Stage cementing tool, consisting of stage collars and port collars, is placed within any types of casing string to provide an intermediate passage to the annulus (Fig. 1). The collars are generally made of match-grade steel. They may be available in special weight ranges to optimize strength and internal dimensions.

Stage cementing tool is generally used to:
Protect weak formations from excessive hydrostatic pressure
Cement widely separated zones
Reduce mud contamination from cement.

Stage collars are available in both mechanical and hydraulic versions. However, even the mechanical method of opening and closing requires hydraulic force.

Mechanical tools are opened and closed using freefall plugs or pumpdown-closing plugs to select and shift the appropriate internal sleeve(s) (Fig. 2). The lower sleeve initially covers the ports. Once the first stage is complete, the lower sleeve is pumped down to uncover the ports by seating the free-fall (or pump-down) opening plug and applying pressure. The second stage is pumped and the ports are closed again by seating and applying pressure to the larger closing plug. Once closed, the stage collar cannot be reopened. The pressure required to open and close varies with manufacturers, but it is generally between 800 and 1,400 psi [5.5 to 9.7 MPa]. When two-stage collars are used, a special upper stage collar is required, and care should be taken to release the correct plugs in the proper sequence. The ID of the upper stage collar seats must be larger than the lower collar seats. For highly deviated holes, the free-fall dart should be replaced with a pump-down plug. Drilling is required to remove the plugs and aluminum seats.