Primary Barrier
The conventional process of drilling a well maintains the pressure (force per unit area) inside the wellbore higher than the formation pressure. The difference between the hydrostatic pressure in the well and the fluid pressure in the formation being drilled is called overbalance. Overbalance is considered the 'Primary Barrier' in a well.
The primary barrier keeps the well under control by ensuring that the specific gravity of the drilling fluid is more than the formation pressure and is aimed at disallowing the flow of formation fluid into the wellbore. Different companies have different policies for a minimum level of required overbalance as a primary barrier depending on the type of well and expected reservoir fluid.