF) are needed in hotter wells. (Note: TCF at 200°C = 0.81 for Nippon steels!)
- In deviated wells, consider the potential for casing wear and the effect on the pressure rating of the casing. Burst strength will be determined by the thinnest part of the casing wall.
- Fluid gradients, temperatures, and potential surface pressures will dictate the strength of the casing required during production. Tubing and packer leaks must also be considered in terms of where the pressures may be exerted and whether in collapse or burst. Temperature correction factors are needed in hotter wells.
In deviated wells, consider the potential for casing wear and the effect on the pressure rating of the casing.
Produced fluids and temperatures could affect the grade of casings used.
Produced fluids could affect the completion fluid chemistry.
Stimulation including gas lift.
- Fluid gradients, temperatures, and potential surface pressures will dictate the strength of the casing required for any treatments car-ried out during stimulations.
- Injection pressures for gaslift must be considered for casing burst; also remember that if the production casing leaks, this pressure will be exerted against the previous casing.
Workovers and recompletions.
- Provision may need to be made for a different completion in the future; for instance as the well depletes it may be desired to run gas lift valves, submersible pumps, or other tools. The casing design will have to account for these future possibilities.
- Where other zones may be produced later on, the casing design will have to ensure that the required zones are accessible (e.g., not behind multiple casings), and that the cement sheath at that depth is high-compressive strength and provides good zonal isolation.
Abandonment. Eventually the well will be abandoned. Government regulations may require certain actions to be taken. For example, in Egypt it is required that all annuli have cement between the casing and open hole, even though no permeable zones may be present. This is not currently a requirement in the North Sea. It is important to know these details at the well design stage to avoid unnecessary work in the future.
Restoration of the site after abandonment should also be considered at the well design stage to minimize the expense later on.