Foam EOR as an Optimization Technique for Gas EOR: A Comprehensive Review of Laboratory and Field Implementations

Abstract

Foam-enhanced oil recovery (EOR) is poised to become one of the most promising tertiary recovery techniques to keep up with the continuously increasing global energy demands. Due to their low sensitivity to gravity and permeability heterogeneities that improve sweep efficiency, foams are the preferred injection fluids over water or gas. Although foam injection has been used in the field to improve oil recovery and control gas mobility, its success is still hindered by several conceptual and operational challenges with regard to its stability and foamability under reservoir conditions. This can be attributed to the insufficient attention given to the mechanisms underlying foam generation and stability at the microscopic level in many studies. For a deeper understanding, this study reviews the most pertinent published works on foam-EOR. The major objective is to provide a broad basis for subsequent laboratory and field applications of foam-EOR. In this work, we highlighted foam classification and characterization, as well as the crucial factors impacting foam formation, stability, and oil recovery. Additionally, the principal mechanisms of foam generation are thoroughly explained. Finally, the most recent developments in foam generation and stability improvement are discussed. Foam-EOR is comprehensively reviewed in this work, with an emphasis on both theoretical and practical applications.

Full text

Citation: Bello, A.; Anastasia, I.;
Cheremisin, A. Foam EOR as an
Optimization Technique for Gas EOR:
A Comprehensive Review of
Laboratory and Field
Implementations. Energies 2023,16,
972. https://doi.org/10.3390/
en16020972
Academic Editor: Rouhi Farajzadeh
Received: 21 December 2022
Revised: 5 January 2023
Accepted: 9 January 2023
Published: 15 January 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
energies
Review
Foam EOR as an Optimization Technique for Gas EOR:
A Comprehensive Review of Laboratory and Field
Implementations
Ayomikun Bello †,* , Anastasia Ivanova †and Alexey Cheremisin †
Center for Petroleum Science and Engineering, Skolkovo Institute of Science and Technology,
Moscow 121205, Russia
*Correspondence: ayo.bello@skoltech.ru
† These authors contributed equally to this work.
Abstract:
Foam-enhanced oil recovery (EOR) is poised to become one of the most promising tertiary
recovery techniques to keep up with the continuously increasing global energy demands. Due to their
low sensitivity to gravity and permeability heterogeneities that improve sweep efficiency, foams are
the preferred injection fluids over water or gas. Although foam injection has been used in the field to
improve oil recovery and control gas mobility, its success is still hindered by several conceptual and
operational challenges with regard to its stability and foamability under reservoir conditions. This
can be attributed to the insufficient attention given to the mechanisms underlying foam generation
and stability at the microscopic level in many studies. For a deeper understanding, this study reviews
the most pertinent published works on foam-EOR. The major objective is to provide a broad basis
for subsequent laboratory and field applications of foam-EOR. In this work, we highlighted foam
classification and characterization, as well as the crucial factors impacting foam formation, stability,
and oil recovery. Additionally, the principal mechanisms of foam generation are thoroughly explained.
Finally, the most recent developments in foam generation and stability improvement are discussed.
Foam-EOR is comprehensively reviewed in this work, with an emphasis on both theoretical and
practical applications.
Keywords: foam; surfactants; nanoparticles; oil; recovery; mobility
1. Introduction
Historically, the main (and most important) source of annual energy consumption has
been petroleum. Petroleum-based products are used to heat buildings and power vehicles,
such as cars and planes that carry passengers and cargo. Plastics, polyurethane, solvents,
and thousands of other intermediate and finished goods for the industrial sector are all
produced by the petrochemical industry using petroleum as the raw material [
1
,
2
]. To put
this into perspective, the global average oil consumption in 2020—including biofuels—was
about 91 million barrels per day [3].
While a decrease in global oil demand is predicted to have a negative impact on
energy exports, domestic oil consumption is anticipated to rise in the next decades despite
increased interest in and investments in renewable energy. According to the Organization
of the Petroleum Exporting Countries (OPEC), global oil demand and consumption will, in
fact, recover from the COVID-19 pandemic by 2036 and rise steadily to roughly 109 million
barrels per day by 2045 [
4
]. From this, it can be deduced that the petroleum sector will
be prompted to develop new techniques and improve existing ones which are capable of
increasing oil production rates from conventional and unconventional deposits to fulfill
the rising energy demand.
Oil production is divided into three stages: primary, secondary, and tertiary. The
first process in recovering oil from deposits is called primary production, and it comprises
Energies 2023,16, 972. https://doi.org/10.3390/en16020972 https://www.mdpi.com/journal/energies

Energies 2023,16, 972 2 of 52
oil extraction from a reservoir by using its natural driving forces. These driving forces
can be gravity drainage, gas drive, and water drive, among others [
5
]. During primary
recovery, only about 5–10% of the original oil in place (OOIP) may be recovered [
6
,
7
].
The second stage of oil recovery is known as secondary production. At this stage, oil
is displaced and driven toward the producing wells by injecting water or gas to replace
generated fluids. This stage aims to maintain or raise reservoir pressure while displacing oil
toward the wellbore. The third and last stage of recovery is known as tertiary production.
Additionally, it is frequently known as enhanced oil recovery (EOR). In order to extract oil
from formations, primary and secondary recovery methods rely on the pressure difference
between the surface and underground wells. However, in order to facilitate production,
tertiary production entails modifying the reservoir rock properties and also the composition,
and physical and chemical characteristics of oil, such as viscosity, specific gravity, and
interfacial tension. Tertiary production can be divided into three major groups: chemical
EOR, gas EOR and thermal EOR [5].
Chemical EOR involves injecting chemical solutions into reservoirs, such as surfactant,
polymer, and alkali solutions, to enhance waterflooding performance generally and help
with the release of trapped oil. Gas EOR involves injecting gases—such as natural gas,
nitrogen, carbon dioxide, methane, or other hydrocarbon gases—into oil reservoirs to
dissolve oil and improve its flow to production wells. Thermal EOR is used in deposits
with high-viscosity oils (API gravity under 20) [
8
]. The reservoir is typically heated with
steam or hot water injections to reduce oil viscosity and increase its mobility to the surface
for production. Figure 1provides a summary of the various techniques.
Figure 1. Stages of oil recovery.
Enhanced oil recovery generally aims to boost sweep efficiency both on a macroscopic
scale and a microscopic scale. The overall displacement efficiency, E, % can be used to
describe its effectiveness:
E=EvEd(1)
where
Ev
= macroscopic sweep efficiency (volumetric sweep), %;
Ed
= microscopic sweep
efficiency (displacement sweep), %.
Macroscopic displacement refers to the region of the reservoir that the injected fluid
has been able to contact, whereas microscopic displacement specifies the volume of the
oil that has been displaced from the reservoir by the injected fluid [
5
]. The volume of oil
left behind in the swept zone and the remaining oil in the unswept zone make up the
trapped oil in the reservoir. Therefore, EOR techniques are used to increase the efficiency
of microscopic and macroscopic sweeps to enhance oil recovery.

Energies 2023,16, 972 3 of 52
Typically, large capillary numbers are aimed to enhance
Ed
with EOR techniques. The
capillary number shows the relationship between viscous and interfacial forces:
Nc=µV
cosθ∗σ(2)
where
µ
= viscosity of injected fluid, mPa
·
s;
σ
= interfacial tension (IFT), mN/m;
V= velocity of injected fluid, m/s; θ= contact angle (wettability), degrees.
From Equation (2), it can be inferred that, an increase in the injected fluid viscosity
and/or a decrease in IFT is required to produce a high capillary number. Surfactants are
used for these purposes because they are known to increase viscosity and decrease IFT by
up to four orders of magnitude [
9
]. According to Hirasaki et al. [
10
], it is also possible to
inject gases as solvents to reduce the IFT and improve microscopic displacement efficiency.
In order to improve
Ev
, polymer flooding can better serve as a mobility control agent by
increasing viscosity but not IFT. To obtain favorable mobility control,
Ev
must be improved.
The following formula describes the mobility, Mof a phase:
M=k
µ(3)
where k= permeability, mD; µ= viscosity of injected fluid, mPa·s.
The mobility ratio of the displacing and displaced fluids is one of the most crucial
elements determining the effectiveness of EOR techniques. The mobility ratio should
be below 1 for an effective flooding process [
11
]. However, it is typically greater than
1 during gas EOR since the reservoir fluid has a higher viscosity than gas. Therefore,
gravity override and viscous fingering frequently result in poor volumetric (macroscopic)
sweep efficiency in gas field operations and subsequently gas breakthrough, which would
ultimately reduce the efficiency of the EOR process [
12
]. Hence, foam-EOR is suggested as
an effective method of mitigating this. This stems from the formation of foam lamella that
can restrict gas mobility for a more efficient sweep. To displace reservoir oil, the formation
of a strong foam can result in a uniform propagation front and, thus, directing gas and
liquid phases to the previously unswept zones [9].
Foam-EOR is currently a developing technique in the petroleum industry and, thus,
just a few review papers are published on it. The majority of them are concentrated on
the use of foam-EOR only in unconventional formations, such as hydraulic fracturing
fluids, surfactant reviews for foam-EOR, CO
2
foam reviews, or nano-stabilized foams.
Wang et al. [
13
] provides an in-depth analysis of the development of technologies for
increased oil recovery. This review, however, only considered how well foam can be
applied to shale and tight reservoirs. Although it covered every other EOR approach in
detail, foam received little attention. The foam was categorized by the authors as a gas
mobility control agent that can aid in the improvement of the viscosity of the injected
gas. The authors went on to explain that although CO
2
miscible flooding is a proven and
promising EOR technique, it is still unable to achieve a perfect sweep efficiency because
macroscale reservoir heterogeneity can lead to fingering and channeling, which can cause
the less viscous phase (gas) to only flow toward the high permeability zones.
Moortgat et al. [
14
] provide additional evidence in favor of this. In order to investigate
how heterogeneity affects miscible gas flooding, the authors developed a three-phase
compositional model through numerical simulation experiments. The authors discovered
that a significant volume of reservoir fluids would be left behind once the miscible phase
propagates across a high permeability zone.
By concentrating on foam-EOR, Wanniarachchi et al. [
15
] moved a bit further away
from Wang et al. [
13
]; yet, the state-of-the-art assessment focuses on the use of foam for
shale gas extraction. The effectiveness of foam-based fracturing fluids for recovering shale
gas was explicitly reviewed by the authors. The capacity to reuse foam was cited as a major
benefit since, following the flooding process, the injected foam may simply be collected
through a well because it is compressed under reservoir conditions. The little foam bubbles

