Used with permanent type packers to provide isolation between the producing zone and the annular space above the packer when the tubing is located into the packer.

The seal assemblies are designed with external seals on the tubing which pack-off in the polished bore of the packer or a packer extension (used to retain the same packer bore diameter as the tubing). Basic seal units include two seal stacks, but any number of seal units can be screwed together to increase the length of the assemblies.

Types of assemblies

Locator seal assemblies:

The tubing locator seal assembly does not lock into the packer. The tubing string can't be landed in tension except that of its own weight, however, the tubing can be landed with setdown weight. Careful calculation is required to ensure that with maximum shortening the seal remains in the packer bore. Tubing expansion above the original design estimate is liable to cause buckling.

Anchor seal assemblies:

The anchor tubing seal assembly is latched to the packer. the string can be landed in tension to take up expansion during production. helical buckling is prevented if sufficient tension is pulled. The risk of leakage is reduced.

Latching the tubing can produce problems in tubing recovery (deviated holes, corrosive wells, production zone above the packer, solids settlement in annulus) If packer fluid is used with latched tubing, a solids-free fluid should be used.

Stingers:

used with the hydraulically set, single string permanent packer.

Seal nipple assemblies:

normally used as the method of sealing the production tubing at the lowest of two or more seal bore packers, set one above the other in the well bore. Since the seal nipple does not have a locating shoulder, it can pass through the seal bore in the upper packer and seal in the lower packer. It can be spaced out such that as it seals in the lower packer, an anchor seal or locator seal assembly can be landed in the upper packer.

Polished Bore Receptacle (PBR):

This is frequently used for wells where a production liner has been installed whereby the production tubing can be run and located in a seal bore at the top of the liner. It offers a simple and cheap completion technique but the seals may be prone to damage from solids settling out of the fluid in the annulus and to be a safe completion it requires an effective cement bond in the liner lap region to provide fluid isolation.

Installing a moving or dynamic seal assembly is dictated by the predicted maximum string movement and its ability to tolerate the stresses.

For cases where the triaxial stress/tubing movement is small a number of alternatives are feasible:

1. use an anchor seal assembly whereby a static seal is inserted into the packer bore;
2. use production tubing of increased wall thickness or tensile strength and higher strength tubing couplings;
3. if the tubing movement is less than 2 ft, use can be made of a telescopic joint.

If the above alternatives are not viable, or if the predicted tubing movement and stress are too great, a dynamic seal assembly will be required.

Recommendations

The selection is influenced by past equipment records (seal durability and reliability), by the extent to which the running and retrieval of the string is affected by the incorporation of the seal assembly. For example the polished bore receptacle and locator seal assembly both have the advantage of theoretically being easily retrievable, although operators have reported seal sticking after extended periods of installation. However, since it is the moving seal system which has to be run into the seal bore during installation, the possibility of seal damage must be considered.