Primary Well Control

During normal drilling operation, we always keep the wellbore full of drilling fluid (mud). The drilling fluid column in the wellbore exerts a pressure at the bottom of wellbore, called hydrostatic pressure.  Hydrostatic pressure is the pressure acting on wellbore when the mud pumps are off and there is no circulation of fluid in wellbore. 

Also, the compressed formation fluid contained in the formation has a trapped pressure called formation pressure. If the hydrostatic pressure of drilling fluid column is greater than or equal to the formation pressure, the formation fluid cannot enter the wellbore and we say that the well is under control.If the hydrostatic pressure of drilling fluid column falls below the formation pressure, the formation fluid starts flowing into the wellbore. In this situation we say that the wellbore has taken a kick.

Thus, the use of hydrostatic pressure of drilling fluid column to balance the formation fluid pressure is termed as 'PRIMARY WELL CONTROL'.

If the primary well control fails, the well takes a kick.

Not to forget one of the most common question asked in IWCF examination:

Question: What is the primary means of preventing kick from occuring?

a.    Using reducing pump rate and circulating pressure to kill the well.

b.    Use of instrumentation to detect kick in early stage.

c.    Use of Blow Out Preventer (BOP) to control the kick

d.    Using combination of BOP and drilling fluid hydrostatic pressure.

e.    Using drilling fluid hydrostatic pressure to balance the formation pressure.