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University of Groningen
Synthesis and evaluation of novel linear and branched polyacrylamides for enhanced oilrecoveryWever, Diego-Armando Zacarias
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Chapter 1
11
Chapter 1
Introduction
Abstract
Current crude oil extraction techniques are briefly introduced along with
enhanced oil recovery (EOR), particularly polymer flooding. The fundamentals
of polymer flooding are explained together with the requirements which the
polymers have to meet to be applied in this technology. An overview of
recent developments in the field of water soluble polymers aimed at
enhancing the solution viscosity is given. The currently polymers are
discussed in terms of their advantages and limitations. Eventually the aim
and scope of this thesis are presented.
Based on: D.A.Z. Wever, F. Picchioni, A.A. Broekhuis. Polymers for enhanced
oil recovery: A paradigm for structure-property relationships in aqueous
solution. Progress in Polymer Science, 2011, 36, 1558-1628.
Introduction
12
1.1. Oil recovery
The population of the world is projected to increase beyond 8 billion
people by 2035.1 In line with this increase and the corresponding raise in the
living standards in the developing countries, the energy consumption is
expected to grow by 34% between 2015 and 2035.2 The global primary
energy supply in 2010 comprises several different sources (Figure 1.1).3
Coal/peat
27,3%
Other (solar, wind, geothermal, etc)
0,9%
Biofuels and waste
10%Hydro
2,3%Nuclear
5,7%
Natural Gas
21,4%
Oil
32,4%
Figure 1.1: World primary energy supply in 20103
Oil covers approximately 30% of the primary energy supply. The increase in
energy consumption will exert a relevant pressure at industrial level towards
a more efficient exploitation of the current sources. New ones, such as
renewables, have not been demonstrated to be reliable yet and as of 2010
account 2013 for 10 % (projected to reach 14% by 2035) of the total world
energy consumption.2 Therefore, to guarantee the supply of energy and
provide a transition period between current sources and the renewables ones,
current sources have to be exploited in a more efficient manner.
Current oil production has reached >90 million barrels per day (bpd).4 In
the early 1950’s, the so called “peak oil” theory was developed. This
predicted in first instance a peak in oil production followed by a steady
decline. On the other hand, evidence for the underestimation of a field’s
productivity, on which the theory is based, has been recently published.5
Nevertheless, new technologies can increase and/or extend current oil
production. Easily (by current technologies) recoverable oil is running out;
however significant amounts of oil remain in the reservoirs after the
conventional methods have been exhausted.5 Therefore, in order to
guarantee its continuing supply, enhanced oil recovery (EOR) has to be
implemented. After the current recovery methods have been depleted
Chapter 1
13
approximately 7.0·1012 barrels6 of oil will remain in the oil fields, which
represents a production of more than 200 years at the current rate. An
overview of the total amount of oil (composed of conventional oil (light oils)
and unconventional oil (heavy oil and tar sands)), the remaining amount of
oil that can be recovered by current techniques (proven conventional
reserves), the remaining unconventional oil that can be recovered by new
techniques (recoverable resources) and what has been recovered so far is
presented in Figure 1.2.5
Figure 1.2: World’s oil resources5
Oil reservoirs can be classified into three main categories according to the
American Petroleum Institute gravity index (API): light oil reservoirs (API >
31), heavy oil reservoirs (API < 22) and tar sands. Light oil reservoirs are the
most common in the world. In light oil reservoirs the oil is embedded in
porous media (Figure 1.3). In most cases gas (small hydrocarbons) is also
present. In addition saline water is also present as an aquifer (water in a
porous media) or as connate water (water present in the pores of
sedimentary rock as they were being formed) in the oil deposit. The amount
of salt (or dissolved solid) is denoted as total dissolved solid (TDS).
The conventional techniques for extracting the oil out of a reservoir
consist of primary and secondary methods. The primary technique uses
natural forces to produce the oil. Three different mechanisms are utilized to
extract the oil: the aquifer drive, the gas cap drive and the gravity flow. The
aquifer drive, according to which the pressure that is exerted on the oil by
the aquifer represents the driving force for extraction, is the most efficient
mechanism. The production of oil leads to a decrease in pressure of the
reservoir, and the aquifer moves towards the production well. The oil cut
Introduction
14
decreases as more and more water is produced along with the oil. The gas
cap drives the oil in a similar fashion as the aquifer drive. Gas production
(along with the oil) is not seen as a disadvantage, since it also can be used
as an energy source. Finally, gravity is the important factor in the gravity
flow, for which the well placement is obviously. The use of this method is
limited and is heavily dependent on the geology of the reservoir. The primary
techniques recover, depending on the oil reservoir, on average between 15-
25 % of the original oil in place (OOIP).6, 7
The secondary method involves the injection of either water or gas to
increase the pressure in the reservoir, which in turn drives the oil out. After a
given time, the injected water breaks through in the production wells. As the
production well ages, after the water breakthrough, the water cut increases.
