The intermediate string is a string which is used to isolate weak formations, to case off loss zones, sloughing, caving and reservoir formations. Such strings are also set in transition zones to abnormal formation pressure to provide blowout protection by upgrading the strength of the well.
Cement fill is required to shut off hydrocarbon zones and flowing salt sections but not necessarily into the surface string.
- ensure adequate blowout protection for drilling
- isolate formations or deeper hole profile changes that can cause drilling problems.
- recommended whenever there is a chance of encountering an influx that could cause breakdown at the previous casing shoe, and/or severe losses in the open hole section.
- nearly always set in the transition zone above or below significant overpressures, and in any potential cap rock below a severe loss zone.
- it is good practice when appraising untested, deeper horizons, to case off the known hydrocarbon intervals as a contingency against the possibility of encountering a loss circulation zone. This advice applies primarily to massive reservoir sections rather than sand-shale sequences with numerous small reservoirs and sub-reservoirs.
- may also be set to shut off a swelling shale, a brittle caving shale, a creeping salt, an over-pressured permeable stringer, a build-up or drop-off section, a high-permeability sand or partly depleted reservoir that causes differential sticking.
- design the well to combine many of these objectives in a single casing point.
- a liner may be used instead of a full intermediate casing
- difficult wells may contain several intermediate casing strings and/or liners.