134
The Chemistry and Technology of Petroleum
Polymer flooding (polymer augmented water flooding) is water flooding in which organic polymers are injected with the water to improve horizontal and vertical sweep efficiency. The process is
conceptually simple and inexpensive, and its commercial use is increasing despite relatively small
potential incremental oil production. Surfactant flooding is complex and requires detailed laboratory testing to support field project design. As demonstrated by field tests, it has excellent potential
for improving the recovery of low-viscosity to moderate-viscosity oil. Surfactant flooding is expensive and has been used in few large-scale projects. Alkaline flooding has been used only in those
reservoirs containing specific types of high-acid-number crude oils.
The terms microemulsion and micellar solution are used to describe concentrated, surfactantstabilized dispersions of water and hydrocarbons that are used to enhance oil recovery. At concentrations above a certain critical value, the surfactant molecules in solution form aggregates called
micelles. These micelles are capable of solubilizing fluids in their cores and are called swollen
micelles. Spherical micelles have size ranges from 10 −6 to 10 −4 mm. The micellar solution or microemulsion is homogeneous, transparent or translucent, and stable to phase separation. The term soluble oil is often used to describe an oil external system having little or no dispersed water.
Although used to describe the process, neither microemulsion or micellar solution accurately
describes all compositions that are used in emulsion flooding. In fact, many systems used do not
have an identifiable external or continuous phase and these terms do not always apply.
Microemulsion flooding (micellar/emulsion flooding) refers to a fluid injection process in which
a stable solution of oil, water, and one or more surfactants along with electrolytes of salts is injected
into the formation and is displaced by a mobility buffer solution (Reed and Healy, 1977; Dreher and
Gogarty, 1979). Injecting water in turn displaces the mobility buffer. Depending on the reservoir
environment, a pre-flood may or may not be used. The microemulsion is the key to the process.
Oil and water are displaced ahead of the microemulsion slug, and a stabilized oil and water bank
develops. The displacement mechanism is the same under secondary and tertiary recovery conditions.
In the secondary case, water is the primary produced fluid until the oil bank reaches the well.
In microemulsion flooding, two approaches have developed to enhance oil recovery (Gogarty,
1976). In the first process, a relatively low-concentration surfactant microemulsion is injected
at large pore volumes of 15%–60% to reduce the interfacial tension between the water and oil,
thereby increasing oil recovery. In the second process, a relatively small pore volume, from 3% to
20% of a high-concentration surfactant microemulsion, is injected. With the high concentration of
surfactant in the microemulsion, the micelles solubilize the oil and water in the displacing microemulsion. Consequently, the high-concentration system may initially displace the oil in a misciblelike manner. However, as the high-concentration slug moves through the reservoir it is diluted by
the formation fluids and the process ultimately or gradually reverts to a low-concentration flood.
However, this initial displacement forms an oil bank, which is very important in establishing displacement efficiency. Low-concentration systems typically contain 2%–4% surfactant, whereas
high-concentration systems contain 8%–12% w/w.
Mobility control is important to the success of the process. The mobility of the microemulsion
can be matched to that of the stabilized water–oil bank by controlling the microemulsion viscosity. The mobility buffer following the microemulsion slug prevents rapid slug deterioration from
the rear and thus minimizes the slug size required for efficient oil displacement. Water external
emulsions and aqueous solutions of high molecular weight polymers have been used as mobility
buffers.
Microemulsion flooding can be applied over a wide range of reservoir conditions. Generally,
wherever a waterflood has been successful, microemulsion flooding may also be applicable. In
cases in which waterflooding was a failure because of poor mobility relationships, microemulsion
flooding might be technically successful because of the required mobility control. Of course, if
waterflooding was a failure because of certain reservoir conditions, such as fracturing or very high
permeability streaks, microemulsion flooding will most likely also fail.
The Chemistry and Technology of Petroleum
Polymer flooding (polymer augmented water flooding) is water flooding in which organic polymers are injected with the water to improve horizontal and vertical sweep efficiency. The process is
conceptually simple and inexpensive, and its commercial use is increasing despite relatively small
potential incremental oil production. Surfactant flooding is complex and requires detailed laboratory testing to support field project design. As demonstrated by field tests, it has excellent potential
for improving the recovery of low-viscosity to moderate-viscosity oil. Surfactant flooding is expensive and has been used in few large-scale projects. Alkaline flooding has been used only in those
reservoirs containing specific types of high-acid-number crude oils.
The terms microemulsion and micellar solution are used to describe concentrated, surfactantstabilized dispersions of water and hydrocarbons that are used to enhance oil recovery. At concentrations above a certain critical value, the surfactant molecules in solution form aggregates called
micelles. These micelles are capable of solubilizing fluids in their cores and are called swollen
micelles. Spherical micelles have size ranges from 10 −6 to 10 −4 mm. The micellar solution or microemulsion is homogeneous, transparent or translucent, and stable to phase separation. The term soluble oil is often used to describe an oil external system having little or no dispersed water.
Although used to describe the process, neither microemulsion or micellar solution accurately
describes all compositions that are used in emulsion flooding. In fact, many systems used do not
have an identifiable external or continuous phase and these terms do not always apply.
Microemulsion flooding (micellar/emulsion flooding) refers to a fluid injection process in which
a stable solution of oil, water, and one or more surfactants along with electrolytes of salts is injected
into the formation and is displaced by a mobility buffer solution (Reed and Healy, 1977; Dreher and
Gogarty, 1979). Injecting water in turn displaces the mobility buffer. Depending on the reservoir
environment, a pre-flood may or may not be used. The microemulsion is the key to the process.
Oil and water are displaced ahead of the microemulsion slug, and a stabilized oil and water bank
develops. The displacement mechanism is the same under secondary and tertiary recovery conditions.
In the secondary case, water is the primary produced fluid until the oil bank reaches the well.
In microemulsion flooding, two approaches have developed to enhance oil recovery (Gogarty,
1976). In the first process, a relatively low-concentration surfactant microemulsion is injected
at large pore volumes of 15%–60% to reduce the interfacial tension between the water and oil,
thereby increasing oil recovery. In the second process, a relatively small pore volume, from 3% to
20% of a high-concentration surfactant microemulsion, is injected. With the high concentration of
surfactant in the microemulsion, the micelles solubilize the oil and water in the displacing microemulsion. Consequently, the high-concentration system may initially displace the oil in a misciblelike manner. However, as the high-concentration slug moves through the reservoir it is diluted by
the formation fluids and the process ultimately or gradually reverts to a low-concentration flood.
However, this initial displacement forms an oil bank, which is very important in establishing displacement efficiency. Low-concentration systems typically contain 2%–4% surfactant, whereas
high-concentration systems contain 8%–12% w/w.
Mobility control is important to the success of the process. The mobility of the microemulsion
can be matched to that of the stabilized water–oil bank by controlling the microemulsion viscosity. The mobility buffer following the microemulsion slug prevents rapid slug deterioration from
the rear and thus minimizes the slug size required for efficient oil displacement. Water external
emulsions and aqueous solutions of high molecular weight polymers have been used as mobility
buffers.
Microemulsion flooding can be applied over a wide range of reservoir conditions. Generally,
wherever a waterflood has been successful, microemulsion flooding may also be applicable. In
cases in which waterflooding was a failure because of poor mobility relationships, microemulsion
flooding might be technically successful because of the required mobility control. Of course, if
waterflooding was a failure because of certain reservoir conditions, such as fracturing or very high
permeability streaks, microemulsion flooding will most likely also fail.