temperature and reduce the viscosity of oil, and
to produce gas from coal.
II. Chemical processes: These involve injection of
chemicals into the reservoir. The most important
types are:
(a) Injection of caustic (alkaline) chemicals which
reduce surface tension (surfactants).
(b) Polymers which are injected into water to
increase its viscosity to prevent water
breakthrough.
III. Injection of various types of gas to increase the
miscibility of hydrocarbon phases. These may
include carbon dioxide, neutral gases (e.g. nitrogen) or hydrocarbon gases.
I. Thermal Processes: These are only used with
reservoirs with highly viscous oil, where an increase
in temperature will greatly reduce the viscosity. This
will normally be heavy, asphaltic oil which has been
subjected to bacterial breakdown and evaporation. Oil
reservoirs of this type are normally fairly shallow (less
than 1,500 m). Deeper reservoirs will have a high
temperature anyway, so there is less to be gained by
artificially heating the oil.
In situ combustion can be induced by pumping air
down into the reservoir, and the combustion can be
regulated by addition of oxygen. The hydrocarbons
which do not burn will then become hotter and less
viscous. Burning also produces gases, which may
increase production. Special wells can be used for
injecting air, and combustion spreads from these to
the production wells, forcing a zone of hot water, gas
and light oil ahead of it to the production wells. Combustion may also increase the porosity of the reservoir
rock, making it possible to produce later on from the
wells in which combustion has taken place.
Steam injection is responsible for 90% of all production through secondary recovery methods apart
from water injection. One of the major reasons for
the low recovery percentage from oil reservoirs is
that the oil is too viscous. This is particularly true of
oil which has been subjected to bacterial degradation
(biodegradation), leaving the viscous asphaltic part of
the oil behind as heavy oil (low gravity 16–17 API).
The Cretaceous Athabasca tar sand in Alberta,
Canada, is the most famous example, and contains
huge oil reserves. When oil reservoirs with such
heavy biodegraded oil are exposed at the surface, the
oil is almost solid at normal temperatures, and is called
tar sand. For it to be possible to produce oil, the
pressure difference between the oil in the reservoir
rock and the well must exceed the resistance to flow
exerted by capillary forces (in water-wet reservoirs)
and viscosity forces (friction).
Biodegraded oil is normally found at relatively
shallow depths, and the temperature of the oil is low
and the viscosity consequently high. By injecting
steam the temperature can be raised and the viscosity
lowered considerably. There are examples of the viscosity of oil being reduced from 300 to 10 cP after
steam injection, and biodegraded oil is often too viscous to flow at all without steam injection. Steam
injection is particularly important in California,
where 300,000 barrels/day are produced using this
method. Steam injection must be repeated at regular
intervals because the viscosity increases again as the
reservoir cools off. Sandstones which are to be
subjected to steam injection must also have high oil
saturation, because otherwise water and steam can
flow past the oil. Steam is recovered from the produced oil, and in some fields one out of every three
barrels produced is used for steam production.
II. Chemical Processes: When oil flows out of a
reservoir rock into a producing well, it must overcome
the capillary forces involved in two-phase flow. By
reducing the surface tension between oil and the water
which is injected into the wells, the capillary forces can
be reduced. The addition of surfactants, a sort of soap,
will reduce the surface tension so that the wettability
changes. The wettability can be measured by means of
the contact angle, i.e. the angle which the liquid phases
make with a mineral surface, for example (Fig. 21.3).
If an oil reservoir is oil-wet, it means that oil is
more strongly bound to the mineral surface than water.
By reducing the surface tension in the water phase, the
wetting angle is altered and the system may become
water-wet, and the oil will become more mobile so
that the relative permeability to oil is increased. A
mixture of different chemicals (slug) is injected into
oil reservoirs to reduce the surface tension. It includes
micro-emulsions, soluble hydrocarbons, micelles and
frequently also alcohol and various salts.
One serious drawback of this method is that
chemicals can easily be adsorbed onto the reservoir
rock. Clay minerals in particular have high ion
exchange and adsorption capacities. The specific surface (m
2 /g) of rocks is also of significance.
