Petroleum
occurs in the microscopic pores of sedimentary rocks
that form a reservoir typically,
reservoir rock
consists of sand, sandstone, limestone, or dolomite.
However, not all of the pores in a rock will
contain petroleum
some will be filled with water or brine that
is saturated with minerals.
Both oil and gas have a low specific gravity
relative to water
and will thus float through the more porous
sections of reservoir rock from their source area to the surface unless
restrained by a trap.
A trap is a reservoir that is overlain and
underlain by dense impermeable cap rock
or a zone of
very low or no porosity that restrains migrating hydrocarbon.
Reservoirs vary from being quite small to
covering several thousands of acres,
and range in
thickness from a few inches to hundreds of feet or more.
In general,
petroleum is extracted by drilling wells from an appropriate surface
configuration into the hydrocarbon-bearing
reservoir.
Wells are designed to contain and control all
fluid flow at all times throughout drilling and producing operations.
The number of
wells required is dependent on a combination of technical and economic factors
used to determine the most likely range of
recoverable reserves relative to a range of potential investment alternatives.
There are three
phases for recovering oil from reservoirs:
1. Primary
recovery occurs as wells produce because of natural energy from expansion of
gas and water within the producing formation,
pushing fluids into the well bore and lifting
the fluids to the surface.
2. Secondary
recovery requires energy to be applied to lift fluids to surface
this may be accomplished by injecting gas down
a hole to lift fluids to the surface,
installation of
a subsurface pump, or injecting gas or water into the formation itself.
3. Tertiary
recovery occurs when a means is required to increase fluid mobility within the
reservoir
this may be accomplished by introducing
additional heat into the formation to lower the viscosity and improve its
ability to flow to the well bore.
Production
rates from reservoirs depend on a number of factors,
such as reservoir geometry, reservoir
pressure, reservoir depth, rock type and permeability, fluid saturations and
properties, extent of fracturing,
number of wells
and their locations, and the ratio of the permeability of the formation to the
viscosity of the oil.
The geological
variability of reservoirs means that production profiles differ from field to
field.
Heavy oil
reservoirs can be developed to significant levels of production and maintained
for a period of time by supplementing natural drive force,
while gas
reservoirs normally decline more rapidly.
The primary
production from a reservoir when the driving force is the expansion of oil plus
the solution gas,
will depend on
the pressure being above or below the bubble point.
As long as the
reservoir pressure is above the bubble point,
only oil is produced and the solution factor
of gas in the produced oil remains constant.
When the pressure falls below the bubble
point,
the liberated
gas may be produced into the wells together with the oil, and the produced gas
to oil ratio starts to increase.
After some
time, the driving force is exhausted and the production curve starts to fall.
At the same time, water production may start,
which is unfavorable.
The total
recovery for such reservoirs is small (5–25 %).
Preferentially,
the gas should have remained in the reservoir to maintain the driving force.
This can to some extent be achieved by a well
strategy which allows the liberated gas to migrate away from the production
wells to the top of the reservoir.
In mediaeval
days, oil was collected from seepages and even hand-dug wells,
but before the
nineteenth century had closed, drilling technology, which had already been
developed for salt extraction,
was adapted to the oil industry.
The cable-tool, consisting of no more than a
bullet-shaped weight on the end of a rope which thumped its way into the earth,
was followed by the more efficient rotary rig,
comprising a bit on the end of a rotating shaft, allowing the search to go
deeper.
Great technological progress was made in all
aspects of the operation.
While much
early exploration was undertaken by the so-called wildcatters,
it did not take
long to discover the essential geological controls of source, reservoir, trap
and seal.
At first, petroleum geologists relied on
surface observations to identify promising prospects,
endowed with the rare, right combination of
circumstances,
but before long
they developed geophysical techniques to scan the depths.
Both the
technology and the interpretation became ever more sophisticated,
assisted in
more recent years by massive computing power.
Perhaps the most important development of all
was a
geochemical breakthrough in the 1980s
which
elucidated the conditions for oil generation itself,
making it possible to map accurately where oil
was formed and where it was not.
In
technological terms, a major development was the semi-submersible rig,
mounted on
relatively stable pontoons beneath the wave-base,
which opened up
the continental shelves of the world to exploration,
bringing in new production to replace the
traditional onshore fields that were depleting.
Even more elaborate floating production
facilities later tapped the few Deepwater areas having the necessary geological
conditions to yield oil.
The technical
achievements of installing wellheads on the seabed and developing floating
production facilities have been truly impressive.
The operations are constrained by the limit of
the floating facilities, giving a plateau rather than a peak of production.
Only large
fields are commercially viable, given the high operating costs.
Secondary
recovery techniques, such as water-injection, are also constrained in the
circumstances.
It is even more difficult and expensive to
produce deep water gas.
Deepwater
operations test technology to the limit and there have been occasional
accidents,
including the serious Macondo accident in the
Gulf of Mexico in 2010,
when 11 men lost their lives and widespread
pollution had a serious economic and environmental impact on the US coastline.
The move to the
deep water heralded another wave of optimism,
as economists,
looking at their office atlas, concluded that there were vast oceans about to
deliver a limitless new supply of oil,
but again, the
geological constraints began to manifest themselves,
as it became evident that very special
combination of geological circumstances had to be met.
The deep water
finds off South America, Africa and in the Gulf of Mexico rely on oil generated
in the rifts
that opened as the continents began to move
apart, 150 million years ago.
At first, the
rifts were filled with fresh water to become lakes, resembling those of East
Africa today, but then the sea broke in.
It was subject to a high level of evaporation
under the warm climate of the time,
which led to
the deposition of a thick layer of salt, which sealed the underlying oil.
Later, about 60 million years ago, sands and
clays, which had been deposited at the mouths of rivers on the adjoining
continents,
slumped down the continental slope.
In some areas they were then taken back into
suspension by ocean currents
which winnowed out the fine-grained material
depositing pods of porous sand on the ocean floor.
Still later, structural movements locally
ruptured the salt seal to allow the oil to migrate upwards and collect the pods
of sandstone,
which formed
excellent reservoirs for oil.
The remarkable combination of circumstances is
obvious.
Successful attempts are now being made to
penetrate the salt seal itself
and find what is left beneath it, with some
promising results in Brazil, albeit at a depth of about 5,000 m.
If you want
to learn more about the Primary production you could
do so in my book,
economic
study of oil and gas well drilling.
which is
published on amazon, check it out
https://www.amazon.com/dp/B07BST8YCC
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