If due to any reason the hydrostatic pressure exerted by drilling fluid column falls below the formation fluid pressure, the formation fluid starts flowing into the wellbore. In such case we say that 'the primary control is lost' and the well has taken a kick.
What next?
Once the kick is detected, the Driller should immediately close the well using his company's well control policy. In such cases the well can be closed by closing Blow Our Preventer (BOP).
Thus, 'SECONDARY WELL CONTROL' is defined as the use of BOP and well control procedures to control the kick when the primary well control is lost.
The figure below shows a case where the wellbore has taken a kick and the primary well control is lost. In the second figure, the secondary well control is used and the well has been closed using BOP.
Remember, the ultimate aim of secondary well control is to somehow re-establish primary well control.
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