1. n. [Drilling]

A length of pipe used below the surface casing string, but before the production casing is run, to isolate one or more zones of the openhole to enable deepening of the well. There may be several intermediate casing strings. Depending on well conditions, these strings may have higher pressure integrity than the prior casing strings, especially when abnormally pressured formations are expected during the drilling of the next openhole section.

See related terms: casing string,  production casing