Liners are run on drillpipe and therefore the conventional cementing techniques cannot be used for cementing a liner. Special equipment must be used for cementing these liners.
As with a full string of casing the liner has a float collar and shoe installed. In addition there is a landing collar, positioned about two joints above the float collar (Figure 16). A wiper plug is held on the end of the tailpipe of the running string by shear pins.
The liner is run on drillpipe and the hanger is set at the correct point inside the previous casing string. Mud is circulated to ensure that the liner and the annulus is free from debris, and to condition the mud. Before the cementing operation begins the liner setting tool is backed off to ensure that it can be recovered at the end of the cement job. The cementing procedure is as follows:
1 Pump spacer ahead of cement slurry
2 Pump slurry
3 Release pump down plug
4 Displace cement down the running string and out of the liner into the annulus
5 Continue pumping until the pump down plug lands on the wiper plug.
6 Apply pressure to the pump down plug and shear out the pins on the wiper plug. This releases the wiper plug
7 Both plugs move down the liner until they latch onto landing collar
8 Bump the plugs with 1000 psi pressure
9 Bleed off pressure and check for back flow
Since there is no bottom plug in front of the slurry the wiper plug cleans off debris and mud from the inside of the liner. This material will contaminate the cement immediately ahead of the wiper plug. The spacing between the landing collar and the shoe should be adequate to accommodate this contaminated cement, and thus prevent it from reaching the annulus where it would create a poor cement job around the shoe.
To promote a good cement job, cement in excess of that required to fill the annulus between the liner and the borehole is used. This excess cement will pass up around the liner top and settle on top of the liner running assembly. Once the cement is in place the liner setting tool is quickly picked up out of the liner. With the tail pipe above the liner top the excess cement can be reverse circulated out. The setting tool can then be retrieved.
In practice it is very difficult to obtain a good cement job on a liner. The main reasons for this are:
(a) Minimal annular clearances
A 7″ OD liner run in an 8 1/2″ hole gives a clearance of only 3/4″ (assuming the
liner is perfectly centred). This small clearance means that:
• It is difficult to run the liner (surge pressure)
• High pressure drops occur during circulation (lost circulation problems)
• It is difficult to centralise the liner
• Cement placement is poor (channeling)
(b) Mud contamination
When the cement comes in contact with mud or mud cake it may develop high viscosity. The increased pump pressure required to move this contaminated cement up the annulus may cause formation breakdown. Fluid loss additives must be used to prevent dehydration of the cement which may cause bridging in the annulus.
(c) Lack of pipe movement
Due to risk of sticking the setting tool, most operators want to be free of the liner before cementing begins. By disconnecting the setting tool the liner cannot be moved during the cement job. This lack of movement reduces the efficiency of cement placement. Due to these problems it is often necessary to carry out a remedial squeeze job at the top of the liner (Figure17). It is becoming more common these days to remain latched on top of the liner and rotate the liner whilst the cement is being displaced into position. A special piece of liner running equipment, known as a rotating liner assembly, is used for this purpose.
Institute of Petroleum Engineering, Heriot-Watt University