Overbalanced Drilling
Overbalanced drilling is conventional way of drilling, where the pressure (force per unit area) in the wellbore while drilling is maintained above the formation pressure. 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 for well control and is maintained to disallow flow of formation fluid into the wellbore. Different companies have different policies for maintaining minimum level of overbalance depending on type of well and expected reservoir fluid.
Excessive overbalance can significantly slow the drilling progress and limit removal of drilled cuttings below the bit. High overbalance coupled with poor mud properties across porous formation can also result in thick mud cake risking differential sticking of drill string. It also risks formation damage across reservoir rock.