Liner Cementing
A liner is assembled in the same manner as conventional casing and lowered into the well, including float equipment and centralizers. A liner hanger is placed at the top of the liner to enable its connection to the larger casing string from which it will be hung. A liner is hung under tension, which prevents it from buckling under its own weight.
The liner is usually run into the well on drill pipe, using a retrievable setting tool that is removed with the drill pipe once the liner has been cemented in place.
Mud cake and gelled mud removal, always a priority in cementing, can be particularly difficult in the case of liner cementing due to what are commonly smaller annular spaces. For example, a 5-inch OD liner hung from a 7-inch casing will have a maximum clearance of 9/16 inch (1.4 cm) if perfectly centered. Special small clearance centralizers that also permit liner rotation during cementing are used to help alleviate eccentric liners.
Liner movement during cementing is generally considered to be critical, despite concerns about premature unlatching from the setting tool, casing parting, swabbing of the pay zone, or wellbore sloughing and annulus bridging.
Also, adequate volumes of washes and spacers are even more important in liner cementing than with conventional casing strings. Smaller annular spaces make it easier to achieve turbulent flow at lower pump rates, helping to support efficient mud removal ahead of the slurry.
Liner Cementing Procedure
A liner cementing head is installed on the top of the drill pipe with a manifold for holding pumpdown darts and ball, analogous to the wiper plugs pumped down the casing and held in the casing cementing head (Figure 1). As with conventional casing, a top drive cementing head system can also be used to enable pipe movement during cementing.
Drilling mud is conditioned and circulated as with any cementing procedure. After the cementing lines are rigged up and tested, the preliminary chemical wash and spacer is pumped down the drill pipe (Figure 2). A bottom wiper plug may or may not be used ahead of the spacer or slurry. After the slurry is mixed and pumped into the drill pipe, a pumpdown dart is dropped and displaced with mud. The dart passes through the setting tool and latches into the hanger (or the previously dropped wiper plug), after which pumping pressure rises to about 1,200 psi (8.4 MPa). The pressure rise is an indication that the drill pipe has been displaced and the liner is full of cement slurry.
The pressure causes shear pins to break, releasing the pumpdown plug to continue through the setting tool and down through the liner. When the internal volume of the liner has been displaced and the cement slurry is all within the liner/borehole annulus, the plug seats in the float collar and the pressure rises again, indicating that the job is completed. Once the job is completed, the setting tool and drill pipe are pulled out of the hole, leaving the cement to cure.
