Consequently chemicals are often pumped down in
advance (preflush) to reduce adsorption of the
chemicals which are intended to reduce surface tension. This treatment ought also to take into account
which clay minerals occur in the rock.
Polymers. In oil reservoirs with an undesirably high
relative water permeability, large amounts of oil will
remain in the reservoir rock. Water will flow where
there is least resistance to its movement, e.g. along
cracks and permeable sand beds, and injection of
water will then have little effect. This is particularly
relevant with oil reservoirs with low oil saturation
(high water saturation). If the viscosity is increased
so that the pressure gradient in the water phase
increases, the gradient in the oil phase will also
increase. Polymers reduce the mobility of the water,
causing a piston-like replacement mechanism. With
increased viscosity, the polymer-bearing water will
exert greater shear forces on the oil phase, so that oil
drops are more easily carried along. Water containing
polymers is no longer a Newtonian liquid, but a
pseudoplastic liquid in which the apparent viscosity
is reduced by the rate of deformation.
Polymers will also have a tendency to be adsorbed
onto the surface of minerals so that their effect decreases
with increasing distance from the injection well.
Addition of Alkaline (Caustic) Chemicals. Strong
alkaline solutions, for example NaOH, will also
reduce the surface tension of aqueous phases. Injection
of a mixture of these alkaline solutions may therefore
increase both the water-wetness of the reservoir
and oil production. The most common chemicals
include sodium and potassium hydroxide, sodium
orthosilicate, sodium carbonate and sodium phosphate. The method is used largely in sandstone
reservoirs. Small amounts of gypsum or anhydrite
will cause Ca(OH) 2 to precipitation and neutralise
caustic soda (NaOH).
The object of gas injection is to produce a fluid
phase which will dissolve the reservoir oil. The flow of
such a fluid through reservoirs with low oil saturation
may dissolve the oil and carry it along to the production well. Hydrocarbon gases which are in the liquid
phase at reservoir pressure (Liquid Petroleum Gas –
LPG) will be fully miscible with oil. This requires that
the reservoir temperature be below the critical temperature for the gas. At higher temperatures the gas is in
gaseous form regardless of pressure, and is not fully
miscible with oil. Propane is often used in the oilexpelling mixture, and gas and water are injected
subsequently.
CO 2 has a critical temperature of about 31
C, and is
therefore gaseous at all reservoir pressures. Carbon
dioxide is very soluble in oil, which increases in volume when it is saturated with respect to CO 2 . This
creates a sort of gas drive.
Increasing the CO 2 content also reduces the viscosity and density of oil, making it very mobile. The
method is used for oil reservoirs with oil saturation
of 25–55%. Neutral gases such as nitrogen may also be
injected into reservoirs, but far greater pressure is then
required to make the gas soluble in oil.
21.11 Changes During Production
With production from sandstone reservoirs, the physical properties of sandstone must be taken into account,
e.g. distribution of porosity and permeability. The
actual production, however, will lead to changes in
the reservoir rock which may have an adverse effect
on the reservoir properties, i.e. cause reservoir damage. This may happen in two ways:
1. Chemical reactions between minerals and liquids
which are used in drilling or injection of water.
2. Mechanical damage through relatively loose clay
minerals or other small grains being carried by the
fluid flowing towards the well, and obstructing the
pore throats.
As oil flows into a well, the flux (cm
3 /cm
2 ) is
inversely proportional to the distance to the well. If
we can improve the permeability of the reservoir
nearest the well, we may be able to step up the production rate considerably. For this reason chemicals
are sometimes injected into the reservoir to improve
the permeability. We then need to understand some of
the chemical properties of minerals in order to be able
to predict the reactions which will result from treatment with acids and other chemicals.
As far as chemical reactions are concerned, clay
minerals are important because they have a large specific surface and ion exchange capacity. The specific
surface varies with the size and shape of the mineral
grains. Kaolinite will typically have a specific surface
of 5–30 m
2 /g, chlorite 10–50 m
2 /g, illite and smectite
(montmorillonite) >100 m
2 /g. The high specific surface of illite and smectite is due to the fact that they
often occur as very thin sheets or fibres.
