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When oil was struck for the first time in the Forties Field
under the North Sea in t969 it led to the discovery of at
least 350 million tons of oil. But how is oit actually won
from beneath the sea or the and? Oil wells are drilled with
special cutting tools, known as drill bits which spin around
to chip away at the rock.The steel, or diamond-studded steel
drill bit is at the end of a strong steel pipe called the
drrotated either by a motor at the surface or by a turbine
down the hoie
The rock chippings are carried upwards and out of the hole
by pimping a matenal known as "mud" down through
the drill string. It is not real mud, but a combination of
chemicals and water which brings up the chippings and prevents
the drill bit from becoming too hot from friction
As the hole gets deeper, fresh sections of drill string have
to be added, usually in 3Dft (9mt lengths At the top of the
drill string is the kelly, which fits into a rotating table
on the floor of the drilling derrick, like a nut ti~ a spanner
To fit a new section, the drill string is lifted up enough
to remove the kelly, then the new section e~ attached to the
top of the drill string before the kelly is replaced, allowing
drilling to continue
From time to time, every few hours, or every few days, depending
on the rock the bit itself has to be replace. Then the entire
drill string has to be pulled up, separated into 9Oft (27m)
stands, each consisting of three lengths, and stacked vertically
on the derrick. When the bit finally emerges and is replaced
with a new one, the whole string has to be resembled and lowered
down the hole again. The process is known as a round trip
can take up to hours if the well is already deep. To prevent
the hole caving the hole caving in, it is lined with casing,
heavy steel pipes are lowered in as drilling proceeds and
cement is pumped around them to fix them in place.
The casing gets progressively narrowei as the well deepens.
A 15,000ft (45m) well may have 30 in (760mm) diameter casing
at the surface decreasing in steps to 7in (180mm) at the bottom.
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If
the drill strikes oil, the weight of the mud ensures that
the cii cannot escape, but there is an addiitional safeguard
a special valve called a blowout preventer is fix,ed to
the top of the casing .The rate at which a well is drilled
depends entirely on the type of rock. It can as slow as
l2in (300mm); an hour in the impervious cap rock, or as
fast as 200ft (60m) in soft, sands rock.
When oil is found, a whole series of production wells
has to he drilled to bring it to the surface
Offshore and in difficult terrain, the first step is to
drill a number of wclls designed to reach all corners
ot the oil-bearing rock. This can be done from a single
derrick by angling the holes to different parts of the
oil tiee. In a large field, several derricks or drilling
platforms may be usen each drilling directionally according
to aso that the whole area is exploited When the production
wells have been drilled and lined with casing, a perforating
gun is lowered down them to drive 4' Explosive charges
through the casing and cement and into the rock beyond,
allowing the oil to get into the wells As the oil is extracted
pressure may be maintained by injecting water or gas into
the reservoir rock to displace the oil towards the production
wells.
Later, electrical or mechanical pumps may ne used. But
even with the help of such techniques it is seldom possible
to extract more than about 30 to 50 percent of the oil
in a field.
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