A liner is a short (usually less than 5000ft) casing string which is suspended from the inside of the previous casing string by a device known as a liner hanger. The liner hanger is attached to the top joint of the casing in the string. The liner hanger consists of a collar which has hydraulically or mechanically set slips (teeth) which, when activated, grip the inside of the previous string of casing. These slips support the weight of the liner and therefore the liner does not have to extend back up to the wellhead. The overlap with the previous casing (liner lap) is usually 200ft – 400ft. Liners may be used as an intermediate string or as a production string.
The advantages of running a liner, as opposed to a full string of casing, are that:
1. A shorter length of casing string is required, and this results in a significant cost reduction;
2. The liner is run on drillpipe, and therefore less rig time is required to run the string;
3. The liner can be rotated during cementing operations. This will significantly improve the mud displacement process and the quality of the cement job.
After the liner has been run and cemented it may be necessary to run a casing string of the same diameter as the liner and connect onto the top of the liner hanger, effectively extending the liner back to surface. The casing string which is latched onto the top of the liner hanger is called a tie-back string. This tie-back string may be required to protect the previous casing string from the pressures that will be encountered when the well is in production.
In addition to being used as part of a production string, liners may also be used as an intermediate string to case off problem zones before reaching the production zone. In this case the liner would be known as a drilling liner (Figure 2). Liners may also be used as a patch over existing casing for repairing damaged casing or for extra protection against corrosion. In this case the liner is known as a stub liner.
Institute of Petroleum Engineering, Heriot-Watt University