What is Secondary Well Control?

Primary Well Control is the hydrostatic pressure
in the wellbore that prevents reservoir influx while performing drilling
operations (drilling, tripping, running casing/completion, etc).
When primary well control is failed,  
Then,  this situation needs special equipment which is called
“Blow Out Preventer” or BOP to control kick.

We can call that
“Blow Out Preventer” or BOP is Secondary Well Control.
Please also remember that BOP must be used with specific
procedures to control kick such as Driller method, Wait and weight,
Volumetric, Lubricate & bleed and Bull heading.

Without well control practices for using BOP’s,
it will just be only heavy equipment on the rig.
There are several types of “Blow Out Preventer” (BOP)
which have different applications.