The function of the well control system is to prevent the uncontrolled flow of formation fluids from the wellbore. When the drillbit enters a permeable formation the pressure in the pore space of the formation may be greater than the hydrostatic pressure exerted by the mud colom. If this is so, formation fluids will enter the wellbore and start displacing mud from the hole. Any influx of formation fluids (oil, gas or water) in the borehole is known as a kick.

The well control system is designed to:
1. Detect a kick
2. Close-in the well at surface
3. Remove the formation fluid which has flowed into the well
4. Make the well safe

Failure to do this results in the uncontrolled flow of fluids – known as a blow-out
– which may cause loss of lives and equipment, damage to the environment and the
loss of oil or gas reserves. Primary well control is achieved by ensuring that the
hydrostatic mud pressure is sufficient to overcome formation pressure. Hydrostatic
pressure is calculated from:
P = 0.052 x MW x TVD
where:
P        =     hydrostatic pressure (psi)
MW   =     mud weight (ppg)
TVD =      vertical height of mud column (ft)

Institute of Petroleum Engineering, Heriot-Watt University

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