Contrary to the
image portrayed in old movies, drillers do their best to avoid uncontrolled
releases, known as gushers or blowouts. Blowouts waste valuable resources and
often damage the environment. Also, some may release foul-smelling sour gas
containing toxic hydrogen sulphide, which would be a major hazard for workers,
nearby populations and the environment. Blowouts can be enormously expensive to
bring under control. Crews are trained to use blowout preventers and drilling
fluid to reduce the frequency and severity of blowouts.
The drill bit
may be several kilometres deep by the time high-pressure gas deposits are
reached. The weight of the drilling fluid can be increased by adding heavy
minerals, such as barite, to the mixture. Drillers adjust the weight so the mud is
heavy enough to hold back gas from entering the hole, but not so heavy that the
mud will penetrate into the reservoir and damage it.
If the reservoir
pressure is higher than the pressure exerted by the mud column, some gas may
enter the wellbore. This is known as a kick and must be controlled to prevent a
blowout. Kicks are detected by sensitive instruments which monitor the mud flow
and composition and the mud tank levels. Drillers control most kicks simply by
managing the mud flow and increasing the weight of the mud.
Petroleum Communication Foundation. Our Petroleum Challenge: Exploring Canada's Oil and Gas Industry, Sixth Edition. Calgary: Petroleum Communication Foundation, 1999. With permission from the Centre for Energy.