Page 267 - Drilling Technology in Nontechnical Language

258 Drilling Technology in Nontechnical Language Second Edition

The objective now is to restore primary control. Two things are
necessary to do this:

1. Remove all of the influx out of the well

2. Replace the mud in the well with a fluid that is heavy enough to

again exert sufficient hydrostatic pressure to control the downhole
formation pressures with the BOP open

Tertiary control

It sometimes happens that the blowout preventer equipment fails or
the hole starts to allow fluid to leak away into an underground formation.

Secondary control cannot be maintained, and formation fluid again starts

to enter the wellbore. This is now a dangerous situation calling for extreme
measures to restore control. If control is not restored, the end result is a
blowout. Tertiary control has to be applied to try to stop the flow.

Tertiary control involves pumping substances into the wellbore to try

to physically stop the flow downhole. This may involve pumping cement
(with a high risk of having to abandon the well afterwards). However, there
is another method that may be employed, called a barite plug.
A barite plug is set by mixing a heavy slurry of barite in water or diesel
oil. It has to be kept moving while mixing and pumping. Once the slurry
is in position downhole and pumping stops, the barite rapidly settles out to

form an impermeable mass that will hopefully stop the flow of formation

fluid. The main risk is that if pumping stops with the slurry inside the pipe,
barite will settle out in the pipe and plug the drillstring.

Blowout Preventer Stack

When planning and drilling wells, the assumption is made that a kick
is always possible. Even if the well is the 100th drilled in the immediate
area, primary control can still be lost for some reason. This is why blowout
preventers (BOPs) are always used once surface casing has been cemented
in place.

The primary function of the BOP is to form a rapid and reliable seal
around the drillstring or across the empty hole (if no pipe is in the hole)
so as to contain downhole pressures. There are currently two types of
preventer available that allow this seal to be formed. Most BOPs contain

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