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considered what would happen if the bit drilled into a porous and
permeable formation, in which the pressure was so great that the
weight of the mud column could not hold the formation fluids
back? What would happen is that the fluids in the formation would
enter the annulus and start heading for the surface. The first
indication at the surface would be that the mud level would start to
rise in the mud tanks because the mud would be coming out of the
hole faster than it is being pumped back down the drill pipe. This
is called a kick. If proper and decisive action is not taken by the
driller, such a kick could lead to a blowout. When a kick occurs
the driller immediately activates the backup system - the Blowout
Preventor (BOP).

1 - body, 2 - ram replacement cylinder, 3 - hydraulic
cylinder, 4 - lock screw

Figure 4.6 - Blowout Preventor (BOP)

The BOP (Figure 4.4) is a system of powerful hydraulic
rams anchored to the casing which can, at the flick of a switch, be
closed and sealed around the outside of the drill pipe preventing
any further flow of mud or formation fluid up the annulus. When
the flow has stopped, weighting material is added to the mud
making it heavy enough to hold back the formation fluids. When
this is accomplished, drilling resumes.

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