Energies 2023,16, 972 4 of 52
will become larger, degenerate, and eventually escape through the wellbore as a result
of the pressure being released following the flooding process. The authors asserted that
foam-EOR is more efficient than most injection fluids, including water, although requiring
larger capital investments due to the equipment and technology needed.
The work of Pal et al. [
16
] concentrates on the mechanisms of surfactant recovery and
related processes such as wettability alteration, surfactant adsorption, and IFT reduction.
The use of foam as an alternative to the surfactant flooding technique in carbonate reservoirs
was discussed. According to the authors, when polymers, gas, or water-alternating-gas
injection methods are impractical because of unfavorable conditions, such as low perme-
ability, high temperature, or high salinity, among others, foams can be used for mobility
control. Additionally, they emphasized that while gas has a very high microscopic sweep
efficiency, its viscosity is substantially lower than that of water and the majority of reservoir
oils, which affects conformance and mobility on the displacement front. In order to lower
the mobility ratio, foams can be generated by dispersing gas in a continuous liquid solution.
As it is evident, none of the aforementioned studies was able to clarify the arguments
around certain findings relating to the key interaction processes of foam-EOR. In addition,
there are important findings from fresh articles published in recent years that were left out
of earlier review papers. The recent findings, notably the laboratory results, have further
highlighted several research gaps that accounted for some failed field and pilot tests. As
a result, this study provides a thorough and in-depth description of foam and its wide
applicability at laboratory and field scales. By highlighting the results, difficulties, and
potential outcomes, this study provides information on the current status and development
of foam-EOR. The fundamental research gaps are given the most attention. The outcomes
of several field pilot tests, lab-scale flooding experiments, and insights from modeling and
simulation studies were examined, along with recommendations for enhancing oil recovery
in future foam-EOR experiments.
The goal of this review paper is to present an all-inclusive opinion about the feasibility
of foam to recover oil in various reservoirs, as well as an academic viewpoint on the practice
and analysis of the problems and counterarguments surrounding its recovery processes.
2. From Gas EOR to Foam-EOR: Problems and Motivations
Gas EOR involves injecting gases into existing and depleted oil deposits, as was
discussed in Section 1. Injected gases, which may include mixtures of light hydrocarbons
(C
1
–C
4
) or non-hydrocarbons (nitrogen, CO
2
, flue, or even hydrogen sulfide), are usually
vapors at atmospheric pressure and temperature. At reservoir pressures and temperatures,
these gases could, however, transform into supercritical fluids and start to behave similar
to liquids [
5
]. As a result, a key component of the oil recovery mechanism is the mass
interchange of components between the injected gas and oil phase [
17
]. If miscibility
is attained, the interchange rate increases and oil recovery improves. By decreasing oil
viscosity when some gas components condense into the oil, gas EOR can achieve high
oil recovery [
17
]. The oil trapped by capillary forces is expected to be miscible with the
gases that are injected. Following that, the resulting phase forces the oil components to the
surface for production.
In the majority of oil production operations at the secondary or tertiary stages, the
gas injection has been frequently used for several reasons. Among the gases previously
described, CO
2
and nitrogen injections have been considered the most successful and
promising [
18
–
21
]. It is interesting to point out that nitrogen may be preferred due to its
cheaper price and availability compared to CO
2
[
19
,
22
]. However, according to Abdelaal et
al. [
23
], CO
2
has an advantage over nitrogen since it is easily miscible with oil, causing oil
to swell and have less viscosity and interfacial tension under specific reservoir conditions.
As a result, CO2is the gas of choice for injection into light and medium oil reservoirs [18].
Zhang et al. [
24
] further stated that CO
2
is used for significant field activities since
it is readily available in natural resources and may be generated through a variety of
industrial processes. This is a beneficial topic for environmentalists because injecting CO
2

Energies 2023,16, 972 5 of 52
into formations to improve oil recovery can also aid in CO
2
storage in the reservoir, having a
positive effect on the environment. Therefore, CO
2
injection can be seen as a carbon storage
technique that is crucial for reducing CO
2
emissions, which are the main contributors
to global warming. However, in other formations, nitrogen is preferred because, when
injected under high pressure, it may result in a miscible slug that helps release trapped oil
from the reservoir rock [25].
Laboratory tests on improved oil recovery using nitrogen were carried out in the work
of Siregar et al. [
26
]. Nitrogen was injected at different rates into core samples containing
crude oil with an API gravity of 38.5. The results showed that increasing the injection rate
results in higher oil recovery. Additionally, gas analysis tests before and after injection were
carried out, and a change in oil composition was found. After injection, it was found that
the components of the oil were carbon and oxygen. The fraction of gas that was free of
oxygen decreased with increasing injection rates as a result of the oxidation process. The
authors asserted that the oxidation process aided the recovery process.
Gas has many promising advantages as a displacement agent, but its application is
subject to several limitations. The majority of EOR methods aim to recover the most oil
by having the injected fluid make as much contact as possible with the reservoir. In other
words, it is important to improve both the sweep efficiency and the pore-scale displacement
efficiency [
27
]. Since gases are often less dense and viscous than formation water and
reservoir oil, they eventually separate and break through. This is usually referred to as
gravity override. It not only leads to an early breakthrough of the injected gas but also
causes gas wastage and reduces sweep efficiency [28].
Additionally, oil recovery is particularly effective when contact between gas and
reservoir fluids is formed and miscibility is attained since the mobility ratio will be lower
than 1 [
29
,
30
]. Gravity, on the other hand, causes the propagation front to become unstable,
which leads to the formation of finger-like structures made of the oil and gas mixtures and
the massive accumulation of oil that follows. The fingers are more obvious as the mobility
ratio exceeds 1 [
30
]. This is called viscous fingering and it strongly affects oil recovery
by increasing pore-scale displacement efficiency while decreasing sweep efficiency. The
illustration of this phenomenon is shown in Figure 2below:
Figure 2. Viscous fingering during gas EOR.
To improve the poor volumetric (macroscopic) sweep efficiency of gas injection, dif-
ferent gas flooding optimization techniques have been tried. For instance, it is possible
to optimize gas in a water-alternating gas system (WAG). In this process, gas and water
injections are alternated (i.e., gas is injected first, followed by water, gas is injected again,

Energies 2023,16, 972 6 of 52
etc.) [
31
]. Water has a higher viscosity than gas, thus as the gas and water mix, the gas
mobility ratio decreases, improving sweep efficiency.
The implementation of WAG to increase sweep efficiency has been proven to be an
efficient means of controlling gas mobility, which leads to a higher oil recovery [
31
,
32
].
However, as seen in Figure 3below, gas still tends to move to the higher portion of the
formation during the WAG injection cycle [
32
]. According to Gauglitz et al. [
33
], segregation
happens as a result of gravity when there is a significant permeability and density difference
between the injected gas and formation fluids. As rock heterogeneities increase, water
and gas will flow through the high permeability layers, and segregation will become more
visible [
32
]. Therefore, in order to achieve optimal oil recovery, it is crucial to take into
account the volume of water and gas that must be injected throughout a WAG operation.
Figure 3. Gas and water injection cycles during WAG.
Another mobility control technique is foam-assisted WAG (FAWAG), also known as
foam injection [
34
]. Foam is a mixture of liquid and gaseous phases in which the liquid
phase continuously wets the rock formation while the gaseous phase, which is preserved
as bubbles, is discontinued by thin liquid coatings [
33
]. The oil recovery process can be
considerably enhanced by increasing sweep efficiency and pore-scale displacement by
injecting foams. The presence of foam lamella, which decreases gas mobility, enables this.
By making the gas more viscous, foams can stabilize the oil displacement process.
Additionally, as seen in Figure 4, gas flooding alone cannot effectively recover oil; the
WAG method is superior but still leaves some unswept zones. Foam, on the other hand,
has the best sweep efficiency because it can redirect gas flow from high to low permeability
zones and minimize gas mobility there [
9
,
35
]. Furthermore, the surfactant which is present
in the liquid phase of the foam is capable of reducing interfacial tension and capillary
forces. In Ferno et al. [
36
], CO
2
foam was found to have a higher recovery efficiency when
compared to the injection of CO
2
gas in fractured core samples. The authors attributed this
to an improved viscosity of the propagation front.

Energies 2023,16, 972 7 of 52
Figure 4. Comparison of gas, water-alternating gas (WAG), and foam injections.
3. Overview of Foam
3.1. Foam Structure
Foam has been given a variety of definitions and descriptions by various authors.
However, they all agreed on one thing—“the result of a gaseous phase dispersing into a
liquid phase”. In essence, the foam will be formed if a gas and a liquid are mixed. Usually,
a foaming agent such as surfactant is used to prepare the liquid phase. Due to their broad
use in everyday life and industrial importance, foams have long been of significant interest.
A few examples are included in Table 1below:
Table 1. Applications of foam in everyday life.
Group Uses
Food and beverages Beer, ice cream, coffee, champagne [37–39]
Detergent Dishwashing soaps, bottle cleaning process [40]
Waste treatment Sewage treatment, foam fractionation [41,42]
Agriculture Fumigant, insecticides, disinfectants [43,44]
Cosmetics
Shaving cream, bubble bath foams, hair shampoo [
45
]
Pharmaceutical andmedical application treatment of psoriasis, sunburn, eczema, seborrheic,
acne [46–48];
drug delivery [49,50]
The foam may be applied in the petroleum industry at several stages, including well
drilling, reservoir injection, and oil production. Foam as an oil recovery agent is the main
emphasis of this work. Bond and Holbrook [
51
] first proposed the idea of using foam
as an oil recovery technique, when the authors showed how to generate foam in an oil
reservoir by sequentially injecting gas and an aqueous surfactant solution to improve
sweep efficiency.
Foam bubbles are linked by very thin liquid layers, known as lamellae [
52
]. The
plateau boundaries resemble liquid tubes at the junctions of the lamellae. Each lamella is
in contact with the liquid phase and its neighboring lamella [
53
]. The lamellae result in
continuous liquid phases since lamellae are connected throughout the foam channels and
make up the primary components of foam generation and stability. An illustration of the
foam structure is shown in Figure 5below:

Energies 2023,16, 972 8 of 52
Figure 5. Foam structure.
3.2. Classification of Foams
3.2.1. Based on the Structure and Phase Arrangements—Wet and Dry Foams
When gas bubbles are injected into a liquid more quickly than they can evaporate,
a foam structure will always form. A brief dispersion can still develop even though the
bubbles will coalesce as soon as the liquid between them drains away. Wet foam is made
up of sphere-shaped bubbles that are spaced apart [
54
,
55
]. The spherical bubbles in foam
change into foam cells with polyhedral forms and are separated by almost flat liquid
layers. This results in dry foam [
56
]. The surface tensions along the liquid films cause the
arrangements of films to align at equal angles. The bubbles organize themselves into a
polyhedral structure so that along the lamella border, three lamellae always align at an
angle of 120
◦
(Figure 6). The point where they meet is referred to as the plateau border [
57
].
Figure 6. Polyhedral structure of the foam.
3.2.2. Based on the Formation Volume—Bulk Foam and Foam in Porous Media
Bulk foam is defined as a volume of foam that is significantly larger than the size
of the individual bubbles. Because the bubble size is very small compared to the size of
flow channels, bulk foam is typically considered a homogeneous phase in which the liquid
and gas phases have similar velocities [
58
]. Bulk foam is usually used in characterizing

Energies 2023,16, 972 9 of 52
foamability, which is the ability of a foaming solution to form a foam. Bulk foam can be
generated to carry out static tests that can be used for screening different parameters, such
as the type of surfactant, concentration, salinity, temperature, and the effect of crude oil
composition.
Foam in porous media, on the other hand, refers to the generation of foam for eval-
uating its conformity and performance in porous media. Usually, glass bead pack, core
samples or microfluidic chips are used to represent the porous media [
59
]. In the porous
media, foam can be produced in three different modes [9]:
•
In situ: direct injection of the liquid and gas phases into the porous medium simulta-
neously;
•
Pre-generated: injection of pre-generated foam using a foam generator or another
mechanical device;
•
Surfactant-alternating gas (gas): alternate injections of gas and liquid phases into the
porous media.
3.2.3. Based on Gas Quantity—Low Quality and High Quality
Foam quality refers to the ratio of gas volume to foam volume at a particular pressure
and temperature [60].
Γ=VG
VG+VL
·100 (4)
where Γ= foam quality, %; VG= volume of gas, mL; VL= volume of liquid, mL.
Low-quality foam is typically described as having a quality of up to 52%. In this
instance, the gas bubbles are spherical and rarely come into contact with one another. Since
there is a lot of free fluid in this system, foam viscosity is low and will affect the capacity
of the system for fluid loss. High-quality foam, on the other hand, has a quality between
52% and 96% [
61
]. Because the gas bubbles are coming into contact with each other more
frequently, the viscosity will increase the foam of high-quality polygonal bubbles, and
is known as Polyederschaum; a foam of low quality produces bubbles that are roughly
spherical and are separated by a thick layer known as kugelschaum [62,63].
3.2.4. Based on The Lamella Numbers
The lamella number (L), a measure proposed by Schramm et al. [
64
], is used to
determine the extent of oil imbibition into foam lamella. Different types of crude oil cause
foam to behave differently, and oils are normally considered to have a negative impact on
foam stability. The following three metrics, spreading coefficient (S), entering coefficient
(E), and bridging coefficient (B), can be used to conduct a qualitative investigation of the
phenomena. Below is a definition of the formulas:
S=σw−g−σw−o−σo−g
E=σw−g+σw−o−σo−g
B=σw−g2+σw−o2−σo−g2
(5)
where σ= interfacial tension, mN/m; o,w,g= oil, water, and gas indices, respectively.
Thus, Schramm et al. [64] suggested the relationship below:
L=0.15σw−g
σw−o(6)
Foams were divided into A, B, and C (based on the values), representing L < 1, 1 < L >
7, and L > 7, respectively. The most stable foams, those that do not react with oil, are Type
A foams. Both E and S have negative values for them. Type B foams are relatively stable
and have negative S and positive E values. In this case, foam lamellae interact with oil,
yet they do not collapse. Type C foams have positive values for S and E and are unstable.
These foams frequently absorb oil, which causes the foam lamellae to separate.

