Liners
Liners are the pipes that do not usually reach the surface, but are suspended from the bottom of the next largest casing string. Usually, they are set to seal off troublesome sections of the well or through the producing zones for economic reasons. Basic liner assemblies currently in use are shown in Fig. 1.2. these
include: drilling liner, production liner, tie-back liner, scab liner, and scab tie- back liner (Brown — Hughes Co., 1984).
TIE BACK
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SCAB TIE BACK LINER
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SCAB LINER |
(a) LINER (b) TIE BACK LINER (c) SCAB LINER (d) SCAB-TIE BACK LINER
Fig. 1.2 : Basic liner system. (After Brown — Hughes Co., 1984.)
Drilling liner: Drilling liner is a section of casing that is suspended from the existing casing (surface or intermediate casing). In most cases, it extends downward into the openhole and overlaps the existing casing by 200 to 400 ft. It is used to isolate abnormal formation pressure, lost circulation zones, heaving shales and salt sections, and to permit drilling below these zones without having well problems.
Production liner: Production liner is run instead of full casing to provide isolation across the production or injection zones. In this case, intermediate casing or drilling liner becomes part of the completion string.
Tie-back liner: Tie-back liner is a section of casing extending upwards from the top of the existing liner to the surface. This pipe is connected to the top of the liner (Fig. 1.2(b)) with a specially designed connector. Production liner with tie-back liner assembly is most advantageous when exploratory drilling below the productive interval is planned. It also gives rise to low hanging-weights in the upper part of the well.
Scab liner: Scab liner is a section of casing used to repair existing damaged casing. It may be cemented or sealed with packers at the top and bottom (Fig. 1.2(c)).
Scab tie-back liner: This is a section of casing extending upwards from the existing liner, but which does not reach the surface and is normally cemented in place. Scab tie-back liners are commonly used with cemented heavy-wall casing to isolate salt sections in deeper portions of the well.
The major advantages of liners are that the reduced length and smaller diameter of the casing results in a more economical casing design than would otherwise be possible and they reduce the necessary suspending capacity of the drilling rig. However, possible leaks across the liner hanger and the difficulty in obtaining a good primary cement job due to the narrow annulus must be taken into consideration in a combination string with an intermediate casing and a liner.