Page 268 - Drilling Technology in Nontechnical Language

Chapter 11 – WELL CONTROL 259

at least one of each type as the different characteristics of each are useful
for different operations.

Bag-type preventer (or annular preventer)

A bag preventer contains a large rubber seal, which is circular if

viewed from above and conical if viewed from the side (fig. 11–2). This is
held inside a steel chamber. Below the rubber element is a hydraulically
operated piston with a matching conical shape on top to fit the rubber

underside. As the piston moves up, the rubber element is compressed by
the cover above it and pushed inwards by the cone profile. The element

can distort to allow it to seal around any smooth object in the wellbore,
whether it is a round pipe or tool joint or a square kelly. It can also seal
on the open hole, but the level of rubber element distortion necessary will
seriously shorten the useful life of the element. The rubber seal, called a
packing, is expensive and awkward to replace.

Fig. 11–2. Annular preventer rubber and piston

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