Energies 2023,16, 972 10 of 52
4. Role of Surfactants in Foams Generation
Surfactants arrange themselves at the gas–liquid interface when they are dispersed in
water, with the hydrophilic heads staying in the liquid phase and the hydrophobic tails
extending out of the liquid phase (Figure 7). The surfactant monolayer then acts as an
additional mass to oppose the diffusion of a substance across it, which helps to reduce
foam coarsening [65].
Due to their ability to change the wettability of rocks, surfactants have a significant
impact on foam generation. Wettability is a crucial factor that significantly influences oil
recovery in reservoirs that fully or partially use chemical EOR techniques [
66
,
67
]. The
propensity of a fluid to spread preferentially onto a solid phase when additional fluids are
present is known as wettability.
Typically, the contact angle is used to describe the wetting properties. The rock is
considered water-wet if the contact angle is between 0
◦
and 75
◦
; intermediate-wet if it
is between 75
◦
and 115
◦
; and oil-wet if it is between 115
◦
and 180
◦
[
68
]. In foam-EOR,
wettability alteration is crucial, especially when the foam is intended to recover oil after
primary and secondary EOR procedures, which typically fail to mobilize trapped oil due
to high capillary forces. Furthermore, since the viscous forces in the porous media are
ineffective at displacing oil, the majority of unswept oil is retained in the low permeability
zone of the rock, leaving an imbibition process as the only effective method of oil recovery.
Surfactants can considerably alter the wettability of rock surfaces depending on their
hydrophilic head charges.
Figure 7. Arrangement of surfactant molecules at the foam lamella.
Two processes of surfactant-induced wettability modification in carbonate reservoirs
were presented in the works of Standnes et al. [
66
,
69
]. They include removing the oil-wet
layers to reveal the original water-wet layers beneath, and adding a water-wet layer on
top of the oil-wet ones. The first is better suited for cationic surfactants and the second
for anionic.
Reduction of the interfacial tension is another function of surfactants in foam-EOR.
In oil reservoirs, the capillary, viscous, and gravitational forces interact to some extent
to control the rate of oil recovery [
70
]. The following equations describe the relationship
between these three types of forces:
Bond number(NB) = Gravitational forces
Capillary forces (7)
Capillary number(NC) = Viscous forces
Capillary forces (8)

Energies 2023,16, 972 11 of 52
A decrease in capillary forces results in a desirable value for the bond and capillary
numbers in each of these equations. The following equation describes the capillary force as
a function of the IFT between oil and water:
FC=2σo−wcosθc
r(9)
where
σo−w
= interfacial tension between oil and water, mN/m;
θ
= contact angle, degrees;
r = pore radius, mm.
Morrow et al. [
71
] assert that the bond number is crucial and that lowering IFT
may have either a good or negative impact on imbibition. Although a drop in capillary
imbibition may result from a reduction of IFT, imbibition can still happen as a result
of gravitational forces. This demonstrates the strong correlation between capillary and
gravitational forces and the IFT value. Surfactants decrease IFT, which weakens the capillary
adhesive forces that hold trapped oil in the porous media. As a result, oil droplets pass
through pore throats more easily and move in the direction of the displacement front [19].
A recent test on spontaneous imbibition by Mohammed et al. [
68
] involved a cationic
surfactant and limestone core samples. The findings showed that when the core samples
were subjected to distilled water, the oil-wet nature of the core samples and the negative
capillary forces resisted the gravitational forces. When the core was subjected to a cationic
surfactant solution, the same outcomes were attained, and oil recovery was not noticed
until 10 days had passed. This is a result of the wettability alteration, which improved
capillary imbibition.
5. Governing Mechanisms of Foam-EOR
5.1. Mobility Control Mechanisms of Foam-EOR
Foam exists as a gas–liquid mixture in the porous media, where the liquid continu-
ously wets the rock and the lamella causes the gas to become discontinuous. The foam is
introduced to develop a system that would enable uniform gas mobility for oil recovery in
porous media. Its mechanisms are discussed below.
Gas mobility can be controlled with foam by stabilizing the displacement front. As
previously discussed, viscous, capillary, and gravity forces are difficult to manage in gas
EOR, therefore, surfactants are introduced as chemicals to minimize viscous instability.
Foam stabilizes the displacement front by bubble movement and rearrangement of the
bubble interfacial area [
72
]. A bubble requires more pressure to move at a constant rate
than an equivalent volume of liquid. This, therefore, increases the actual velocity of the gas.
Similar to how surfactant movement at the gas–liquid interface causes a surface tension
gradient to delay bubble motion, viscosity is also increased by surfactant mobility [
73
]. At
the pore scale, the wetting phase takes up the smallest pore channels while foam continues
to flow through the high permeability and porosity zones. There is a significant amount of
gas trapped in the averagely-sized pores. As a result, the pore space that is expected to be
filled up with gas will be blocked, which lowers the relative gas permeability [74].
Foam can also be used to control gas mobility by reducing capillary forces. In
Zhang et al. [
75
], the authors injected CO
2
into a micromodel. The methodology em-
ployed in this work consists of a core flooding setup where cores packed with cleaned
sand at a back pressure of 3.5 MPa and temperature of 27
◦
C was used to evaluate the
performance of foam injection. Their findings, however, indicated that the residual oil
trapped in the model could not be displaced by CO
2
since oil and gas had a high mobility
ratio. This result underlined the necessity to first emulsify the oil in order to separate it
from the porous media, which can be done by introducing a chemical phase that can reduce
interfacial tension and result in the development of oil-water emulsions at the displacement
front. More oil and water are solubilized to form emulsions when the chemical phase
moves through the porous media, which leads to the mobilization of oil.
There are other parameters involved besides IFT reduction to achieve a considerable
change in the capillary number. The parameter, known as the contact angle in Equation (9),

Energies 2023,16, 972 12 of 52
can be used to characterize wettability. The interactions between surfactants and the solid
surface of the porous medium cause wettability to change during a foaming operation. The
adsorbed surfactant on the surface lowers surface tension and changes the ability of the
rock for wetting with water or oil. Although surfactant adsorption on the rock is important,
it is not necessarily sufficient to change wettability. The degree to which this is necessary
depends on several parameters, including the original wettability of the rock, the structure,
the type and the concentration of the surfactant, residual oil properties, and brine salinity.
In [
76
], visualization of the oil-wet micro-cell and core flood tests were performed. It
was revealed that when the porous media was oil-wet, much fewer bubbles were produced
due to lamellae detachment and collapse. Similarly, investigations by Sanchez et al. [
77
] in
oil-wet porous media showed that foam formation is a result of the wettability alteration
from oil-wet to water-wet. These arguments suggest that a water-wet system is the best
wettability condition for foam generation. However, the results of Lescure et al. [
78
]
contradict this. The experiments involved a foam flood and a comparison of wettability
measurements. The findings revealed that as a result of significant surfactant adsorption
in a water-wet medium, foam is more effective in an oil-wet medium than in a water-wet
one. However, in order to develop a valid theory, a wide range of concentrations must be
thoroughly investigated. Future research should also take into account a foaming agent
that performs well in both water- and oil-wet environments.
The last mobility control mechanism with foam is improvement in interfacial mass
transfer. It is known that foam increases the possibility of gas interactions with oil since
it can control gas mobility in a porous medium. This is a beneficial mechanism for gases,
particularly CO
2
, because mass component interchange greatly boosts miscibility and,
consequently, oil recovery. Furthermore, foam might result in large flow resistance and
a longer retention period for gas in the porous media. This means foam can prevent or
completely stop gas fingering, mobilizing oil by reducing its viscosity and swelling. This
is consistent with the findings of Farajzadeh et al. [
79
], where the authors stated that the
addition of surfactant at a high pressure significantly improved the mass transfer of CO
2
into water and oil.
5.2. Mechanism of Foam Generation in the Porous Media
The size and distribution of the pore throats have a significant impact on foam gen-
eration in the porous media, because foam bubbles can be filled into one or more of the
pores. In the porous media, foam exists as a discontinuous phase. To visualize and pinpoint
the primary mechanism of foam generation in the porous medium, Ransohoh et al. [
80
]
conducted glass bead tests at the pore scale level. These mechanisms were further verified
by Almajid et al. [81] in their study using silicon micromodels.
The first of these mechanisms is Snap-off (Figure 8). It is the dominant mechanism
for generating foam bubbles in porous media and the most probable foam generation
mechanism during the co-injection of liquid and gas phases. It is a mechanical process that
takes place repeatedly during a multiphase flow in porous media [
82
]. It is referred to as a
mechanical process because it involves the formation of foam bubbles in the pore throat by
the wetting phase due to the flow of gas bubbles through the pore throat [80].
(a) (b) (c)
Figure 8.
Snap-off mechanism process of foam generation in the porous media (adapted from
Ransohoh et al. [80]).

Energies 2023,16, 972 13 of 52
When the gas phase reaches the liquid phase-filled throat, it is blocked at the upstream
side by the pore throat. When the capillary entry pressure is surpassed, it causes the
interface to become curved and increases the capillary pressure, allowing the generated
foam bubble to flow through the pore throat. The pressure inside the foam bubble lowers
as expansion occurs at the interface, and a wedging layer is created as a result of a negative
capillary pressure gradient [
83
,
84
]. Thus, a foam bubble is snapped off and the liquid is
forced into the pore throat from the pore body. An important criterion for snap-off to occur
is that capillary pressure at the throat must be higher than capillary pressure at the front
of the interface. Notable, the ratio of pore body to pore throat needs to be approximately
higher than 2 for this to happen [80].
The second mechanism is lamella division (Figure 9). Large bubbles split into smaller
ones during lamella division. This happens when the interface stretches around the
branching point as illustrated in Figure 9, and the flow of foam bubbles branches in
different directions as a result of this [
80
]. Foam generation with the lamella division
mechanism requires a pre-generated foam with a large bubble size to pore body ratio to be
injected into the porous medium. Moreover, there must be a significant pressure gradient
for the lamella to divide [85].
(a) (b)
Figure 9.
Lamella division mechanism of foam generation in the porous media (Adapted from
Ransohoh et al. [80]).
As seen in Figure 10, the leave-behind occurs when two gas menisci enter the adjacent
branching pore bodies that are filled with liquid.
(a) (b)
Figure 10.
Leave-behind mechanism of foam generation in the porous media (Adapted from Ranso-
hoh et al. [80]).
In the snap-off and lamella division mechanisms, the formed lamellae are relatively
perpendicular to the direction of flow [
84
]. The lamellae from leave-behind, on the other
hand, are parallel and are frequently related to a continuous gas phase that is present at
the start of a foam flooding operation [
80
]. The invasion of two or more gas fronts from
different directions into a liquid-saturated medium causes the leave-behind mechanism,
unlike the snap-off and lamella mechanisms, to produce less stable and weaker foams. This
causes the generation of continuous gas bubbles, which build up to create parallel rows of
many lamellae and obstruct the passageways, creating dead-end paths. Generally, foam

Energies 2023,16, 972 14 of 52
generation by the leave-behind mechanism is not recommended because it results in the
generation of weak and unstable foam [82].
Pinch-off is another mechanism for foam generation that was found in microfluidic
channels by Liontas et al. [
86
]. While neighbor-neighbor pinch-off is caused by two neigh-
boring bubbles, neighbor-wall pinch-off happens when a bubble is pinched off by the
adjoining bubble and the walls of the channels [
86
]. These mechanisms occur at a high
capillary number.
5.3. Mechanism of Foam Instability
All foams have significant surface tension, which makes them thermodynamically
unstable. Foam requires more effort to be generated and becomes less stable when surface
tension increases [
87
]. Gas diffusion (Ostwald ripening, bubble disproportionation) is
the most crucial mechanism since it underlies all the other mechanisms (drainage and
coalescence), making it the cause of foam instability. The stages that foam can go through
before being completely destroyed are depicted in Figure 11. While some foams undergo
the whole stages, others collapse as a result of an external disturbance [88].
Figure 11. Progression of foam stability from generation to coalescence.
In order to explain gas diffusion, let us consider Figure 12 below:
P2
P3
R
P1
Foam film
Foam bubble
Plateau border
Figure 12. The pressure difference between foam bubbles ( Adapted from Langevin et al. [89]).
Points P1, P2, P3 represent arbitrary points within a foam bubble, outside of a foam
bubble, and in the foam film respectively. Thus, the differential pressure between points in
and outside of a foam bubble can be represented by the Laplace equation presented below:
P1−P2=2γ
R(10)
where
P1
and
P2
represent pressure in and outside the foam bubble respectively, MPa;
γ
is
the surface tension, N/m and R is the bubble radius, m.
The theory of surface tension postulates that the pressure inside the bubble is higher
than the pressure outside due to surface tension, which tries to reduce the surface area

