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12 THE WELL CONTROL SYSTEM

The well control system prevents the uncontrolled flow of
formation fluids from the well bore. When the bit penetrates a
permeable formation that has a fluid pressure in excess of the
hydrostatic pressure exerted by the drilling fluid, formation fluids
will begin displacing the drilling fluid from the well. The flow of
formation fluids off the well in the presence of drilling fluid is
called a kick. The well control system prevents detecting the kick,
closing the well at the surface, circulating the well under pressure
to remove the formation fluids and increase the mud density,
moving the drill string under pressure and diverting flow from rig
personnel and equipment.
Failure of the well control system results in an uncontrolled
flow of formation fluids and is called a blowout. This is perhaps
the worst disaster that can occur during drilling operations.
Blowouts can cause loss of life, drilling equipment, the well, much
of the oil and gas reserves in the underground reservoirs, and
damage to the environment near the well. Thus, the well control
system is one of the most important systems on the rig.
Kick detection during drilling operations is usually achieved
by the usage of the pit-volume indicator or a flow indicator. Both
devices can detect an increase in the flow of mud returning from
the well over that which is being circulated by the pump.
Pit-volume indicators usually employ floats in each pit that
are connected by means of pneumatic or electrical transducers to a
recording device on the rig floor. The recording device indicates
the volume of all active pits. High- and low-level alarms can be
present to turn on lights and horns when the pit volume increases
or decreases significantly. An increase in surface mud volume
indicates that formation fluids may be entering the well. A
decrease indicates that drilling fluid is being lost to an
underground formation.

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