Pore Pressure
Pore pressure is the pressure of the fluid inside the pore spaces of the rock. It is also known as Formation pressure. If the pore pressure is higher than 'Normal Hydrostatic Pressure', it is called Abnormal and if it is lower than the normal hydrostatic pressure at a particular depth, it is called Subnormal pressure. Normal hydrostatic pressure is the pressure exerted by a vertical column of water with salinity normal for the specific geographic area.
The specific gravity of drilling fluid is planned such that the hydrostatic pressure created by the column of mud is more than pore pressure at any point in the well. This difference is called over-balance. Every company has safety policies to maintain pre-defined minimum over-balance depending on the type of well and reservoir fluid expected in a well.