Energies 2023,16, 972 15 of 52
and needs to be counterbalanced so that the bubble does not collapse. Thus, since there is
no pressure difference over a flat contact, the pressure in the film must be higher than the
pressure outside the bubble [89].
P1=P3>P2(11)
The repulsive interaction between the two surfaces, which may have an electrostatic
origin as a result of the ionization of the surfactant head groups, is solely responsible
for the pressure increase in the film [
87
,
90
]. As a result, the plateau border experiences
lower pressure, which acts to drain liquid from films between bubbles and increase the risk
of collapse.
Additionally, for smaller bubbles, the pressure and solubility of the dispersed gas
phase are higher. Diffusion from small gas bubbles to bigger ones or the bulk liquid phase is,
therefore, driven by a force [
88
]. The solubility of the dispersed gas phase in the continuous
liquid phase affects the rate of diffusion. For instance, foam in carbonated drinks exhibits
this behavior, where more bubbles quickly arise after the first foam generation occurs when
the drink is poured into a cup.
Note that a bubble has a higher pressure than the gas around it and that this pressure
increases as the bubble becomes smaller. A wider bubble size distribution results from gas
diffusion from small to large bubbles over time. This can be quantified using Henry’s law
to assess gas solubility in the liquid [91].
6. Factors Affecting Foam Stability
Several changes start to occur as soon as foam is generated. Through gas diffusion in
the continuous phase, the smaller bubbles continue to enlarge and may eventually dissolve
to result in disproportionation and Ostwald ripening. The bubbles emulsify quickly and
separate to form a foam layer on top of a bulk liquid. The liquid then drains from the
foam into the bulk liquid and the lamella between foam bubbles breaks, which causes
the bubbles themselves to distort one another and form a polyhedral foam, leading to
bubble coalescence.
6.1. Gibbs–Marangoni Effect
As was noted in earlier sections, pure liquids cannot produce stable foam in the
absence of a surfactant. When gas is put into a liquid phase (just water), as soon as all
the water has been drained away, it coalesces almost instantly. Contrarily, the liquid–gas
interface expands in the presence of surfactant solutions, upsetting the equilibrium at the
surface, and a restoring force known as the Gibbs–Marangoni effect emerges to restore the
equilibrium [
92
,
93
]. A dilatational elasticity is created because of a surface tension gradient
that is present, due to the surfactant. This is known as Gibbs elasticity [
94
]. The flow of
surfactant molecules from the bulk liquid to the interface is then caused by the surface
tension gradient. This phenomenon is referred to as the Marangoni effect [
92
]. Thus, the
Gibbs–Marangoni effect stabilizes the foam by preventing the thinning and disturbance of
the liquid film between the gas bubbles.
At various surfactant concentrations, the maximum foaming behavior is perfectly
explained by the Gibbs–Marangoni effect. Surfactant molecules near the interface cause
the surface area of the foam bubble to increase, which in turn creates an inward strain.
Because the gradient moves the surfactant to the areas with thinner layers, the concentration
of surfactant at the interface decreases proportionally as the surface area increases [
95
].
Foamability will, therefore, be quite low. This is why the surfactant concentration reduces
in a foaming system when a stable film expands.
Due to the substantial supply of surfactant that diffuses to the surface at high surfactant
concentrations, the differential surface tension relaxes too quickly. It also creates a very
thin coating with low foamability, giving the restoring force enough time to balance out the
conflicting forces. The maximum foaming ability is produced at the intermediate surfactant
concentration range [
95
,
96
]. Gibbs–Marangoni effect can therefore affect foam stability due
to the surface tension phenomenon.

Energies 2023,16, 972 16 of 52
6.2. Interfacial Tension
The gas–liquid interface, which separates the dispersed and continuous phases, is a
tiny intermediate boundary, as was previously mentioned. Due to the large surface area of
the gas bubbles, they have high surface energy [
94
]. However, relatively low interfacial
energy is required to produce stable foams. Surfactants or any other additive that can
reduce interfacial tension is added to achieve this. Another method to achieve this is to
supply enough mechanical energy by mixing or agitation.
Due to the synergy between surface energy and foam stability, interfacial tension is
crucial to foam-EOR. Hosseini-Nasab et al. [
97
] describe a foam technique for mobilizing
residual oil following water flooding. This was accomplished by combining two anionic
surfactant formulations, the first of which was used to achieve a low IFT and the second
was used to create a stable foam by co-injecting CO
2
and N
2
gases. According to their
findings, a lower IFT resulted in significantly higher oil recovery with a smaller mobility
reduction factor. In reality, the authors claimed in their conclusion that the ultralow IFT
reduction was the main mechanism for one of the generated foam variants. This proves
how crucial IFT reduction is during foam-EOR.
Similar findings were made by Yekeen et al. [
98
], where it was discovered that a
significant decrease in decane-liquid IFT improved foam stability. This improvement was
attributed to adsorption and tighter packing of the adsorbed surfactant molecules at the
gas–liquid interface, foam lamella, and plateau borders. The strength of the film is necessary
for a foam system to be stable. Indeed, IFT needs to be decreased to an ideal amount in
order to improve oil displacement via foam injection. By increasing the concentration of
the surfactant, IFT reduces and a more stable foam can be generated.
6.3. Gravitational Drainage
Gravitational drainage is a crucial process in foam stability because it involves the
interaction of viscous gas flow, surfactant molecule diffusion and convection, and their ad-
sorption at the film interface [
99
]. Gravitational effects are very important in the generation
of stable foams because they control how much liquid is drained from the foam. When
the foam is sufficiently stable, it dries out and a vertical pressure gradient in the liquid bal-
ances the gravitational force, resulting in a vertical profile of the liquid component. Liquid
drainage causes the liquid layers to thin out, which causes the foam bubbles to coalesce.
Additionally, the gravitational impact of liquid moving from top to bottom due to gas
diffusion through the liquid layers causes the foam to dry up and become unstable [100].
6.4. Wettability
Surfactants can change the rock wettability from being oil-wet to more water-wet. On
oil-wet rock surfaces, foam formation via a snap-off mechanism is typically restricted. This
is because the film on the surface needs to be continuously wetted, which is necessary
for the passage of liquid from the pore body to the pore throat, and a prerequisite for
the snap-off [
80
]. If the surfactant adsorption is insufficient, the lamella may still become
unstable on the oil-wet surface following foam generation. Since the film is continuously
wetted, the lamella is initially stable across the water-wet surface. The surfactant monomers
can, however, adsorb at both the gas–water and water-oil interfaces as the lamella moves
toward and over the oil droplet or a wet surface. This leads to a larger disjoining pressure
and a more continuous film on the surface [
101
]. Alternately, the oil droplet may enter
the gas–liquid interface if the film that separated the gas bubbles from the oil droplet is
unstable, which would result in the lamella pinching off.
The effect of wettability on foam stability and generation has been investigated experi-
mentally by several authors. Flooding tests were performed by Kanda et al. [
102
] using
glass bead packs that were either oil- or water-wet. The results showed that the foam in the
oil-wet system was unstable. They asserted that the oil-wet surface had caused the foam
films at the pore throats to become unstable.

Energies 2023,16, 972 17 of 52
Similar to this, Kuhlman et al. [
103
] and Kristiansen et al. [
104
] reported poor foam
stability in oil-wet pores. In oil-wet porous media, foam generation and stability can be
limited, but it is also feasible that foam can be produced and stabilized. This may occur if
the surfactant adheres to the surface and changes the wettability toward more water-wet.
This argument is supported in the work of Sanchez et al. [
105
] where it was revealed that
wettability modification toward water-wet is a crucial condition for generating stable foams.
According to the findings, the residual oil prevented the oil-wet system from changing its
wettability since the surfactant components were less effective at doing so because they
were concurrently adsorbing at the oil-solid and water-solid interfaces. Similar findings
were made by Mannhardt et al. [
106
] where it was illustrated that weak and unstable foams
were generated when the cores were intermediate-wet with residual oil.
6.5. Oil Presence
The ability of foams to remain stable for as much time as possible in the reservoir when
coming into contact with oil is one of the difficulties of foam-EOR. This is important for
achieving favorable mobility control as well as a favorable oil recovery rate. Experimental
studies have demonstrated that some of the components in the oil phase cause foam
to become more unstable in the reservoir. Low molecular weight oil dispersed in the
surfactant solution, according to Aveyard et al. [
88
], shortens the half-life of the foam.
However, experiments by Mannhardt et al. [
106
] demonstrated that stability in the presence
of oil can be increased if the foaming agents are properly selected and screened. The authors
noted that the tolerance of foam to oil increased with the addition of fluorinated surfactants
to various types of hydrocarbon surfactants. Additionally, a group of foam flow studies was
carried out by Pu et al. [
107
] in which foam was generated in the porous media and showed
greater stability when oil with a larger molecular weight was present. The spreading of oil
droplets is a common mechanism by which oil droplets affect foam stability. According to
the proposed theory, oil droplets squeeze between the film surfaces during foam generation
spread over one of the film surfaces, breaking the lamella. Nikolov et al. [
108
] provided
evidence for this. The authors reported that the process of oil solubilization from the
oil phase into the micelles contained in the liquid phase might play an important role in
foam stability.
Farzaneh et al. [
65
] explained that the pseudo-emulsion film influences how much oil
affects foam stability. This is the aqueous film that forms between the gas and oil phases
in the foam system. While an unstable pseudo-emulsion film will have a detrimental
stabilizing effect on foam, a stable pseudo-emulsion film can stabilize foam and even
increase stability as oil saturation increases. The authors explained that the oil droplets will
drain more slowly in the plateau borders than in the surrounding aqueous phase due to
the buoyant force of the oil droplets and the obstacle to their movement within the plateau
borders. Therefore, the plateau borders are rather thick and can delay liquid drainage when
a stable pseudo-emulsion film is present. If the films are unstable, however, the oil droplets
agglomerate on the plateau edges and grow larger as a result of more foam drainage.
6.6. Thermobaric Conditions
The two key reservoir parameters that determine the stability of foam are pressure
and temperature. An increase in pressure is advantageous to foam stability, according
to many studies. This is because there will be more resistance to liquid drainage and
Ostwald ripening as pressure rises since the liquid layers grow larger and stronger [
9
,
58
].
Additionally, as pressure rises, less gas will diffuse between the lamellae, thus increasing
foam stability. Wang et al. [109] demonstrate this (Figure 13) phenomenon.

Energies 2023,16, 972 18 of 52
6 7 8 9 10 11 12 13 14
1
2
3
4
5
Pressure (MPa)
Dimensionless Half-life
N2
CO2
Figure 13. Effect of pressure on CO2and N2foams stability. (Adapted from Wang et al. [109]).
In this study, the performance of foam under high pressure and temperature conditions
was investigated using a visualizing foam meter. The setup mainly consists of a high-
pressure cell, a fluid injection system, and observation windows for measuring foams
generated in the visual chamber. Under pressure varying from 6 to 14 MPa, six surfactants
were examined, and their half-lives were used to assess the stability of the foam. CO
2
and
N2were co-injected to generate foam.
According to the authors, increasing pressure will generally increase the density of
gas and make it more hydrophobic, which can aid in the embedding of the surfactant
hydrocarbon tails into the oil phase.
Oppositely, temperature increase generally causes a decrease in foam stability because
the half-life is reduced. This is due to the fact that when the temperature rises, the liquid
phase has a greater tendency to evaporate, which causes the bubbles to quickly collapse
and release the gas they contain [
60
,
92
]. Furthermore, when the temperature rises, the gas
phase becomes more soluble and the interfacial strength between the gas and liquid phases
decreases. This causes the foam to become more unstable by increasing liquid drainage [
60
].
The viscosity and elasticity of the foam lamella will decrease at higher temperatures, which
will have a significant impact on the performance of the foam although the extent of the
deterioration of foam stability may depend on the chemical composition or hydrocarbon
chain length of the foaming agent used (Figure 14).
60 70 80 90 100 110 120
0
20
40
60
80
100
SDS - 86% Sodium Dodecyl Sulfate
SDBS - Sodium Dodecyl Benzene Sulfonate
SDS’ - 98% Sodium Dodecyl Sulfate
Temperature (°C)
Half-life (min)
SDS
SDBS
SDS’
Figure 14. Effect of temperature on foam stability (Adapted from Wang et al. [109]).

