In some cases, a device called a circulating (or sliding) sleeve is placed in the producing string. A wireline tool or a surface mechanism is used to manipulate the sliding sleeves to open or close ports between the tubing and annulus. An annulus is the open area between the outside diameter of an inner string and the inside diameter of an outer string. The circulating sleeve can be used to selectively turn on and off production from several levels in a borehole.
The development of production technology has resulted in different types of production strings:
� Multilateral completions - are similar to multiple zone completions, and use the same wellbore to produce from multiple deviated or horizontal wellbores in different zones.
