It is also possible to produce oil and sand from
wells and then separate the sand out as is done when
mining the tar sand.
Another method is to pump air down boreholes and
burn some of the oil to produce heat.
Fluvial sandstones like the lower Cretaceous
McMurray Fm of the Athabasca Tar sand in Alberta
represent a very complex reservoir with respect to
migration and filling of hydrocarbons. Each channel
sand may represent a small trap which will be first
filled with early mature oil from the source rock. As
the oil becomes more mature and less dense it may
displace the first oil to arrive and finally gas may
migrate in and displace most of the oil into shallower
structures. This is a good example of fill and spill in a
series of rather small compartments.
Fluvial channels which are abandoned by channel
avulsion are filled with clay and mud and they may
represent clay plugs which are barriers for fluid flow
and sometimes seals in these reservoirs (Fustic et al.
2012). When oil and gas has migrated into shallow
sandstones like the McMurray Fm where the
temperatures are below 80
C, bacteria are present.
The sand will be loose and indurated due to little or
no quartz cement and the temperatures will also be low
enough for biodegradation at a high rate. Most of the
biodegradation occurs near the oil/water contact and
since new oil/water contacts may develop during
migration there may be complex zones of biodegradation. The highest viscosity oil is usually found near the
base of the oil column where the biodegradation has
been most intense (Jones et al. 2008). The OWC
contact may however have changed during the fill
and spill, resulting in more than one intensively
biodegraded zone. Variations in the oil viscosity
within the reservoir play an important role in the
planning of production.
Only about 20% is close enough to the surface to be
economically mined and the rest must be heated in
place.
A cubic metre of oil, mined from the tar sands,
needs 2–4.5 m
3 of water.
Oil may be extracted by steam-assisted gravity
drainage (SAGD) (Fig. 23.3b). Two parallel horizontal
wells are drilled so that one is about 5 m below the
other. Steam is injected in the shallowest well and the
heated, less viscous, oil is drained into the lower well
where it is produced. The steam will remain near the
top due to its low density but will sink when
condensed to water. The shales will reduce the vertical
permeability and the drainage, but the heating will
cause some fracturing which may allow more vertical
flow. This method requires rather thick and homogeneous sand. Steam can also be injected into one well
for a few weeks until the oil is heated and then produced from the same well. This is called cyclic steam
stimulation (CSS).
Large amounts of energy and water are required to
produce the steam for this method. If petroleum is
burned to produce the heat, high emissions of CO 2
will result not only from when the oil is burnt as fuel
but also from producing it. The steam may be mixed
with solvents (gas) and this may reduce the amount of
heat (steam) required. Much of the water used to
produce steam can be recovered and used again.
For each 3 bbl of oil produced one barrel is burned
to produce the steam required for the production.
50–70 kg of CO 2 is typically produced and emitted
for earch barrel of oil produced mainly due to the
production of steam. This heavy oil should also be
mixed with gas during the refining and a supply of
gas is therefore important.
Heating the oil with electricity has also been proposed and hydroelectric or nuclear power would
reduce the CO 2 emissions.
173 billion bbl (27.5Â10
9 m
3 ) of crude bitumen,
which is 10% of the total bitumen in place in Alberta,
has been considered to be, economically recoverable
using current technology at oil prices above 60–100
USD/barrel. In recent years the price of gas has
dropped due to shale gas. Heating the oil with steam
produced by gas is now more economical than using
oil. In some cases there may a negative energy balance
so that the energy from gas exceeds the energy in the
produced oil.
The environmental problems associated with oil
production from tar sand are considerable and very
large amounts of water are required both for the production of steam for subsurface operations and for the
processing of loose sand excavated from surface pits.
As a result the release of CO 2 is higher by 20–25%
than with normal petroleum production if the CO 2 is
not captured and sequestrated.
In Venezuela there are also very large reserves of
heavy oil and tar sand such as the Orinoco tar sand.