Energies 2023,16, 972 19 of 52
The work of
Wang et al. [110]
also demonstrates how temperature affects foam stability.
Experiments were carried out to investigate the influence of temperature on the stability
of foams generated from different types of surfactants. Although the authors highlighted
that anionic surfactants are less impacted by temperature compared to cationic and non-
ionic surfactants, foam stability decreased with increasing temperature in all cases. They
attributed this to decreased surface viscosity, which led to quicker liquid drainage at higher
temperatures. The results of
Liu et al. [111]
further support this. The authors of this study
demonstrated that at higher temperatures, bubble size increased uncontrollably, which
caused a rapid collapse.
6.7. Salinity
The percentage of salt ions in the liquid phase is another parameter that has a deciding
impact on the stability of foams. At high salinity, the presence of salt ions in the bulk fluid
reduces the surface potential at the gas–liquid interfaces, hence reducing the repulsion
between the two layers [
60
,
112
]. Increased salinity, in this case, causes a decrease in double-
layer repulsion, which promotes liquid to drain from the foam lamella. As a result, foam
stability decreases as salinity increases.
The ionic interactions between the foaming agent (surfactant) and salt can determine
how foam behaves. Consequently, depending on the strength of the interactions, foam
stability can be improved or worsened [
113
]. To illustrate the effects of salt on foam stability,
both positive and negative effects are discussed below.
In the work of Bello et al. [
60
], the foam was generated using 0.3 wt.% of AOS surfactant
with salinity ranging from 0 wt.% to 15 wt.% MgCl
2
. Figure 15 below demonstrates that
foam stability is only improved at low salinity, and stability degrades when a salt content
goes above 2%. The authors attributed this to the forceful deposition of additional ions on
the surface of the foam film, which led to a partial neutralization of the electric charges.
This decreased the electrostatic repulsion and hence made the foam films unstable.
0 2 4 6 8 10 12 14 16
12
14
16
18
20
Salt concentration (wt.%)
Foam height (cm)
Figure 15. Effect of temperature on foam stability (Adapted from Wang et al. [109]).
The findings of the study by Le et al. [
114
] appear contrary. The authors investigated
the stability of foam under dynamic conditions for salinity ranging from 5 to 25 wt.% NaCl.
Ethomeen C
12
was employed as the foaming agent. According to their findings, the foam
was noticeably more stable at higher salinity (Figure 16).

Energies 2023,16, 972 20 of 52
0 2 4 6 8 10 12 14 16 18 20 22 24
0
0.2
0.4
0.6
0.8
1
Time elapsed, hours
Normalized foam height
5% NaCl
15% NaCl
20% NaCl
25% NaCl
Figure 16. The positive influence of salinity on foam stability (Adapted from Le et al. [114]).
According to the authors, the success of this interaction at higher salinity was due to
the compression of the electrical double layer and an increase in the maximum disjoining
pressure at higher salinity. Bello et al. [
60
] asserted that bubble coalescence typically begins
early in many surfactants at high salinity conditions, which worsens the foam stability.
However, in the work of Le et al. [
114
] where Ethomeen was used, the ionic interactions
between the protonated amine headgroups with the Cl
−
counter ion strengthened the
molecular density of the surfactant.
7. Laboratory Studies of Foam-EOR
The harsh conditions of reservoirs, as was covered in earlier sections of this work,
have the biggest negative impact on the stability of foam and its use as an oil recovery
agent. This has piqued the curiosity of researchers, who are stepping up attempts to find
solutions for foam instability. This section only focuses on the investigations of foam-EOR
in the laboratory and thus, advances in improving foam stability will be discussed in a
section below.
In order to assess the potential of foam as an EOR agent, Haugen et al. [
115
] described
tests including the application of foam in low permeability, oil-wet core samples. According
to the findings, less than 10% of the oil was recovered with water, surfactant, or gas injection.
On the other hand, pre-generated foam injection was able to recover up to 78% of the
original oil in the core. The authors attributed this to poor gas mobility in the fractured
rock, which led to an increase in differential pressure and a diversion of flow toward the
oil-saturated matrix.
To study the impact of injection mode on foam-EOR, John et al. [
116
] presented
investigations comprising laboratory experiments and numerical modeling. Pre-generated
foam was injected into the core sample at 34 MPa and 55
◦
C. The findings indicated that
CO
2
foam, as opposed to pure CO
2
and WAG, can recover more oil after flooding with
water. The results of their numerical simulation were also similar. The calculated mobilities
demonstrated the improved efficiency of CO
2
foam as a blocking agent, reducing CO
2
mobility while increasing its effective viscosity.
An ultra-low interfacial tension foaming agent was used by Kang et al. [
117
] for the
dual purpose of reducing oil-water IFT and enhancing foam stability in core samples
containing heavy oil. In order to lower IFT and increase foam stability, the authors em-
ployed commercially available surfactants, modified natural carboxylates, and partially
hydrolyzed polyacrylamides. The gas phase used was nitrogen. Their results showed
that foam flooding can also be applied in heavy oil as ultimate oil recovery was improved
to approximately 23% initial oil in place over waterflooding recovery. The authors at-
tributed this to the ultra-low interfacial tension between oil and water as well as an effective
mobility control.

Energies 2023,16, 972 21 of 52
The foam was used as a thermal EOR technique by Chen et al. [
118
]. The paper focuses
on an investigation of the viability of a foam-assisted SAGD (FASAGD) through numerical
simulation. Their simulation results demonstrated that a significant volume of stable foam
was generated and accumulated in the area between the wells. Additionally, the inclusion
of steamed foam helped in limiting steam breakthrough and produced superior recovery
results than the traditional SAGD. Cumulative oil recovery was increased by roughly 30%
with FASAGD compared to the cases without foam (Figure 17).
0 200 400 600 800 1 000 1 200 1 400 1 600 1 800 2 000 2 200
0
20
40
60
80
100
120
140
160
180
SAGD - Steam-Assisted Gravity Drainage
FASAGD - Foam-Assisted SAGD
Cumulative steam injected, CWE m3
Cumulative oil produced, m3
SAGD
FASAGD
Figure 17.
Comparison of the recovery efficiency between SAGD and FA-SAGD (Adapted from Chen
et al. [118]).
A comprehensive summary of laboratory foam-EOR experiments is given in Table 2be-
low:

Energies 2023,16, 972 22 of 52
Table 2. Laboratory studies of foam-EOR.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous Medium Permeability
(mD) Porosity (%) Main
Mechanism Results
[12] Pre-generated 0.3 wt.% Neodol CO2and N2
Extra heavy
crude oil API
11.5◦
4.1 MPa; 44 ◦C N/A Microfluidic
model N/A N/A Increased
viscosity
Improved CO2
foam recovery
performance;
N2foam was
more stable than
CO2foam.
[35] Pre-generated 0.2 wt.% Coco
betaine surfactant Air N/A Ambient N/A PDMS
microfluidic chip N/A 45 Sweep profile
improvement
Foam tends to
become drier
when diverted
into the low
permeable region
especially in the
case of wet foam;
Foam effectively
improved the gas
sweep efficiency.
[36] Pre-generated 1 wt.% AOS
surfactant CO2n-Decane with
viscosity 1.01 cp 9 MPa; 20 ◦C4 wt% NaCl,
3.4 wt% CaCl2,
0.5 wt% MgCl
Edwards
limestone 11–60 18–25 Increased
viscosity
Foam
significantly
increased oil
recovery
compared to pure
CO2injection by
adding a viscous
component to the
oil recovery
process.
[58] Pre-generated
1:1 blend of
cocamidopropyl
betaine and
sodium dodecyl
sulfate (SDS)
surfactants
N2
Isopar G (0.843
cp)and Isopar V
(10.840 cp) Ambient 0.25 M NaCl Microfluidic chip 5200 57.5 N/A
There was
formation of a
less stable front
as the foam
quality increased,
leading to less oil
recovery.
[119] Pre-generated 0.3 wt.% Neodol CO2
Medium-heavy
crude oil with
API 19.1◦
4.1 MPa; 44 ◦C 30,000 ppm NaCl Microfluidic chip N/A N/A Gas breakthrough
reduction
Significant
improvement in
oil recovery of up
to 90%;
Reduction of
injected CO2.

Energies 2023,16, 972 23 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[120] Pre-generated CTAB, DTAB,
AOS, AES CO2and N2Crude oil 80 ◦C3500–
179,863 mg/L Indiana
limestone 50–100 15–20 Increased
viscosity
N2foam
exhibited better
foamability
and stability
than CO2foam;
CTAB showed
excellent oil
recovery of 92%
cumulative
recovery factor.
[121] Pre-generated
0.5% Surfactant
solution N2
Crude oil with
viscosity
11,310 cp
5 MPa;
50–300 ◦CN/A Sand pack 1124 and 5735 33 Uniform
propagation
front
Foam
displacement
efficiency was
81.24%, which
was 37.94%
higher than
steam flooding;
Foam increased
the flow
resistance of
steam and
improved
micro-sweep
efficiency;
Thermal foams
can
significantly
enhance the
injection profile
by propagating
hot water and
steam evenly
across the
reservoir.

Energies 2023,16, 972 24 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[122] Pre-generated GINF-1 and
LINF-2
surfactants Air Crude oil with
viscosity of
23 cp 25 ◦C 6150 mg/L Sand pack 1208 31 Diversion of
gas flow
HSF foam
generates large,
stable bubbles
in presence of
oil, beneficial to
block high
permeability
areas
[123] Pre-generated IOS surfactant N2Crude oil with
API 37.82◦5 MPa; 60 ◦C 1.6 wt.% NaCl Bentheimer
sandstone cores
1300 20 IFT reduction
AS formulation
was found to
have a
well-defined
optimum
salinity for
which the IFT
drops by at
least four
orders of
magnitude
[124] Pre-generated AOS surfactant N2
Hexadecane
with viscosity
3.06 cp 2 MPa; 20 ◦C 0.5 M NaCl Bentheimer
sandstone cores
2500 21 N/A
Foam strength
increased with
surfactant
concentration;
Surfactant
pre-flushing is
a better foam
injection
strategy.
[125] Pre-generated Anionic
surfactant N2Crude oil API
28◦5 MPa; 65 ◦C 106.75 g/L Core samples
from sultanate
of Oman 0.5 N/A IFT reduction
Foam
formulation
shows an
enhanced oil
recovery ability
in low
permeability
carbonate core

Energies 2023,16, 972 25 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[126] SAG Industrial
surfactant Air Heavy crude
oil with
viscosity 500 cp
3.5 MPa;
Ambient
temperature 160,599 mg/L Sand pack 165.71–1696.51 32.75–41.64 Increased flow
resistance
Air-foam
generated in
the porous
medium could
improve the
resistance to
the flow of the
injected fluid;
Improvement
of sweep
efficiency and
oil recovery.
[127] Pre-generated 0.5 wt.% SDS,
ABS, AAS
surfactants N2
Crude oil with
viscosity
3170 cp Ambient 3712 ppm
NaHCO3
2D quartz glass
plates model 4320 37.5 Sweep
efficiency
improvement
Sweep
efficiency was
increased from
46.18% to
77.93% after
foam injection;
Oil
displacement
efficiency
increased from
72.76% to
84.01%.
[23] Pre-generated 0.5 wt%
surfactant CO2+ N2Crude oil 12.4 MPa; 90
◦C253.88 g/L Berea
sandstone cores
71.83 19.79 Increased
viscosity
The mixed
CO2/N2foam
system enables
the increase of
oil recovery by
62.5% over the
pure CO2foam
system