The use of the secondary methods enables the extraction of 20-50 % of the
OOIP depending on the reservoir.6, 7
At most 55 % of the OOIP can be recovered (in most cases this value is
much lower) using the primary and secondary techniques. Therefore a large
portion of the OOIP remains embedded in the reservoir. Since the 1970’s,
many different methods have been developed to increase the oil recovery as
a response to the oil crisis.6, 7 These all belong to the category improved oil
recovery (IOR). Improved oil recovery implies improving the oil recovery by
any means6, such as operational strategies. Enhanced oil recovery, a
subgroup of IOR, is different in that the objective is to reduce the oil
saturation below the residual oil saturation (the latter being defined as the oil
saturation after a prolonged waterflood).
1.1.1. Reservoir properties
Oil reservoirs are porous media of which part of the total volume (the
porosity, φ) is occupied by a fluid, either oil or water. The permeability of the
porous media is defined as the ability of a specified fluid to permeate (flow
through) the porous media. The permeability can be determined by Darcy’s
Law (equation 1.1).
(1.1)
= permeability (mD), = fluid flowrate (ml/s), = pressure drop (bar), = fluid
viscosity (cP), = section length (cm), = cross sectional area (cm2)
The permeability of the porous media of oil reservoirs varies significantly
depending on the type of reservoir. Reservoirs with sandstone (e.g.
Bentheim) as the porous media display permeability values higher than 1000
Chapter 1
15
mD while carbonate porous media display values lower than 10 mD (e.g.
Rote Mainz). Permeability values above 105 mD are considered pervious, 105
– 10 mD are semi-pervious and values lower than 10 mD are impervious.8
Most oil reservoirs contain either sandstone or carbonate as the porous media
and the range of permeability values encompasses the semi-pervious class as
well as a part of the impervious types.
An increase in the viscosity of the fluid is synonymous to a more difficult
flow through the porous media. This will lead to a lower permeability. One
way of increasing the viscosity of the displacing fluid is through the use of
water-soluble polymers (as an Enhanced Oil Recovery agent).9 A layer of
polymer might build up on the surface of the rock (e.g. by
precipitation/adsorption), thus leading to a further decrease of the
permeability of the porous media.9
1.2. Enhanced oil recovery
Enhanced oil recovery (EOR) involves different techniques that were
developed to extend the oil field’s life. Most of the EOR techniques have been
developed early in the twentieth century as an answer to the low oil
production in combination with the oil crisis. However, the price of a barrel of
oil was at that time much lower (20$ per barrel) compared to the current one
(~100$ per barrel). Given the low oil price at that time, the EOR techniques
were not fully developed and their application was limited to only a few
projects. Nevertheless, much experience has been gained through the EOR
projects. The focus of oil companies has now turned back to EOR because of
the steep increase of the oil prices and the increased demand for oil products.
In addition, the dwindling number of easy recoverable oil reserves is also
crucial in stimulating the development of better EOR techniques. Two main
categories of EOR technology exist; thermal and non-thermal (the focus of
this thesis). For a complete overview of all the techniques the reader is kindly
referred to the literature.6, 7
1.2.1. Non-thermal
Non-thermal methods use either gas or chemicals to improve the
recovery of oil from an oil field. Chemical EOR consist of mainly polymer
flooding and alkaline surfactant polymer (ASP) flooding. Non-thermal EOR is
more suitable for light oils rather than the heavy (viscous) oils. Chemical EOR
has been developed early in the twentieth century and has been implemented
in several oil reservoirs with different (mixed) results. So far, polymer
flooding is by far the most used chemical EOR technique.
Introduction
16
1.2.2. Polymer flooding
The conventional methods (primary and secondary) can at best extract
55% of the OOIP.6 The oil crisis of the 1970’s sparked the efforts in
developing better oil extracting technologies. The development of polymer
flooding also started during this period. Many different polymer flooding
projects have been carried out with mixed results.10 Nonetheless, a
significant number of polymer flooding projects have been recently started in
many different countries11 indicating the maturity of this technique. The
polymer flooding projects have been performed in reservoir temperatures
ranging from 8 – 110 °C, reservoir permeability values in the range 0.6 –
15000 mD, oil viscosity between 0.01 – 1500 cP and resident brine salinities
ranging between 0.3 – 21.3 % TDS.10
When water is used as a secondary recovery method two different problems
arise, a macroscopic one related to the volumetric sweep efficiency ( as
defined in equation 1.2) and a microscopic problem related to the
displacement efficiency of oil. When a water-flood is performed not all of the
OOIP is contacted by the displacing fluid.
