Page 265 - Drilling Technology in Nontechnical Language

256 Drilling Technology in Nontechnical Language Second Edition

The well is planned and drilling operations are controlled with the
intention that primary well control is always maintained. The exception to
this is in underbalanced drilling, which is discussed later in this chapter.

When a kick is taken, primary control has been lost for some reason.
There are four main reasons why primary control might be lost during
drilling operations:
1. The well penetrates an overpressured zone with a higher
formation pressure than mud hydrostatic pressure.
2. A weak downhole formation allows sufficient mud to leave

the wellbore that the level of mud in the annulus drops. Since
hydrostatic pressure = gradient × depth (of the fluid column),

if the top of the column drops, hydrostatic pressure along
the wellbore decreases. If it drops far enough, hydrostatic
overbalance on a permeable formation exposed somewhere else
may be lost, allowing fluids to enter the well.

3. The hole is not properly filled when pulling out of the hole. As

steel is pulled out of the well, it has to be replaced by mud. If the
driller does not keep the hole full while pulling out, the mud level
in the annulus will drop, and hydrostatic pressure will reduce.
4. Swabbing operations may also affect primary control. (Swab
pressures were described in chapter 7.) If the drillstring is pulled

up with sufficient speed, the reduction in pressure at the bottom

can be enough to allow formation fluids to enter the well.

Secondary well control

If primary control is lost and formation fluids start to flow into the

well, secondary well control is initiated by closing the blowout preventers
to seal off the annulus. This stops mud leaving the well at the surface.

As fluid enters the well from the kicking formation, pressure in the well
will increase until the total pressure exerted by the mud on the kicking
formation equals the formation pore pressure. The pressure exerted by the
mud equals mud hydrostatic pressure plus the surface pressure held by
the BOP:
Mud hydrostatic pressure + Surface pressure =
Formation pore pressure

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