Energies 2023,16, 972 26 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[128] in situ 1 wt.%
Ethomeen C12
surfactant CO2
reservoir oil
with viscosity
of 8 cP
24.1 MPa;
120 ◦C220,000 ppm
NaCl Carbonate
outcrops 50–240 15.9–40 Gas flow
diversion
Foam strength
decreases in the
presence of oil;
Foam has a
greater effect
on gas relative
permeability in
higher
permeable
zones.
[129]in situ, SAG,
Pre-generated AOS 0.4 wt.%
surfactant N2Crude oil with
viscosity 9.6 cp
12.9 MPa; 45
◦
C
3722 mg/L Artificial
3-layer sand
pack core 1300 N/A N/A
in situ injection
is the most
effective mode
and the
alternate
injection is the
poorest.
[130] Pre-generated
500 ppm HSF-X
surfactant
solution N2
Crude oil with
viscosity
40000 cp 100 ◦C–200 ◦C N/A Sand pack 2280 N/A Gas mobility
reduction
Foam reduced
steam mobility
by more than
40%;
Foam increased
oil recovery by
20%.
[131] Pre-generated
IOS,
ethoxylated
carboxylate,
decyl glucoside
surfactants
Ethane
Wolfcamp dead
crude oil
viscosity 5.2 cp
10.3 MPa; 75
◦
C
22.7 wt% NaCl Carbonate
outcrop cores <15 28.6 Sweep profile
improvement
Improvement
of microscopic
displacement
was attributed
to the
attainment of
miscibility
between the
injectant and
crude oil

Energies 2023,16, 972 27 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[132] Pre-generated AOS + betaine
surfactants CO2
Malaysian
crude oil with
API 43◦
10.3 MPa; 80
◦
C
N/A Berea
sandstone 314 18.69 IFT reduction
Foam
successfully
improved the
cumulative oil
recovery from
57.58% to
74.08%.
[133] Pre-generated 0.5% AOS
surfactant CO2, N2
n-decane
viscosity 0.92
cP
8.2 MPa; 48 ◦C
for CO2and
4 MPa; 20 ◦C
for N2
5 wt.% NaCl;
3.8 wt.% CaCl2
Edwards
limestone 5–20 18–26 Sweep profile
improvement
Oil recovery
was higher in
oil-wet cores
than in
water-wet
cores;
Miscible
conditions that
generated CO2
foam were
significantly
more efficient
than
immiscible
conditions,
which
generated N2
foam.
[134] Pre-generated Kerosene and
FC-432
surfactant N2Crude oil 5.7 MPa N/A Berea
sandstone core 139.6 23.1 N/A
Oil foams
behave as non-
Newtonian
fluids and their
behavior
closely fit the
Bingham-
plastic
model.

Energies 2023,16, 972 28 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[135] SAG 2000 ppm AOS
surfactant HC Piedemonte
crude oil 10.3 MPa;
100 ◦CN/A Piedemonte
core samples N/A N/A Diversion of
gas flow
Gas blocking
capacity of
foams can be
correlated with
the gas rate in
the quality and
strength of the
foam
[136]in situ,
Pre-generated 0.5% AOS
surfactant N2Hexadecane 5 MPa; 21 ◦C 3 wt.% NaCl Bentheimer
sandstone 2820 22 N/A
Foam
generation and
propagation is
subject to the
destabilizing
effects of oil on
foam and
depends partly
on the injection
strategy.
[137] Pre-generated
IOS +
Carboxylate +
APG
surfactants
N2Crude oil
viscosity 1.2 cp 6.9 MPa; 69 ◦C 214,000 ppm Limestone
outcrop cores 10 20 Increased
viscosity
Oil recovery of
over 60% was
achieved after
water flooding,
even after
reducing the
surfactant
concentration
by 75%.
[138] Pre-generated 0.5% AES
surfactant Air Crude oil
12.2 MPa; 56
◦
C
2910 mg/L Sand pack
model 320 58.5 Sweep profile
improvement
Oil recovery
factor was
enhanced by
7.4% through
air foam
flooding and
4.9% after
water flooding

Energies 2023,16, 972 29 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[139] Pre-generated Na-DDBS
surfactant CO2Heavy oil with
viscosity 670 cp
10.3 MPa; 22
◦
C
1000 ppm
Sandstone and
carbonate
outcrop core
samples
N/A N/A IFT reduction
Foam injection
improved
heavy oil
recovery after
water flooding
in homogenous
core sample
and could
produce more
than 10% of the
residual oil
after water
flooding.
[140] Pre-generated
Fluorosurfactant
and Triton X
surfactant N2crude oil 7.2 MPa; 60 ◦C 21,600 ppm Limestone
samples 1.65–202.38 11.93–19.36 N/A
Maximum
recovery of
47.7% of
residual oil in
low
permeability
was observed
compared to
43% in high
permeability
core.
[141]in situ,
Pre-generated 0.4% AOS
surfactant N2
Keshang crude
oil with
viscosity 8 cp
9.5 MPa;
28.7 ◦C9038 mg/L Keshang core
samples 159 N/A Increased
viscosity
Oil recovery for
in situ injection
was 4.98%
higher than
that of
pre-generated
injection.
[142]in situ
AOS, Dodecyl
betaine
surfactants CO2
Yangsanmu
crude oil with
viscosity
557.5 cp
12.3 MPa; 63
◦
C
4500 mg/L Sand pack N/A N/A IFT reduction
in situ foam
injection mode
can effectively
reduce oil
viscosity and
oil–water IFT.

Energies 2023,16, 972 30 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[143] Pre-generated
Anionic and
non-ionic CO2
soluble
surfactants
CO2
West Texas
Wasson crude
oil with
viscosity 7.1 cp
10.3 MPa; 35
◦
C
NaCl Silurian
dolomite
outcrop 150 N/A Improvement
of sweep
profile
Recovery factor
of CO2
flooding was
only 24% due
to the
heterogeneous
nature of the
fractured core.
Co-injection of
CO2and water
increased the
RF to 35%,
which was
further
increased to
54% when a
water-soluble
surfactant was
added.
[144] Pre-generated Anionic and
non-ionic
surfactants CO2Crude oil with
viscosity 7.5 cp 9 MPa N/A Tight
sandstone core
samples 5–15 11.64–20.64 Reduction of
gas
breakthrough
A higher
injection rate
and/or
permeability
ratio result in
an earlier gas
breakthrough
and causes less
oil to be
produced
before the gas
breakthrough

Energies 2023,16, 972 31 of 52
Table 2. Cont.
Reference
Foam
Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[145] Pre-generated 0.5% AOS
surfactant CO2, N2
North Sea oil
with viscosity
2.1 cP 22 MPa; 100 ◦C 36,340 ppm Berea
sandstone core
sample 349 and 436 21.8 and 21.9 N/A
An easy way to
improve the
CO2foam
properties at
reservoir
conditions is
suggested as
introducing
nitrogen into
the gas phase.
[146]Pre-generated,
SAG
Industrial
surfactant
Masurf FS-3020
Mixture of
natural gas
liquids
Crude oil with
viscosity 3 cp
5.51 MPa; 32
◦
C
N/A Indiana
limestone core
sample 9.7 18.6 Increased
viscosity
Propagation of
foam was
strongly
influenced by
the injection
strategy
[147]Pre-generated,
SAG Surfactant N2Crude oil with
viscosity 1.1 cp 6.9 MPa; 80 ◦C 20 wt% NaCl Limestone
outcrop core
samples 46.3 N/A Increased
viscosity
A cumulative
oil recovery of
67% was
achieved after
foam injection,
in which foam
was able to
produce 25%
incremental oil
recovery.
[148] Pre-generated Mixed
surfactant Methane Crude oil with
viscosity 5 cp 20 MPa; 94 ◦C10700 mg/ L
Na+Sandstone core
samples 493 25 Increased
viscosity
Foam exhibited
higher
apparent
viscosity in
cores with
higher
permeability.

Energies 2023,16, 972 32 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[149]in situ AOS surfactant N2
Crude oil with
viscosity
3.28 cp 2 MPa; 21 ◦C 0.5 M of NaCl Bentheimer
sandstone cores
2500 21 Sweep profile
improvement
By increasing
the surfactant
concentration
from 0.1% to
1%, foam
exhibited a
better front-like
displacement
[150] Pre-generated Mixture of
surfactants
AOS, SDS, FS CO2, N2Oil with API
20.18◦Ambient 3 wt.% NaCl Bentheimer
sandstone cores
1200 22 Reduction of
gas
breakthrough
With nitrogen
foam, most oil
production was
achieved
within the first
PV of foam
injection. CO2
was after the
1.5 PV foam
injection.
[151]in situ Surfactant N2and Air Crude oil Ambient 1 wt.% NaCl
sandstone cores
32 and 1000 20 N/A
The presence of
oil decreases
the ability of
foam to
decrease the
permeability of
a porous
medium to
water.
[139] SAG
Non-ionic
Surfonic N85
(Huntsman)
surfactant
CO2
Heavy crude
dead oil
1320 cp Ambient N/A Sand pack 38,000 37 Increased
viscosity
The addition of
polymer to the
N85 foaming
solution
accelerates the
foam
generation and
increases its
stability in
heavy-oil-
saturated
porous media.

Energies 2023,16, 972 33 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[152] Pre-generated 1 wt.%
Bio-Terge AS40
surfactant N2n-decane Ambient N/A Water-wet
silicon
micromodels 700 52 Sweep profile
improvement
Foam injection
significantly
enhanced
sweep
efficiency in
fractured
systems in
terms of greater
pore occupancy
by gas and
larger contact
area with
displaced fluid
compared to
continuous gas
injection.
[153]in situ 0.3 wt.% AOS
surfactant CO2
Heavy crude
oil with
viscosity of
8670 cp
4.1 MPa; 50 ◦C
8000 ppm NaCl
and 2000 ppm
CaCl2
Micromodel 10,000 61 IFT reduction
CO2foam
significantly
increased the
contact area
between the oil
and gas and
thus, the
efficiency of the
recovery
process.
[154]in situ SDS, AOS
surfactants N2Crude oil
viscosity 38 cp
13.8 MPa; 65
◦
C
1 wt.%
synthetic
seawater Core sample 7530 36.49 N/A
Pressure has a
positive impact
on the foam
stability,
although the
extent of this
depends on
surfactant type;
A stable foam
has a better oil
recovery
performance
than gas
injection.

Energies 2023,16, 972 34 of 52
Table 2. Cont.
Reference Foam Injection
Mode
Liquid Phase
Component(s)
Gas Phase
Component(s)
Oil
Specifications
Thermobaric
Conditions Salinity Porous
Medium
Permeability
(mD) Porosity (%) Main
Mechanism Results
[155] Pre-generated 0.5 wt.% AOS
surfactant N2n-decane 13.79 MPa;
80 ◦C1 wt.% NaCl Microfluidic
chip 29,130 25 Increased
viscosity
Unlike gas
injection, the
oil recovery
from the matrix
in foam
injection is
improved by
the viscous
cross-flow
mechanism.

