What is Kick?

When the bottom hole pressure (BHP) in the wellbore becomes less than the formation pressure, the formation fluid starts flowing from the formation to the wellbore. Such flow of formation fluid from formation to wellbore is termed as kick.

Kick can be either of water, oil or gas. It is a measurable quantity and is expressed in terms of units of volume like litre or barrel.

The figure below shows a wellbore full of mud. The bottom hole pressure at the bottom of wellbore is 1000 psi. At the same time, the wellbore is in contact with a potential formation containing pressurized formation fluid with formation pressure being 1000 psi. The pressurized formation is indicated by green color.

Since the bottom hole pressure is equal to the formation pressure, the pressure is balanced between the wellbore and formation, and thus, no formation fluid tries to enter from the formation to the wellbore. Such condition of wellbore is called 'balanced condition'.

Below figure shows a situation where the bottom hole pressure has fallen down to 970 psi. In this situation the formation pressure overrules the bottom hole pressure and the kick enters the wellbore. As more and more kick enters the wellbore, it start filling up the wellbore replacing the mud and thus further reducing the bottom hole pressure of the wellbore.  The further reduced BHP gives rise to more kick.

If driller fails to notice the kick in early stage, the wellbore takes more and more kick. 

Once the driller notices the symptoms (early warning signs) of kick on surface, he can close the BOP and stop further invasion of kick.

Once the BOP is properly closed, the complete wellbore system becomes closed and no more new fluid from the formation enters the system. But this is not the end. The well is now to be killed!

In my next blog we shall discuss on what happens when the kick gets uncontrolled.