554
K. Bjørlykke
to produce gas from coal.
II. Chemical processes: These involve injection of
chemicals into the reservoir. The most important
types are:
(a) Injection of caustic (alkaline) chemicals which
reduce surface tension (surfactants).
(b) Polymers which are injected into water to
increase its viscosity to prevent water
breakthrough.
III. Injection of various types of gas to increase the
miscibility of hydrocarbon phases. These may
include carbon dioxide, neutral gases (e.g. nitrogen) or hydrocarbon gases.
I. Thermal Processes: These are only used with
reservoirs with highly viscous oil, where an increase
in temperature will greatly reduce the viscosity. This
will normally be heavy, asphaltic oil which has been
subjected to bacterial breakdown and evaporation. Oil
reservoirs of this type are normally fairly shallow (less
than 1,500 m). Deeper reservoirs will have a high
temperature anyway, so there is less to be gained by
artificially heating the oil.
In situ combustion can be induced by pumping air
down into the reservoir, and the combustion can be
regulated by addition of oxygen. The hydrocarbons
which do not burn will then become hotter and less
viscous. Burning also produces gases, which may
increase production. Special wells can be used for
injecting air, and combustion spreads from these to
the production wells, forcing a zone of hot water, gas
and light oil ahead of it to the production wells. Combustion may also increase the porosity of the reservoir
rock, making it possible to produce later on from the
wells in which combustion has taken place.
Steam injection is responsible for 90% of all production through secondary recovery methods apart
from water injection. One of the major reasons for
the low recovery percentage from oil reservoirs is
that the oil is too viscous. This is particularly true of
oil which has been subjected to bacterial degradation
(biodegradation), leaving the viscous asphaltic part of
the oil behind as heavy oil (low gravity 16–17 API).
The Cretaceous Athabasca tar sand in Alberta,
Canada, is the most famous example, and contains
huge oil reserves. When oil reservoirs with such
heavy biodegraded oil are exposed at the surface, the
oil is almost solid at normal temperatures, and is called
tar sand. For it to be possible to produce oil, the
pressure difference between the oil in the reservoir
rock and the well must exceed the resistance to flow
exerted by capillary forces (in water-wet reservoirs)
and viscosity forces (friction).
Biodegraded oil is normally found at relatively
shallow depths, and the temperature of the oil is low
and the viscosity consequently high. By injecting
steam the temperature can be raised and the viscosity
lowered considerably. There are examples of the viscosity of oil being reduced from 300 to 10 cP after
steam injection, and biodegraded oil is often too viscous to flow at all without steam injection. Steam
injection is particularly important in California,
where 300,000 barrels/day are produced using this
method. Steam injection must be repeated at regular
intervals because the viscosity increases again as the
reservoir cools off. Sandstones which are to be
subjected to steam injection must also have high oil
saturation, because otherwise water and steam can
flow past the oil. Steam is recovered from the produced oil, and in some fields one out of every three
barrels produced is used for steam production.
II. Chemical Processes: When oil flows out of a
reservoir rock into a producing well, it must overcome
the capillary forces involved in two-phase flow. By
reducing the surface tension between oil and the water
which is injected into the wells, the capillary forces can
be reduced. The addition of surfactants, a sort of soap,
will reduce the surface tension so that the wettability
changes. The wettability can be measured by means of
the contact angle, i.e. the angle which the liquid phases
make with a mineral surface, for example (Fig. 21.3).
If an oil reservoir is oil-wet, it means that oil is
more strongly bound to the mineral surface than water.
By reducing the surface tension in the water phase, the
wetting angle is altered and the system may become
water-wet, and the oil will become more mobile so
that the relative permeability to oil is increased. A
mixture of different chemicals (slug) is injected into
oil reservoirs to reduce the surface tension. It includes
micro-emulsions, soluble hydrocarbons, micelles and
frequently also alcohol and various salts.
One serious drawback of this method is that
chemicals can easily be adsorbed onto the reservoir
rock. Clay minerals in particular have high ion
exchange and adsorption capacities. The specific surface (m
2 /g) of rocks is also of significance.
554
K. Bjørlykke