21 Production Geology
555
advance (preflush) to reduce adsorption of the
chemicals which are intended to reduce surface tension. This treatment ought also to take into account
which clay minerals occur in the rock.
Polymers. In oil reservoirs with an undesirably high
relative water permeability, large amounts of oil will
remain in the reservoir rock. Water will flow where
there is least resistance to its movement, e.g. along
cracks and permeable sand beds, and injection of
water will then have little effect. This is particularly
relevant with oil reservoirs with low oil saturation
(high water saturation). If the viscosity is increased
so that the pressure gradient in the water phase
increases, the gradient in the oil phase will also
increase. Polymers reduce the mobility of the water,
causing a piston-like replacement mechanism. With
increased viscosity, the polymer-bearing water will
exert greater shear forces on the oil phase, so that oil
drops are more easily carried along. Water containing
polymers is no longer a Newtonian liquid, but a
pseudoplastic liquid in which the apparent viscosity
is reduced by the rate of deformation.
Polymers will also have a tendency to be adsorbed
onto the surface of minerals so that their effect decreases
with increasing distance from the injection well.
Addition of Alkaline (Caustic) Chemicals. Strong
alkaline solutions, for example NaOH, will also
reduce the surface tension of aqueous phases. Injection
of a mixture of these alkaline solutions may therefore
increase both the water-wetness of the reservoir
and oil production. The most common chemicals
include sodium and potassium hydroxide, sodium
orthosilicate, sodium carbonate and sodium phosphate. The method is used largely in sandstone
reservoirs. Small amounts of gypsum or anhydrite
will cause Ca(OH) 2 to precipitation and neutralise
caustic soda (NaOH).
The object of gas injection is to produce a fluid
phase which will dissolve the reservoir oil. The flow of
such a fluid through reservoirs with low oil saturation
may dissolve the oil and carry it along to the production well. Hydrocarbon gases which are in the liquid
phase at reservoir pressure (Liquid Petroleum Gas –
LPG) will be fully miscible with oil. This requires that
the reservoir temperature be below the critical temperature for the gas. At higher temperatures the gas is in
gaseous form regardless of pressure, and is not fully
miscible with oil. Propane is often used in the oilexpelling mixture, and gas and water are injected
subsequently.
CO 2 has a critical temperature of about 31
C, and is
therefore gaseous at all reservoir pressures. Carbon
dioxide is very soluble in oil, which increases in volume when it is saturated with respect to CO 2 . This
creates a sort of gas drive.
Increasing the CO 2 content also reduces the viscosity and density of oil, making it very mobile. The
method is used for oil reservoirs with oil saturation
of 25–55%. Neutral gases such as nitrogen may also be
injected into reservoirs, but far greater pressure is then
required to make the gas soluble in oil.
21.11 Changes During Production
With production from sandstone reservoirs, the physical properties of sandstone must be taken into account,
e.g. distribution of porosity and permeability. The
actual production, however, will lead to changes in
the reservoir rock which may have an adverse effect
on the reservoir properties, i.e. cause reservoir damage. This may happen in two ways:
1. Chemical reactions between minerals and liquids
which are used in drilling or injection of water.
2. Mechanical damage through relatively loose clay
minerals or other small grains being carried by the
fluid flowing towards the well, and obstructing the
pore throats.
As oil flows into a well, the flux (cm
3 /cm
2 ) is
inversely proportional to the distance to the well. If
we can improve the permeability of the reservoir
nearest the well, we may be able to step up the production rate considerably. For this reason chemicals
are sometimes injected into the reservoir to improve
the permeability. We then need to understand some of
the chemical properties of minerals in order to be able
to predict the reactions which will result from treatment with acids and other chemicals.
As far as chemical reactions are concerned, clay
minerals are important because they have a large specific surface and ion exchange capacity. The specific
surface varies with the size and shape of the mineral
grains. Kaolinite will typically have a specific surface
of 5–30 m
2 /g, chlorite 10–50 m
2 /g, illite and smectite
(montmorillonite) >100 m
2 /g. The high specific surface of illite and smectite is due to the fact that they
often occur as very thin sheets or fibres.
21 Production Geology
555