One of the primary functions of the drilling mud is the control of subsurface formation pressure. This it achieves in either of two ways:
1. Overbalance Drilling
This is currently the most popular technique in which the drilling mud exerts ahydrostatic pressure on the formation which is greater than the formation pressure.
Thus
PH = PF + POB
Where
PH= Hydrostatic pressure
PF = Formation pressure
POB = Overbalance pressure or simply Overbalance,
If the formation pressure becomes greater than the hydrostatic pressure then formation fluid will flow into the wellbore, a phenomenon known as KICK.
2. Underbalanced Drilling
This is a specialised drilling technique in which the influx of formation fluid into the wellbore can be deliberately controlled to minimise or avoid certain borehole problems such a formation damage, thus the hydrostatic pressure would be designed to be less than the formation pressure. This is a controlled kick in which the volume of fluid flowing and Mixing with the wellbore fluid would be known.
Thus, in this case
PH = PF – POB
Most conventional wells are drilled overbalance but recent developments in drilling technology have witnessed the adoption of underbalanced drilling to improve well productivity.
The regulation of the relationships between these pressures is crucial to a
successful drilling operation.
Pressure can be defined as the force exerted on a unit cross-sectional area.
Pressure Gradient is the pressure exerted per unit length.