Energies 2023,16, 972 35 of 52
8. Field Applications of Foam-EOR
Many pilot studies have been carried out on foam flooding as a promising EOR method
for depleted oil fields. According to Yang et al. [
156
], three categories of factors mainly
influence the performance of foam injection during pilot tests. These include characteristics
of the target formation, the chemical composition of the foam and, lastly, the injection
scheme and foam quality.
Blaker et al. [
157
] and Skage et al. [
158
] reported the Snorre foam injection, which is
regarded as the largest foam application in the petroleum industry. The Snorre oil field
is located in the Norwegian part of the North Sea. The formation’s permeability ranges
between 400 and 3500 mD, and the oil viscosity is 0.789 cP. The reservoir temperature
was reported as 90
◦
C. Waterflooding was first implemented in this field; however, after
two years, a WAG pilot project with two injection wells and three production wells was
launched. The injection gas used was a mixture of hydrocarbon gas with a high mole
fraction of intermediate components. After some time, one of the producing wells had
an early gas breakthrough, prompting foam injection. C
14−16
alpha-olefin sulfonate was
used as a surfactant to generate foam. A packer was used to mechanically segregate the
foam in the intended high permeability layer. The injection rate was regulated to ensure
that the fracture pressure was not exceeded. Skauge et al. [
158
] stated that there were no
major operational issues during the injection of foam chemicals and gas and that the oil
rate increased. Foam injection was estimated to have produced around 250,000 sm3of oil.
In the Huan-Xi-Ling oil field, nitrogen foam flooding was implemented. The subsea
depth of this formation is 1080 meters. The average porosity and permeability are 1065
mD and 29.7%, respectively. It is a heavy oil field with oil viscosity ranging from 110–129
cP. Skauge et al. [
158
] reported that steam flooding was initially utilized in this reservoir,
and pilot foam injection was only started after the cumulative recovery factor reached
33%. The pilot injection was allowed to run for 33 months. During that time, there was
a recovery of 5.5% of OOIP. The pilot area was then enlarged to a 9-well pattern for a
total of 54 months. During this time, the total oil recovery was estimated to be 9.75% of
OOIP. Compared to water injection wells, foam injection wells had higher injectivity and a
higher oil production rate. This shows that foam injection is not just for enhancing sweep
efficiency in gas flooding; it can also increase oil production following steam and water
flooding cycles.
Foam flooding in Russian oil fields has only been mentioned in a few publications. In
an oil field in Western Siberia, foam treatments using viscoelastic surfactants, crosslinked
polymer, and nitrogen foam were described in Oussoltsev et al. [
159
]. Even though it was
used as a hydraulic fracturing fluid, where the generated foam gave promising results.
Longer effective lengths of fractures and higher fracture conductivity were achieved due to
the improved rheological properties of the foams and high conductivity due to the high
gas phase.
Saifullin et al. [
160
] provided the closest reference to non-hydraulic fracturing foam
operation. The field studied was the Messoyakhskoye oil field, operated by Gazprom Neft.
Gas channeling from the gas caps in the producing wells has been causing problems for the
operators. The authors explored how foam may be employed as a gas-blocking agent in
both production and injection wells. Their findings showed that foam injection might be
used to mitigate early gas breakthroughs in production wells.
Table 3summarizes some of the foam injection pilot tests that have been conducted.

Energies 2023,16, 972 36 of 52
Table 3. Field studies of foam-EOR.
Reference Field Geological
Characteristics Problem/Aim Liquid Phase Gas Phase Injection Parameters Main Mechanism Results
[161]Rangely Weber Sand
Unit, Colorado
Average porosity = 12%
Average
permeability = 8 mD
Reservoir
temperature = 71 ◦C
Oil viscosity = 1.7 cp
Initial res.
pressure = 18.9 MPa
CO2breakthrough
Poor sweep efficiency
High gas production
Chevron chaser CD1040
with average surfactant
concentration 0.46% CO2
Pre-treatment of 12000
bbl surfactant slug
followed by 55000 bbl
of 79% foam quality
Sweep profile
improvement
CO2production was
much lower;
Oil production was
slightly higher;
Foam reduced CO2
injectivity for as long as
2 months.
[162]North Ward-Estes Field,
Texas
Average porosity = 18%
Average
permeability = 15 mD
Reservoir
temperature = 28 ◦C
Oil viscosity = 1.4 cp
Initial res.
pressure = 7.6 MPa
Poor sweep efficiency
Early CO2
breakthrough
Low tertiary oil
production
Chevron chaser CD1040
0.1–0.5% CO2
Average foam quality
between 50% and 80%
was maintained
Reduction of gas
breakthrough
Foam was generated
without delay;
Foam reduced CO2
injectivity by 40%;
Foam lasted up to
6 months.
[163]Joffre Viking Field,
Alberta, Canada
Average porosity = 13%
Average
permeability = 506.6
mD
Reservoir
temperature = 56 ◦C
Oil viscosity = 1 cp
Initial res.
pressure = 7.8 MPa
Viscous fingering
Gravity segregation
Poor mobility of CO2
0.2 wt% of the
surfactant CO2N/A N/A
Foam did not propagate
a significant distance
into the reservoir;
modest increase in oil
production.

Energies 2023,16, 972 37 of 52
Table 3. Cont.
Reference Field Geological
Characteristics Problem/Aim Liquid Phase Gas Phase Injection Parameters Main Mechanism Results
[164]Rock Creek, West
Virginia
Average
porosity = 21.7%
Average
permeability = 21.5 mD
Reservoir
temperature = 22.8 ◦C
Oil viscosity = 3.2 cp
Low oil production
High gas mobility Alipal CD-128 CO2N/A N/A
Absence of any sign of
oil bank;
Possibility of CO2foam
breaking down
(thermal/chemical
degradation);
Inability to lower the
mobility under
reservoir conditions.
[165]East Seminole field,
Permian Basin, USA
Average porosity = 12%
Average
permeability = 13 mD
Reservoir
temperature = 40 ◦C
Oil API gravity = 31◦
Initial res.
pressure = 17.2 MPa
Brine salinity = 70,000
ppm
Poor areal sweep
efficiency
Unfavorable mobility
Huntsman L24−22
surfactant 0.5 wt.% CO2
70% foam quality in an
inverted 40-acre
five-spot well pattern
Sweep profile
improvement CO2injectivity was
reduced by 70%
[166]East Vacuum Grayburg
San Andres Unit, New
Mexico
Average
porosity = 11.7%
Average
permeability = 11 mD
Reservoir
temperature = 40.5 ◦C
Oil API gravity = 38◦
Initial res.
pressure = 11 MPa
Early CO2
breakthrough
Gas channeling
2500 ppm chevron
Chaser CD-1045 CO280% foam quality Diversion of gas flow
Foam achieved some
diversion of injected
fluid away from high
permeability zone into
lower permeability
zones;
Positive performance of
foam injection was seen
in a reduction in CO2
production and increase
in oil rate performance
[167]
Scurry Area Canyon
Reef Operational
Committee field,
Permian basin, USA
Average porosity = 7.6%
Average
permeability = 19.4 mD
Reservoir
temperature = 57.8 ◦C
Oil API gravity = 42◦
Initial res.
pressure = 24 MPa
Poor mobility control
Poor vertical
conformance ELEVATE CO2N/A Sweep profile
improvement
CO2injectivity
decreased by more than
50% compared to CO2
injection alone;
Foam improved vertical
conformance at the
injector.

Energies 2023,16, 972 38 of 52
Table 3. Cont.
Reference Field Geological
Characteristics Problem/Aim Liquid Phase Gas Phase Injection Parameters Main Mechanism Results
[168]Cusiana field,
Colombia Average porosity = 9%
Oil API gravity = 34◦
Poor tertiary oil
recovery AOS C12/C14 CH4
High injection rate of
>100 MMSCF/d to
promote foam
generation, after
which, the well was
returned to 35
MMSCF/d
Diversion of gas flow
Foam gas blocking
effect and divergence
at near-wellbore level
was achieved;
improvements in
sweep efficiency
[169] Snorre field, North Sea
Reservoir
temperature = 90 ◦C
Permeability = 400–
3500 mD
Oil viscosity = 0.789 cp
Poor mobility control
Gas breakthrough AOS Hydrocarbon gas A total of 2000 tons of
AOS surfactant was
injected
Gas breakthrough
reduction
Reduction in
production GOR;
Foam injection was
estimated to have
produced around
250,000 sm3of oil.
[170]SZ oil field, Bohai Bay,
China
Oil
viscosity = 24–452 cp
Average
porosity = 25%
Average
permeability = 745 mD
Reservoir
temperature = 65 ◦C
Early breakthrough of
injected water;
High water cut Surfactant N2
N2and fluid are
injected as small daily
slugs for 10–15 days
with a cumulative gas
injection of
2,592,000–3,888,000 m
3
,
for 3–6 months.
Increased viscosity
Oil production
increased and the
water cut decreased in
the pilot of the SZ oil
field;
Cumulative oil
production increased
to 30,283 m3per year.
[171] Liaohe oilfield, China
Average
porosity = 29.7%
Oil
viscosity = 110–129 cp
Average permeabil-
ity = 1079 mD
Initial reservoir
pressure = 10.7 MPa
Reservoir
temperature = 49.7 ◦C.
Pressure depletion
after 9 years of steam
huff-n-puff;
Poor sweep efficiency
Surfactant N2
Average injection rate
was 142.1 m3/d, and
average foam quality
was 60%.
Diversion of gas flow
The recovery rate of
foam flooding was
over 1.5%, higher than
that of water drive;
During foam flooding,
average injection
pressure was increased
to 8 Pa. It proves that
the flow resistance of
foam liquid increases
due to gas blockage
effect in foam flooding.
[135,172]Piedemonte field,
Colombia
Initial reservoir
pressure = 10.3 MPa
Reservoir
temperature = 100 ◦C
Poor sweep efficiency;
Low recovery of
residual oil AOS Surfactant Hydrocarbon gas
About 600 bbls of
foaming solution was
injected and the gas
injection rate was
20–30 MMSCF/d.
Sweep profile
improvement
Reduction of injectivity
in the gas injector;
Positive impacts on
GOR and the oil rate of
the producers.

Energies 2023,16, 972 39 of 52
Table 3. Cont.
Reference Field Geological
Characteristics Problem/Aim Liquid Phase Gas Phase Injection Parameters Main Mechanism Results
[173] Kern river field Average porosity = 30%
API gravity = 13 Poor sweep efficiency
0.5 wt.% AOS surfactant
N2
Steam foam was
generated by
continuous injection of
39.7 m3/d
Increased viscosity Improved vertical
sweep
[174] Midway Sunset field
Extremely
heterogeneous sand
rich reservoir
Average porosity = 25%
Average
permeability = 1000 mD
Understand
mechanisms of steam
diversion caused by
foam
Surfactant chaser
SD1000 N2
35 gal/hr of Chaser
SD1020 mixed with 30
SCF/min of nitrogen
was injected.
Sweep profile
improvement
bottom- hole injection
pressure increased from
0.7 to 2 MPa, indicating
good foam generation;
improvements in both
vertical and areal sweep
efficiency
[175]South Belridge Field,
Kern County
Average porosity = 35%,
Average
permeability = 1500–
3500
mD
Reservoir
temperature = 204.4 ◦C
Initial res.
pressure = 2 MPa
Initiation of a foam
steam diversion pilot AOS surfactant N2
About 178,000 lbs of
surfactant and 15
MMSCF of nitrogen
were injected
Sweep profile
improvement
Foam injection resulted
into 183,000 barrels of
incremental oil
production;
Foam diverted steam
and improved vertical
and areal sweep
efficiency
[176]Levantine–Moreni
reservoir, Romania
Average porosity = 29%
Average
permeability = 1000 mD
Initial res.
pressure = 2 MPa
Oil viscosity = 800 cp
Reservoir
temperature = 17 ◦C
Low oil production rate
due to heavy and
viscous oil CAPTOR 4020X N2
CAPTOR 4020X was
diluted in industrial
water to a concentration
of 1–2% active matter.
20–30 m3slugs of
foaming solutions were
injected. N2injection
pressure varied from
40–70 bar
Increased viscosity
Significant decrease in
water cut;
Supplementary oil
production relative to
steam drive was
estimated at 1060
tonnes.
[177] Siggins field, Illinois
Average porosity = 19%
Average
permeability = 75 mD
Oil viscosity = 8 cp
Average
temperature = 18.3 ◦C
Initial res.
pressure = 0.7 MPa
Determine effect of
foam on gas and water
injectivity;
Effect of foam on water
injection profile.
Neutral blend of
anionic surf with a
small amount of
non-ionic surfactant
Air
400 bbl of foaming
agent solution was
injected at a constant
bottom-hole pressure of
3 MPa;
100 bbl of the chemical
tracer solution was
injected at 3 MPa;
Approximately 850,000
SCF of air was injected
at 3 MPa.
Gas mobility reduction
Foam injection caused a
more uniform water
injection profile in the
injection well;
Water mobility reduced
to about 70%;
Air mobility was
reduced by more than
50% and the severe
channeling of air to the
production well
stopped.