While the average surface temperature in Northern
584
K. Bjørlykke
wells and then separate the sand out as is done when
mining the tar sand.
Another method is to pump air down boreholes and
burn some of the oil to produce heat.
Fluvial sandstones like the lower Cretaceous
McMurray Fm of the Athabasca Tar sand in Alberta
represent a very complex reservoir with respect to
migration and filling of hydrocarbons. Each channel
sand may represent a small trap which will be first
filled with early mature oil from the source rock. As
the oil becomes more mature and less dense it may
displace the first oil to arrive and finally gas may
migrate in and displace most of the oil into shallower
structures. This is a good example of fill and spill in a
series of rather small compartments.
Fluvial channels which are abandoned by channel
avulsion are filled with clay and mud and they may
represent clay plugs which are barriers for fluid flow
and sometimes seals in these reservoirs (Fustic et al.
2012). When oil and gas has migrated into shallow
sandstones like the McMurray Fm where the
temperatures are below 80
C, bacteria are present.
The sand will be loose and indurated due to little or
no quartz cement and the temperatures will also be low
enough for biodegradation at a high rate. Most of the
biodegradation occurs near the oil/water contact and
since new oil/water contacts may develop during
migration there may be complex zones of biodegradation. The highest viscosity oil is usually found near the
base of the oil column where the biodegradation has
been most intense (Jones et al. 2008). The OWC
contact may however have changed during the fill
and spill, resulting in more than one intensively
biodegraded zone. Variations in the oil viscosity
within the reservoir play an important role in the
planning of production.
Only about 20% is close enough to the surface to be
economically mined and the rest must be heated in
place.
A cubic metre of oil, mined from the tar sands,
needs 2–4.5 m
3 of water.
Oil may be extracted by steam-assisted gravity
drainage (SAGD) (Fig. 23.3b). Two parallel horizontal
wells are drilled so that one is about 5 m below the
other. Steam is injected in the shallowest well and the
heated, less viscous, oil is drained into the lower well
where it is produced. The steam will remain near the
top due to its low density but will sink when
condensed to water. The shales will reduce the vertical
permeability and the drainage, but the heating will
cause some fracturing which may allow more vertical
flow. This method requires rather thick and homogeneous sand. Steam can also be injected into one well
for a few weeks until the oil is heated and then produced from the same well. This is called cyclic steam
stimulation (CSS).
Large amounts of energy and water are required to
produce the steam for this method. If petroleum is
burned to produce the heat, high emissions of CO 2
will result not only from when the oil is burnt as fuel
but also from producing it. The steam may be mixed
with solvents (gas) and this may reduce the amount of
heat (steam) required. Much of the water used to
produce steam can be recovered and used again.
For each 3 bbl of oil produced one barrel is burned
to produce the steam required for the production.
50–70 kg of CO 2 is typically produced and emitted
for earch barrel of oil produced mainly due to the
production of steam. This heavy oil should also be
mixed with gas during the refining and a supply of
gas is therefore important.
Heating the oil with electricity has also been proposed and hydroelectric or nuclear power would
reduce the CO 2 emissions.
173 billion bbl (27.5Â10
9 m
3 ) of crude bitumen,
which is 10% of the total bitumen in place in Alberta,
has been considered to be, economically recoverable
using current technology at oil prices above 60–100
USD/barrel. In recent years the price of gas has
dropped due to shale gas. Heating the oil with steam
produced by gas is now more economical than using
oil. In some cases there may a negative energy balance
so that the energy from gas exceeds the energy in the
produced oil.
The environmental problems associated with oil
production from tar sand are considerable and very
large amounts of water are required both for the production of steam for subsurface operations and for the
processing of loose sand excavated from surface pits.
As a result the release of CO 2 is higher by 20–25%
than with normal petroleum production if the CO 2 is
not captured and sequestrated.
In Venezuela there are also very large reserves of
heavy oil and tar sand such as the Orinoco tar sand.
While the average surface temperature in Northern
584
K. Bjørlykke