A liner (other than slotted liners) is a string of casing which does not extend all the way to the surface. Liners are installed to permit deeper drilling, to separate the productive zones from the other reservoir formations or for testing purposes. Usually cemented up to the top of the liner.
- Usually cemented over its entire length to ensure a seal with the previous casing string.
- It is important to ensure that the liner overlap has a good seal.
- In cases of suspected cement seal quality a mechanical seal (liner packer), should be installed.
Additional distinction between drilling and production liners
Drilling liners are set:
- to provide a deeper and hence a stronger shoe;
- to keep the hole open from unstable formations;
- to achieve a drilling casing at low cost;
- because of rig limitations on tensional loads;
- to minimise the effect of a reduced internal diameter on drilling hydraulics.
Production liners are set:
- to achieve a production casing at low cost;
- because of rig limitations on tensional loads;
- to allow the installation of a larger flow conduit.
Either type of liner may subsequently be tied back to surface with a string of casing stabbed into the top of the liner