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Scientists can find the conditions that are favourable for the accumulation
of oil or gas in a certain area, but the only sure way to know if oil and
gas are there is to drill.
The costs of drilling
Most oil wells are between 900 and 5,000 metres deep, but it is now
possible to drill 8 km (5 miles) below the surface, an achievement made
possible by skilled operators using powerful equipment and advanced technology.
However, the costs of drilling can double or treble when in very deep water,
hostile environments and when high pressure or temperature is encountered.
Drilling is a very expensive activity, with each well costing several
million pounds. Even with today's technology, there is still a low probability
that oil or gas will be found.
The mechanics of drilling
The rock is drilled with a rotating drill bit, similar to those that
are used to drill a hole in wood. The drill bit is attached to a string
of steel pipes, each approximately 9 metres long. The derrick, the structure
that stands above the hole, must be strong, as the drill pipe and bit are
suspended from it. Only a small proportion of the total weight of the drilling
string is allowed to bear on the drill bit. This proportion will vary depending
on the rock formation being drilled. The derrick must also be tall enough
to enable the individual lengths of drill pipe to be added to or removed
from the string.
The drilling process is lubricated and cooled by a carefully constituted mud. This passes down inside the pipes to the drill bit and then returns to the top of the hole between the pipe and the sides of the hole, bearing rock debris with it. This provides the geologists with rock samples to indicate the kind of rock the drill is passing through. The weight of the mud also prevents the escape of oil or gas if it is found.
If test drilling indicates that oil or gas is present, the drilling of further wells is necessary to determine the area of the reservoir and the amount of hydrocarbons trapped.
| Usually the gas or oil is under pressure in the ground. To stop wasteful and dangerous gushes, a set of valves (called a Christmas tree!) is fitted to the well head to control the flow of fluids from the well. |