Energies 2023,16, 972 40 of 52
Table 3. Cont.
Reference Field Geological
Characteristics Problem/Aim Liquid Phase Gas Phase Injection Parameters Main Mechanism Results
[178] Wilmington field
Reservoir
temperature = 130F
Reservoir
pressure = 6.2–7.6 MPa
Permeability = 100–
1000 mD
Porosity = 24–26%
Oil gravity API = 13–14
Poor distribution and
sweep efficiency of
injected CO2
Alipal CD-128 CO2/N2
About 21,000 bbl of 1%
Alipal CD-128 solution
was used with a volume
of gas designed to
generate a foam quality
of 90%.
Diversion of gas flow
Foam successfully
diverted gas to a
previously unswept
interval;
Increase in oil
production but
excessive gas
production in some
wells.
[179]Painter reservoir,
Wyoming
Reservoir
temperature = 78.8 ◦C
Initial res.
pressure = 31 MPa
Average
permeability = 7 mD
Oil gravity = 44 API
Nitrogen breakthrough;
Gas channeling;
High N2mobility
causing gas coning and
shuts off production
Surfactant 0.5% N2
During foam injection,
the wellhead pressure
ranged from 28.9 to
30.3 MPa
N/A
Foam injection
temporarily reduced
injectivity but was not
effective in controlling
gas channeling.
[180]
East Mallet Unit Well 31,
Hockley county, Texas
Slaughter field
Average porosity = 8.2%
Average
permeability = 2.15 mD
Oil viscosity = 1.078 cp
Average res.
pressure = 17.2–
18.6 MPa
Evaluate SAG foam
generation technique
for reduction of CO2
production and
improvement of oil
production
CD-128 surfactant CO2
CO2injection rate was
1000 MMSCF/day. A
total of 20,200 active
lbm of CD-128
surfactant
Gas breakthrough
reduction
Foam reduced gas
injectivity;
Foam reduced gas
production;
Foam increased overall
oil production by 22%
and 31%.
[180]
McElmo Creek Unit
Well P-19, San Juan
County Greater Aneth
field
Average porosity = 12%
Average
permeability = 10 mD
API gravity = 41.4◦
Initial res.
pressure = 14.7 MPa
Severe gas production;
breakthrough CD-128 and CD-1045 CO2
A total of 80,500 lbm
active surfactant was
injected. The average
wellhead injection
pressure increased from
5.5 to 14.5 MPa during
foam injection
Gas breakthrough
reduction
With foam, gas
production was reduced
by 50% relative to the
rate of CO2injected,
while oil production
was maintained at a
higher level.
[181]Kaybob South Triassic
Unit, Alberta, Canada
Average
porosity = 11.5%
Average
permeability = 92 mD
Oil viscosity = 0.414 cp
Reservoir
temperature = 88 ◦C
Initial res.
pressure = 17.6 MPa
Gas breakthrough;
Gas gravity override;
Poor sweep efficiency. Dowfax surfactant N2
A 10 m3slug of
surfactant solution was
injected at the
beginning of each week
of the water injection
cycle. Alternate
two-week cycle of gas
and two-week cycle of
water followed.
Sweep profile
improvement
Successful reduction of
gas injectivity was
observed after foam
injection;
Slight reduction in
GOR.

Energies 2023,16, 972 41 of 52
Table 3. Cont.
Reference Field Geological
Characteristics Problem/Aim Liquid Phase Gas Phase Injection Parameters Main Mechanism Results
[182]Madisonville West,
Woodbine, Texas
Average
permeability = 15 mD
Reservoir
temperature = 120 ◦C
Initial res.
pressure = 26.2 MPa
Oil API gravity = 39◦
Porosity = 13%
To lower the mobility of
injected gas and
increase its residence
time
Surfactant N2
Gas and water injection
rates were around 3000
MCF/d and 1200
MCF/d respectively
Sweep profile
improvement
Improved volumetric
sweep efficiency;
Oil production rates
were more than
doubled;
Gas utilization was
improved;
Low GOR was
confirmed.
[183]
Cupiagua, Recetor field,
Colombia
Permeability = 0.01–
10 mD
Porosity less than 6%
Initial res.
pressure = 41.4 MPa
Average
temperature = 126.7 ◦C
To temporarily block
high conductivity
layers;
To improve injection
conformance;
To improve sweep
efficiency.
Petrostep C1 N2N/A Diversion of gas flow
Strong gas injection
blockage was observed;
Incremental oil
production of 30% and
12% in 2 pilots;
Decreasing trend of
GOR.

Energies 2023,16, 972 42 of 52
9. Advances in Foam-EOR
The biggest challenge with the implementation of foam-EOR is foam stability, and in
order to solve this instability, various researchers have proposed and investigated different
methods to advance foam-EOR.
Nanoparticles have been introduced into the liquid phase in various experiments
because of their ability to improve foam stability. This is because nanoparticles can be
irreversibly adsorbed into the fluid due to their high adsorption energy as well as high
chemical and thermal stability [
184
]. Thus, they serve as stabilizing agents which can
provide optimal conditions with respect to temperature, pressure, and salinity. Surface-
coated silica nanoparticles have been found to stabilize emulsions, thereby improving
phase mobility [
185
,
186
]. Binshan et al. [
187
] found that hydrophilic silica nanoparticles
may change the wettability of rocks and lower interfacial tension, suggesting that they can
be employed as EOR agents. In the experiments conducted by Hendraningrat et al. [
188
],
their optimum nanoparticle concentration was 0.05 wt.%. The authors further stated that
the smaller the nanoparticle size, the higher the oil recovery, although this does not apply
to all cases. Furthermore, nanoparticles have been shown to create adsorption layers
on the rock surface, considerably altering the wettability and interfacial tension of the
formation [189,190].
Several studies have suggested several mechanisms to explain the enhanced oil re-
covery of nanoparticles. Al-Anazi et al. [
25
] claimed that an increase in recovery factor
was possible because of the tiny size of nanoparticles, which allows them to absorb and
de-adsorb easily while being transported through the reservoir. When the attraction force
is greater than the repulsive force, adsorption occurs. Diffusion and convection also play
a role in nanoparticle transfer in the pores. The authors further stated that pore throat
clogging is caused when the size of the nanoparticle is bigger than the pore throat of the
porous medium.
The mechanism of wettability change in surfactants is similar to that of nanoparti-
cles [
191
]. Wettability is a key factor that affects oil displacement. It refers to the capacity
of a fluid to adhere to a solid surface in the presence of immiscible fluids. Changing the
wettability of the rock from strongly oil-wet to water-wet is highly important for most
EOR operations. In the case of nanoparticles, they change rock wettability by replacing
carboxylic particles on the rock surface and forming a wedge layer that pushes out residual
oil due to disjoining pressure. Furthermore, as nanoparticles adhere to the rock surface,
they create textured surfaces that act as a buffer, separating and releasing oil droplets from
the porous medium, and this results in increased oil production.
In [
192
], the adsorption of nanoparticles on the porous medium resulted in forming a
composite nanostructure surface with improved water-wetting properties. Furthermore,
Jun et al. [
17
] studied the influence of hydrophilic and lipophilic polysilicon nanoparticles
on the wettability of a polished synthetic-silica surface at various concentrations. The
findings are consistent with those of Hendraningrat et al. [
188
], which assert that increasing
the concentration of nanoparticles reduces the contact angle of crude oil and changes the
wettability to a more water-wet state. The authors believe that smaller nanoparticles tend
to lower the contact angle more than larger nanoparticles due to stronger electrostatic
repulsion on smaller sizes.
The disjoining pressure is another interaction mechanism between nanoparticles and
the porous medium that improves oil recovery [
193
,
194
]. This refers to the attraction and
repulsive force between thin layers at the surfaces of two fluids [
195
]. Figure 18 below
shows that nanoparticles form microstructures in enclosed areas, such as foams, gels, and
emulsions. This adds a third interface to the two that already exist. By increasing the
entropy of the total dispersion, nanoparticles distributed in the liquid tend to form wedge-
shaped structures to facilitate a push toward the oil-rock interface, thereby allowing greater
freedom for the nanoparticles in the bulk liquid [196].

Energies 2023,16, 972 43 of 52
Figure 18. Disjoining pressure mechanism of nanoparticles.
A recent work carried out by Bello et al. [
60
] describes a comprehensive set of CO
2
and N
2
foam stability experiments in the presence of silica nanoparticles. The experiments
were conducted at ambient and elevated temperatures and with a wide range of salinity.
As it can be seen in Figure 19, their results showed that indeed, nanoparticles can increase
foam stability by reinforcing the thin liquid film between foam bubbles, thereby reducing
liquid drainage rate and forming a stronger and more stable foam.
with 0% NP with 0.05% NP with 0.1% NP
0
100
200
300
400
Foam half-life (in minutes)
N2foam
CO2foam
Figure 19. Influence of nanoparticles on foam half-life (Adapted from Bello et al. [60]).
Ionic liquids are a new group of chemical blends that can be used to improve foam
stability. According to Khan et al. [
197
], the key benefit of an ionic liquid is its stability in
high temperature and high salinity reservoir conditions. Ionic liquids can also significantly
lower IFT and change wettability. Nandwani et al. [
198
] studied the potential application of
ionic liquids for IFT reduction and enhanced oil recovery. Ionic liquids of 1-alkyl-3-methyl
imidazolium bromide was used and compared with a typical cationic surfactant, CTAB.
Their findings demonstrated that ionic liquid lowered IFT more effectively than CTAB,
even at low concentrations. Additionally, the residual oil that was trapped in the core after
waterflooding, the ionic liquid showed the maximum oil recovery. Further evidence for the
promising effect of ionic liquids can be found in Pillai et al. [
199
]. Ionic liquids demonstrated
an ability to lower contact angle by altering an oil-wet rock surface to water-wet, as seen in
Figure 20.

Energies 2023,16, 972 44 of 52
Water C8mimBF4C10mimBF4C12 mimBF4
0
20
40
60
80
100
Contact angle (°)
Figure 20. Wettability alteration ability of ionic liquids (Adapted from Pillai et al. [199]).
The authors attributed this to the formation of an ion pair between the positive head
groups of ionic liquids and the adsorbed negatively charged carboxylic groups of crude oil
on the rock surface.
Stable active chemicals are required to improve foam-EOR, and research has demon-
strated that ionic liquids can meet the needs of the oil industry for foam-EOR approaches.
10. Conclusions
This work provided information on the current status and development of foam-EOR
applications by emphasizing the results, current potential, and technical challenges being
faced in the lab and the field. The ability to demonstrate how foam can enhance oil recovery
in reservoirs with heterogeneous formations has advanced. Although there existed some
issues with its use in the field, foam is a significant agent for diverse applications in EOR
procedures. The following points can be drawn from this review paper:
•
The processes underlying the foam-EOR process continue to be the subject of discus-
sion among different researchers, because diverse distinct mechanisms are responsible
for various foam applications, injection modes, and foam formulations.
•
A wide knowledge gap still exists in foam generation and stability at high tempera-
tures, pressures, and salinity, despite decades of research on foam-EOR. One of the
biggest challenges of advancing foam-EOR to the field is the fact that several ear-
lier research studies were carried out under ambient conditions. Thus, a thorough
understanding of foam properties under reservoir conditions is required to obtain
optimum results.
•
In porous media, the injection mode of foam plays a great role in oil recovery. Foam
can be generated outside the porous medium before entering the pay zone with the
aid of a foam generator or just downward flow through the tubing (pre-generated
foam). Foam can also be generated in situ by co-injection of surfactant solution and gas.
Lastly is the SAG foam where foam is generated by alternate injection of surfactant
solution and gas. There is a considerable difference between these three methods and
should be optimized before foam-EOR operation.
•
Pressure has an increasing effect on foam stability as foam bubbles at high pressures are
smaller than those formed at low pressures, reducing the propensity to liquid drainage.
The temperature on the other hand has a reverse effect as high temperature increases
gas diffusion and propensity to liquid drainage and hence reduces foam stability.
•
The success of foam-EOR in the field is governed by several criteria, according to the
reported field pilot experiments. Thus, thorough laboratory research should serve as a
prerequisite for choosing the surfactant. The injection technique should be validated
by studies that use a precise reservoir description and foam model.

Energies 2023,16, 972 45 of 52
•
To improve the interaction of foam with crude oil in the reservoir, it is crucial to
identify the ideal wetting conditions and type of oil.
•
Foam bubbles are challenging to maintain due to Ostwald ripening. Thus, it is difficult
to generate stable foam for long periods. However, this review paper suggests several
approaches, which include the addition of nanoparticles, polymers, ionic liquids, and
binary surfactants.
Author Contributions:
All authors contributed equally to this work. All authors have read and
agreed to the published version of the manuscript.
Funding:
This work was supported by the Ministry of Science and Higher Education of the Russian
Federation under agreement no. 075-10-2022-011 within the framework of the development program
for a world-class research center.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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