Heavy oil is highly viscous, “cold syrup” like hydrocarbons which are formed much the same way as the conventional low viscous oil. The oil sands or tar sands are loose sand or partially consolidated sandstone containing naturally occurring mixtures of sand, clay, and water, saturated with a dense and extremely viscous form of petroleum technically referred to as bitumen. In the Athabasca sands in Alberta, Canada there are very large amounts of bitumen covered by little overburden, making surface mining the most efficient method of extracting it. The overburden consists of water-laden muskeg (peat bog) over top of clay and barren sand. The oil sands themselves are typically 40–60 m deep, sitting on top of flat limestone rock.
Due to the high viscosity,
heavy oil is more expensive to extract than conventional oil. The bitumen in
tar sands cannot be pumped from the ground in its natural state.
Therefore, surface mining, requiring large areas, has been the
preferred extraction technique. After excavation, hot water and caustic soda
(NaOH) is added to the sand, and the resulting slurry is piped to the
extraction plant where it is agitated and the oil skimmed from the top.
Provided that the water chemistry is appropriate to allow bitumen to separate
from sand and clay, the combination of hot water and agitation releases bitumen
from the oil sand, and allows small air bubbles to attach to the bitumen
droplets. The bitumen froth floats to the top of separation vessels, and is
further treated to remove residual water and fine solids. About two tons of oil
sands are required to produce one barrel (ca. 1/8 of a ton) of oil. Originally,
roughly 75% of the bitumen was recovered from the sand but today extraction
plants recover well over 90 % of the bitumen in the sand.
Several techniques which restrain from occupying large surface land
areas have also been employed, e.g. Cyclic Steam Stimulation (CSS). In this
method, the well is put through cycles of steam injection, soak, and oil
production. First, steam is injected into a well at a temperature of 300–340 ◦C
for a period of weeks to months. Then, the well is allowed to sit for days to
weeks to allow heat to soak into the formation.
Later, the hot oil is pumped out of the well for a period of weeks
or months. In the Steam assisted gravity drainage (SAGD) process, two parallel
horizontal oil wells are drilled into the formation, one about 4–6 m above the
other. The upper well injects steam, and the lower one collects the heated
crude oil or bitumen that flows out of the formation, along with water from the
condensation of injected steam. The basis of the process is that the injected
steam forms a “steam chamber” that grows vertically and horizontally in the
formation.
The heat from the steam reduces the viscosity of the heavy crude
oil or bitumen which allows it to flow down into the lower wellbore. SAGD has
proved to be a major breakthrough in production technology since it is cheaper
than CSS, allows very high oil production rates, and recovers up to 60 % of the
oil in place.
Several more exotic techniques have also been tried at the tar sand
fields. For example, using solvent instead of steam to separate the bitumen
from the sand, or an in-situ combustion process which ignites oil in the
reservoir and creates a vertical wall of fire moving from the “toe” of the
horizontal well toward the “heel”, which burns the heavier oil components and
upgrades some of the heavy bitumen into lighter oil right in the formation.
The cost of producing one barrel of tar sands or extra heavy oil
amounts to between 30 and 60 Euro2008/bl of oil equivalent, to which transport
costs must be added. With current oil prices (70 Euro2008/bl in September
2013), it makes economic sense to extract this resource. It has been estimated
(perhaps conservatively) that, given the existing technology and current oil
prices, 1–1.5 trillion barrels of tar sands and extra heavy oil can be
recovered economically (IEA 2013b), most of
them being located in Canada and Venezuela.
All production methods of oil from tar sand requires large amounts
of energy, chemicals and water. It also releases considerable amounts ofCO2
into the atmosphere.
An upheaval of large land areas and substantial pollution is often
the result. Needless to say, there are strong environmental concerns regarding
extraction of energy from these resources. However, with increasing oil prices,
tar sands and extra heavy oil are considered important from an industrial
perspective, especially since the resource base is significant.
Oil sands are rocks that consist predominantly of sandstones which
contain bitumen within the pore spaces that has been produced by the
biodegradation of oils in the subsurface. Most oil reservoirs are sufficiently
hot that biogenic activity is curtailed or absent. However, oil that migrates
into shallow reservoir rocks may be altered completely to bitumen that is very
viscous. Although found close to the surface, production of oil from oil sands
is expensive. The bitumen must be heated before it will flow and commercial
extraction requires large amounts of energy. Once extracted the bitumen must
also be upgraded by purification and hydrogenation before it can be refined
like conventional crude oil.
Although oil sands have long been recognized and used in limited
ways, it is only within the last 40 years that commercial production has grown.
Canadian oil sands production began in 1967 and production has grown steadily.
By 2009 production from oil sands was equivalent to over 550 million barrels of
oil, accounting for 49% of Canada’s oil production in 2009. Though production
was initially subsidized, production costs have fallen with technological
advances and is now economically viable at $50/barrel and production is
forecast to more than triple by 2025. Venezuela also has substantial oil sand
deposits and smaller accumulations are known in Russia and the Middle East.
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