(1.2)
The displacement of a viscous fluid (oil) with another (immiscible) less
viscous fluid creates instabilities which lead to viscous fingering. According to
Homsy12 viscous fingering arises for all mixtures of fluids. In porous media
viscous fingering develops due to an increase in the effective permeability of
the porous media to water.13 Due to the viscous fingering, the sweep
efficiency of an oilfield is limited. Early breakthrough (production) of the
displacing fluid (water) typically occurs. As a consequence, large portions of
the oil reservoir are not swept, thus leaving vast quantities of oil behind. In
order to solve this problem and to enhance the oil extraction, the water-oil
mobility ratio has to be improved. This ratio is dependent on the relative
permeability values and the viscosities of both the oil and the water phase.
The water-oil mobility ratio13 ( ) is defined by equation 1.3, with and
being the permeability of the porous media to water and oil respectively. The
viscosities of the oil and water are represented by and .
(1.3)
Chapter 1
17
A high volumetric sweep efficiency is obtained when the water-oil mobility
ratio is less than or equal to unity. At low mobility ratios a piston like sweep
of the reservoir will be obtained while at higher water-oil mobility ratios
viscous fingering will arise. In theory there are several different methods to
affect the water-oil mobility ratio. The permeability of the porous media can
be altered, the viscosity of the oil can be decreased or the viscosity of the
displacement fluid can be increased. In practice only the two latter
techniques are possible. A schematic presentation of the viscous fingering in
a porous medium is given in Figure 1.3. The flow pattern using a polymeric
solution is also displayed in Figure 1.3.
Figure 1.3: Viscous fingering (left) and polymer flow (right) in a porous medium
In most cases the displacement efficiency ( ), as defined in equation 1.4 by
assuming constant oil density9, is relatively low because of the viscosity
difference between the displacing fluid (water) and the oil.
(1.4)
The higher the difference in viscosity between the displacing fluid and the oil,
the more inefficient is the displacement. The displacement efficiency can be
improved by using water-soluble polymers since the viscosity of the
displacing fluid is increased and this will recover the oil at a higher rate than
conventional water flooding. In theory, the residual oil saturation cannot be
reduced by polymer flooding; only the time it takes to reach the residual oil
saturation of the reservoir is reduced. However, this has been contested by
several researchers in China. According to a number of studies14-19 residual
oil can be mobilized by employing visco-elastic materials, i.e. polymers.
Indeed, Lake9 has defined residual oil to be the one remaining behind in a
certain region of a reservoir that has been thoroughly swept with water.
Residual oil is classified into four different types14, 18: oil film on the rock
Introduction
18
surface (1), oil trapped in dead ends (2), oil (ganglia) in pore throats retained
by capillary forces (3) and oil unswept in micro-scale heterogeneous portions
of the porous media (4). The four different types of residual oil are presented
schematically in Figure 1.4.
Figure 1.4: The different types of residual oil
Several studies15, 16 proposed a mechanism according to which the visco-
elastic properties of the water solution play a crucial role in enhancing the
recovery of the different kinds of oil outlined above. This mechanism is
mainly supported by indirect evidence and mathematical models.17, 18, 20, 21
Water-soluble polymers for EOR applications have been successfully
implemented, mainly in Chinese oilfields.22, 23 The purpose of the water-
soluble polymers in this application is to enhance the rheological properties of
the displacing fluid. Oil production increases with the microscopic sweep of
the reservoir and the displacement efficiency of the oil9 Indeed, the use of
water-soluble polymers improves the water-oil mobility ratio9, and leads to
enhanced oil recovery. However, given the harsh conditions present in most
oil reservoirs, new problems and limitations arise with the use of water-
soluble polymers. Besides positively affecting solution rheology, water-soluble
polymers should withstand high salt concentration, the presence of calcium,
Chapter 1
19
high temperatures (> 70 °C) and long injection times (at least 12 months).9,
24 High salt concentrations reduce the thickening capability of most ionic
water-soluble polymers while the presence of calcium leads to flocculation.25
New water-soluble polymers were successfully tested at higher
temperatures26, 27 while associative water-soluble polymers were tested and
showed promising results compared to traditionally used polymers.28, 29
Several studies14-18, 20, 21 demonstrated that the oil is produced faster
(compared to water flooding), but also more oil can be recovered.
Independently of the exact displacement mechanism and efficiency, the use
of water-soluble polymers for EOR still constitutes a challenging research field
at both industrial and academic level.
