Well Control Training - Primary and Secondary
- A Categories Well Control Training extract, taken from the Fearnley Procter Well Control course manual.
The function of Well Control can be conveniently sub-divided into two main categories, namely Primary Well Control and Secondary Well Control. These categories are briefly described in the following paragraphs.
Primary Well Control (hydmud > Pf)
This is the maintenance of sufficient hydrostatic head of fluid in the wellbore to balance the pressure exerted by the fluids in the formation being drilled.
However, it should be noted that balancing formation pressure is a theoretical minimum requirement; good drilling practice dictates that a sufficient excess of hydrostatic head over formation pressure, be maintained at all times to allow for contingencies. This overbalance is generally referred to as ‘Trip Margin’ or ‘Safety Margin’.
Secondary Well Control (Hmud < Pf)
If for any reason the effective head in the wellbore should fall below formation pressure, an influx of formation fluid (kick) into the wellbore would occur, depending on permeability. If this situation occurs, the Blowout Preventers (BOPs) must be closed as quickly as possible to prevent, or reduce the loss of mud from the well.
The purpose of Secondary Well Control is to rectify the situation by either:
- Allowing the invading fluid to vent harmlessly at the surface, or
- Closing the well in - i.e. providing a surface pressure to restore the balance between pressures inside and outside the wellbore
This latter procedure prevents any further influx of formation fluid and allows any one of a variety of ‘Kick Removal’ methods to be applied thus restoring a sufficient hydrostatic head of fluid in the wellbore. This re-establishes the preferred situation of Primary Well Control.