- Drilling liners are used to isolate lost circulation or abnormally pressured zones to permit deeper drilling.
- Production liners are run instead of a full casing to provide isolation across the production or injection zones.
- The tie-back liner is a section of casing extending upwards from the top of an existing liner to the surface. It may or may not, be cemented in place.
- The scab liner is a section of casing that does not reach the surface. It is used to repair existing damaged casing. It is normally sealed with packers at top and bottom and, in some cases, is also cemented.
- The scab tie-back liner is a section of casing extending from the top of an existing liner but does reach the surface. The scab tie-back liner is normally cemented in place.
Advantages of a liner:
The main advantages of a production liner are: (a) total costs of the production string are reduced, and running and cementing times are reduced; (b) the length of reduced diameter is reduced which allows completing the well with optimum sizes of production tubings. Other advantages include:
- Complete wells with less weight landed on wellheads and surface pipe
- A scab liner tie-back provides heavy wall cemented section through salt sections.
- Permits drilling with tapered drillstring.
- Where rig capacity cannot handle full string; when running heavy 9 5/8″ casing.
- To provide a PBR (Polished Bore Receptacle) completion. This type of completion is recognised to be the best casing to tubing seal system.
- Improved completion flexibility.
- To provide an upper section of casing (tie-back liner) which had seen no drilling.
- For testing in critical areas where open hole testing is not practised.
The disadvantages of a liner are:
(a) possible leak across a liner hanger; and
(b) difficulty in obtaining a good primary cementation due to the narrow annulus between the liner and the hole.