Page 290 - Drilling Technology in Nontechnical Language

Chapter 11 – WELL CONTROL 281

Underbalanced drilling

When a reservoir rock is penetrated by the drill bit, damage almost
inevitably occurs at the exposed rock face. The major cause of this

damage is the interaction between the drilling fluid and the formation.
The higher the overbalance, the greater the pressure tending to force mud
into the formation, and the greater the damage. If this overbalance can be
eliminated, so can much or all of the damage from drilling.
Normally, operations are conducted so that an overbalance always
exists—primary well control is maintained. However, using special

equipment, drilling fluids, and operating techniques, it is possible to drill
with the mud hydrostatic pressure below the formation pore pressure. This

means also that the formation fluids will continually flow into the well
while drilling.
On top of the blowout preventer is added a special tool, called a rotating
control head (fig. 11–14).

Fig. 11–14. Rotating control head
Photo Courtesy of Williams Tool Company Inc., Arkansas.

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