132
The Chemistry and Technology of Petroleum
reservoir but may also produce a hydrocarbon exchange so that the concentration of liquid petroleum gases in the oil is increased.
Water injection is still predominantly a secondary recovery process (waterflood). Probably the
principal reason for this is that reservoir formation water is ordinarily not available in volume during the early years of an oil field and pressure maintenance water from outside the field may be
too expensive. When a young field produces considerable water, it may be injected back into the
reservoir primarily for the purpose of nuisance abatement, but reservoir pressure maintenance is a
valuable by-product.
Nevertheless, some passages in the formation are larger than others, and the water tends to flow
freely through these, bypassing smaller passages where the oil remains. A partial solution to this
problem is possible by miscible fluid flooding. Liquid butane and propane are pumped into the
ground under considerable pressure, dissolving the oil and carrying it out of the smaller passages;
additional pressure is obtained by using natural gas.
6.5.3 enHAnCed oIl reCovery
EOR (tertiary oil recovery) (please see Chapter 7) is the incremental ultimate oil that can be recovered from a petroleum reservoir over oil that can be obtained by primary and secondary recovery
methods.
The viscosity (or the API gravity) of petroleum (Chapter 10) is an important factor that must be
taken into account when heavy oil is recovered from a reservoir. In fact, certain reservoir types,
such as those with very viscous crude oils and some low-permeability carbonate (limestone, dolomite, or chert) reservoirs, respond poorly to conventional secondary recovery techniques.
In these reservoirs, it is desirable to initiate EOR operations as early as possible. This may
mean considerably abbreviating conventional secondary recovery operations or bypassing them
altogether. Thermal floods using steam and controlled in situ combustion methods are also used.
Thermal methods of recovery reduce the viscosity of the crude oil by heat so that it flows more easily into the production well. Thus tertiary techniques are usually variations of secondary methods
with a goal of improving the sweeping action of the invading fluid.
Thus, EOR methods are designed to reduce the viscosity of the crude oil (i.e., to reduce the pour
point of the crude oil relative to the temperature of the reservoir), thereby increasing oil production.
EOR methods are applied started when secondary oil recovery techniques are no longer enough to
sustain production.
Thermally enhanced oil recovery methods are tertiary recovery techniques that heat the oil and
make it easier to extract. Steam injection is the most common form of this process and is used
extensively to increase oil production. In situ combustion is another form of thermally enhanced oil
recovery but instead of using steam to reduce the crude oil viscosity, some of the oil is burned to
heat the surrounding oil. Detergents are also used to decrease oil viscosity.
A significant amount of laboratory research and field testing has been devoted to developing
EOR methods as well as defining the requirements for a successful recovery and the limitations of
the various methods (Figure 6.7). The intent of EOR is to increase the effectiveness of oil removal
from pores of the rock (displacement efficiency) and to increase the volume of rock contacted by
injected fluids (sweep efficiency) (Schumacher, 1980).
To understand the phenomenon of EOR, it is helpful to understand the condition in the reservoir
after other recovery operations have been exhausted. The oil remaining after conventional recovery
operations is retained in the pore space of reservoir rock at a lower concentration than originally
existed. In portions of the reservoir that have been contacted or swept by the injection fluid, the
residual oil remains as droplets (or ganglia) trapped in either individual pores or clusters of pores. It
may also remain as films partly coating the pore walls. Entrapment of this residual oil is predominantly due to capillary and surface forces and to pore geometry.
The Chemistry and Technology of Petroleum
reservoir but may also produce a hydrocarbon exchange so that the concentration of liquid petroleum gases in the oil is increased.
Water injection is still predominantly a secondary recovery process (waterflood). Probably the
principal reason for this is that reservoir formation water is ordinarily not available in volume during the early years of an oil field and pressure maintenance water from outside the field may be
too expensive. When a young field produces considerable water, it may be injected back into the
reservoir primarily for the purpose of nuisance abatement, but reservoir pressure maintenance is a
valuable by-product.
Nevertheless, some passages in the formation are larger than others, and the water tends to flow
freely through these, bypassing smaller passages where the oil remains. A partial solution to this
problem is possible by miscible fluid flooding. Liquid butane and propane are pumped into the
ground under considerable pressure, dissolving the oil and carrying it out of the smaller passages;
additional pressure is obtained by using natural gas.
6.5.3 enHAnCed oIl reCovery
EOR (tertiary oil recovery) (please see Chapter 7) is the incremental ultimate oil that can be recovered from a petroleum reservoir over oil that can be obtained by primary and secondary recovery
methods.
The viscosity (or the API gravity) of petroleum (Chapter 10) is an important factor that must be
taken into account when heavy oil is recovered from a reservoir. In fact, certain reservoir types,
such as those with very viscous crude oils and some low-permeability carbonate (limestone, dolomite, or chert) reservoirs, respond poorly to conventional secondary recovery techniques.
In these reservoirs, it is desirable to initiate EOR operations as early as possible. This may
mean considerably abbreviating conventional secondary recovery operations or bypassing them
altogether. Thermal floods using steam and controlled in situ combustion methods are also used.
Thermal methods of recovery reduce the viscosity of the crude oil by heat so that it flows more easily into the production well. Thus tertiary techniques are usually variations of secondary methods
with a goal of improving the sweeping action of the invading fluid.
Thus, EOR methods are designed to reduce the viscosity of the crude oil (i.e., to reduce the pour
point of the crude oil relative to the temperature of the reservoir), thereby increasing oil production.
EOR methods are applied started when secondary oil recovery techniques are no longer enough to
sustain production.
Thermally enhanced oil recovery methods are tertiary recovery techniques that heat the oil and
make it easier to extract. Steam injection is the most common form of this process and is used
extensively to increase oil production. In situ combustion is another form of thermally enhanced oil
recovery but instead of using steam to reduce the crude oil viscosity, some of the oil is burned to
heat the surrounding oil. Detergents are also used to decrease oil viscosity.
A significant amount of laboratory research and field testing has been devoted to developing
EOR methods as well as defining the requirements for a successful recovery and the limitations of
the various methods (Figure 6.7). The intent of EOR is to increase the effectiveness of oil removal
from pores of the rock (displacement efficiency) and to increase the volume of rock contacted by
injected fluids (sweep efficiency) (Schumacher, 1980).
To understand the phenomenon of EOR, it is helpful to understand the condition in the reservoir
after other recovery operations have been exhausted. The oil remaining after conventional recovery
operations is retained in the pore space of reservoir rock at a lower concentration than originally
existed. In portions of the reservoir that have been contacted or swept by the injection fluid, the
residual oil remains as droplets (or ganglia) trapped in either individual pores or clusters of pores. It
may also remain as films partly coating the pore walls. Entrapment of this residual oil is predominantly due to capillary and surface forces and to pore geometry.