When drilling underbalanced you intentionally keep the hydrostatic weight of you drilling fluid at a lesser value than the known reservoir pressure. This pressure can be naturally attained with a light drilling fluid or it can be induced. This induction can be achieved by adding natural gas, nitrogen or air to the liquid phase of the drilling fluid.
If the underbalanced state is attained naturally or by induction the resulting situation at surface is the same. The influx from formation or induction must be circulated from the well and controlled at surface. Lowering the hydrostatic pressure at surface adds many benefits to a drilling operation:
• The well bore is kept cleaner, the gassified fluid moves up hole more consistently as BHP can be controlled at surface.
• The reservoir is not plugged by drilling debris as it is in a constant state of influx.
• This will increase productivity and decrease fluid losses.
Having the well in a state of influx, assisting the drilling operation, greatly enhances the drilling operation as real time date retrieval is available. This allows the drilling process to be site specific allowing the drilling program to be tailored for the well being drilled and not a compilation of past results and/or mistakes.
UNDERBALANCED DRILLING
October 17, 2002 UNDERBALANCED DRILLING
With the well in a constant state of influx control of the annular flow is very important. This is achieved by using a closed drilling system at surface to control the flow qt surface. The BOP system remains closed at surface and flow and cuttings are directed through pressure control equipment so flowing pressure can be regulated.
- BRENT 72,83 
