Oil extraction
Tonnes of crude oil are extracted every day in many oil rigs around the world. What happens to the void left as a result of this?
ARUN MOHAN
Thiruvananthapuram
There is no void on extraction of oil from the reservoir.
Let us see what happens. Oil and natural gas are generated
from marine organisms after their death in the aqueous
medium. The most important condition is that they
should be buried as early as possible, failing which dissolved
oxygen in the sea water decomposes the organisms
and only their skeleton will be deposited.
Therefore, the ideal situation for oil formation is shallow
and calm sea preferably a bay or gulf or a lake. Substantial
settlements, thickness in terms of kilometres, of
sediments along with organisms beneath the earth, undergo
a limited thermal alteration due to geothermal energy
or as a result of different metamorphic processes.
This results in generation of oil and natural gas from the
organisms. As the oil is lighter than water, it moves upwards
and water replaces the space. It keeps on moving
until it comes to the surface of the water or earth.
But there are possibilities that the moving oil gets
trapped in impervious rocks such as clay layers, igneous
rocks in the form of sill or lava. Geologically most of the
sediments are soft, thus undergoing folding and faulting.
The folded domes covered by the impervious rocks trap
the oil and natural gas.
As a lighter material, oil and gas will push the water
down and occupy uppermost region(s) on every addition
of their accumulation within the trap. Depending upon the
depth of this formation, the surrounding water exerts
pressure on the oil and gas.
Therefore, when oil and gas are extracted from these
formations by borewells, they automatically come out to
the surface. If the pressure of water is not sufficient to
push the oil above, then water is pumped down to increase
the pressure in the domes in which oil is trapped.
Unlike mining - excavation of minerals and rocks from
mines - which results in generation of void in the earth
surface or underground, oil and natural gas extraction
does not cause any void space in the earth.
D. GNANA BHARATHI
Scientist
Formerly with Centre for Mining and Fuel Research, Dhanbad, Jharkhand
