In well drilling, particularly for oil or gas, casing is cemented in the well to seal off the formation. For pressure control and the protection of fresh water formations, the well will have multiple strings of casing, each string of casing being smaller in diameter than the one above. In one technique, each string of casing extends completely to the surface where it is supported at a wellhead by a casing hanger. In another technique, liners are employed. A liner is a string of casing that will overlap the next upward string of casing, but will not extend completely to the surface. This avoids additional costs of several strings of concentric casing in the shallower portions of the well.
The upper end of the liner has to be remotely attached by a liner hanger to the casing, normally near the lower end of the casing. When installing a liner, normally a special section of pipe, called a mandrel, will be attached to the upper end of the liner. The mandrel will be machined to accept the liner hanger assembly. Because the mandrel must have a performance rating equal to the pipe of the liner, normally the customer will supply for machining a section of the actual liner to be used. A running tool will engage the mandrel, and the entire assembly will be lowered into the well. After the liner reaches bottom, the operator will actuate the liner hanger to secure the mandrel to the casing.
The operator will cement the liner into the well, in some cases before actuating the liner hanger, on other cases after. Usually a cement bushing will be employed with the running tool to locate inside the mandrel. This cement bushing engages the mandrel to support the running tool and drill pipe against an upward force that occurs while pumping the cement down the drill pipe. After the cement has set and the liner hanger has been set, the operator withdraws the running tool and cement bushing. The mandrel and liner hanger assembly will remain in the well.
There are many different types of liner hangers and different techniques employed for setting liner hangers. In one technique, hydraulic force is used to set the liner hanger. The hydraulic force may be supplied by pumps on the rig pumping down the drill pipe. The flow must be diverted through a fluid port in the running tool and the mandrel to apply the fluid pressure to the liner hanger on the exterior of the mandrel. The flow is diverted by dropping a ball or dart from the surface, which opens and closes various passages once seated. A considerable amount of time is required for the ball to reach the running tool, often several hours. During this time period, the operator is normally unable to circulate fluid through the liner. If the liner has not already been cemented in place, keeping the liner stationary in an open hole without the ability to circulate can be dangerous.
In another hydraulic type, hydraulic force is supplied by a hydraulic chamber exposed to wellbore fluids. The operator must raise and lower the drill pipe to provide the increased pressure. This system may prevent the operator from raising and lowering the drill pipe prior to setting for other purposes.
Other liner hangers are operated mechanically, which involves rotating and moving the drill pipe axially to actuate the liner hanger. For example, J-pins and J-slots may be employed which, when rotated, move the liner hanger to different positions. In highly deviated wells, it is difficult to rotate the drill pipe. Other mechanical running tools do not have the ability for the operator to stroke the liner up and down after the liner reaches bottom without causing setting of the hanger. As a result, the operator may be unable to reciprocate the liner during cementing, which often is a good procedure to follow.
The requirement of specially prepared mandrels is a disadvantage. The machining is done at a shop, not in the field. Consequently, delays may occur, and it is costly to ship the liner section to a shop and back to a wellsite. Moreover, in some costly corrosion-resisting casing strings, machining may damage the properties of the mandrel. Inspections are necessary to make sure that the machining operations have not harmed the metal of the pipe.
Additionally, in very long liners it may be desired to spread the load support by the liner hanger to the casing at various points along the overlapped region. Liner hangers used, however, support the liner only at the top of the liner. Other liner hanger systems and running tools may not have the ability to automatically fill the drill pipe while the liner is being run. The customer may prefer to have the drill pipe filled while being run. If so, then the customer must manually fill it from the drill rig, which takes additionally time.