CASING AND LINER

When a well is being drilled, exposed formations must be periodically covered and protected by steel pipe. This is done for several reasons — to keep the hole from caving in, to protect the formations being drilled and/or to isolate different geological zones from each other. These protective pipes are called casings and liners. Casing refers to pipe that starts at the surface or mud line and extends down into the borehole. The term liner applies to pipe whose upper end does not reach the surface or mud line but is inside and overlaps the bottom of the last casing or liner. Casing and liners are either totally or partially cemented in place.

Casing. Two, three or more casing strings may be run in a well, with the smaller pipe being run inside the larger sizes, and the smaller ones going deeper than the larger. The “surface casing” is run and cemented at a depth to protect freshwater aquifers and to avoid mud seepage into shallow sand and gravel beds; it might be set at about 2,000 ft. The next string is the “intermediate” casing. It is run and cemented when there’s a need to change the mud to a density that can’t be tolerated by the exposed formations or by the surface casing. Below the intermediate casing may be another string of casing or a liner.

Liners. It may not be necessary, economical or practical to line the entire, already-cased hole all the way to the surface just to protect the lower open hole. This is especially true as the hole nears total depth and becomes smaller. So a liner is run from the bottom of the hole, up into the casing, overlapping it by several hundred feet. Liners are held in place inside the casing by special tools called liner hangers. The practice of running a liner protects the last open hole interval, which often includes the reservoir section.

Date: 2015-02-28; view: 1044