1.3. Currently used polymers
1.3.1. Polyacrylamide (PAM)
Polyacrylamide was the first polymer used as thickening agent for
aqueous solutions. The thickening capability (increase of the corresponding
solution viscosity) of PAM resides mainly in its high molecular weight, which
reaches relatively high values (> 1·106 g/mol). In the general framework of
EOR processes, PAM is mainly used as the reference “model system” for
chemical modification. Many authors have reported different attempts to alter
the chemical structure of PAM or to synthesize new acrylamide-based
copolymers with improved properties, i.e. shear resistance, brine
compatibility and temperature stability.30-33 The synthesis of the copolymer
N,N-dimethyl acrylamide with Na-2-acrylamido-2-methylpropanesulfonate
(NNDAM-NaAMPS) has been accomplished and the polymer was tested for its
performance in EOR applications.30, 31 The stability of the polymer at high
temperature was demonstrated by aging at 120 °C for 1 month.30 By using a
sand pack, an improved performance in terms of EOR for the NNDAM-
NaAMPS copolymer31 as compared to an unmodified partially hydrolyzed
polyacrylamide, HPAM, was demonstrated. In another example, the oil
recovery rate through the use of starch-graft-poly(acrylamide-co-(2-
acrylamido-2-methylpropanesulfoacid)) was higher compared to HPAM, and
the novel polymer displayed better temperature and shear stability.32 These
two examples already define a common research theme in the general field
of water-soluble polymers for EOR. That is, a strategic approach involving the
chemical modification of commercial polymers (in this case PAM) to tailor and
improve the corresponding solution properties and eventually EOR
performance.
Introduction
20
1.3.2. Partially hydrolyzed polyacrylamide (HPAM)
HPAM, by far the most used polymer in EOR applications, is a copolymer
of AM and acrylic acid (AA) obtained by partial hydrolysis of PAM or by
copolymerization of sodium acrylate with acrylamide (AM).34 The chemical
structure of HPAM is provided in Figure 1.5.
Figure 1.5: Chemical structure of HPAM
In most cases the degree of hydrolysis of the acrylamide monomers is
between 25-35%.9, 35 The fact that a relevant fraction of the monomeric units
needs to be hydrolyzed (lower limit of 25 %) is probably related to the
formation of the corresponding salt. According to the general theory of
polyelectrolyte solutions36, the presence of electrostatic charges along a
polymer backbone is responsible for prominent stretching (due to electric
repulsion) of the polymeric chains in water which eventually results in a
viscosity increase compared to the uncharged analogue. On the other hand,
the degree of hydrolysis cannot be too high because the polymer solution will
become too sensitive to salinity and hardness of the brine (electrolytes
present in solution have a “shielding effect” on the electrostatic repulsion).37
Indeed, polyelectrolytes, i.e. polymers bearing charges, show significantly
different rheological behavior compared to their neutral analogues.38-40 The
thickening capability of HPAM lies in its high molecular weight and also in the
electrostatic repulsion between polymer coils and between polymeric
segments in the same coil.9 When polyelectrolytes are dissolved in water
containing electrolytes (salts) a reduction in viscosity is observed.35, 41-43 It
has been demonstrated that the specific viscosity of HPAM solutions depends
on the amount of salt present.44 This effect is attributed to the shielding
effect of the charges9, 42 leading in turn to a reduction in electrostatic
repulsion and consequently to a less significant expansion of the polymer
coils in the solution. This results in a relatively lower hydrodynamic volume,
which is synonymous with a lower viscosity.43 A few decades ago,
Chapter 1
21
substitution of one or both hydrogens on the amide nitrogen with alkyl
groups has been presented as a solution to the salt sensitivity of HPAM45, 46,
although the exact reasons for this behavior have not been fully elucidated.
The addition of monovalent NaCl leads to a reduction in the level of
aggregation. However, at higher ionic strengths (higher salt concentration)
the addition of NaCl leads to macroscopic flocculation.47 It has also been
demonstrated that multivalent cations can form polyion-metal complexes that
affect the viscosity of the resulting solution.48-50 The dependence of the self
complexation of HPAM on the Ca2+ concentration and the degree of hydrolysis
of HPAM has been investigated. It was demonstrated48 that depending on the
Ca2+ concentration intra- and inter-chain complexation takes place (Figure
1.6).
Figure 1.6: Complexation behavior of HPAM under different conditions48
Besides the salt dependency, other factors influencing the viscosity of HPAM
solutions are the degree of hydrolysis, solution temperature, molecular
weight, solvent quality and pressure.44
The increase in the viscosity of the HPAM solutions cannot solely be
accounted for by the increase in viscosity of the solvent.51 The intrinsic
viscosity and the radius of gyration are both invariant with pressure, albeit
with a 10% experimental uncertainty.51 In principle, the average dimension
Introduction
22
of the polymer coils does not change while the solvent volume decreases (i.e.
by increasing the concentration). Therefore the volume fraction of the
polymer coil per unit volume of the solvent increases, hence a higher
viscosity.51 Another parameter that affects the solution viscosity of the
polymer solution is shear.52 Under high shear the HPAM polymer chains are
reduced in size due to chain scission, i.e. fragmentation.53 This leads to a
reduction in the solution viscosity.
HPAM is preferred in EOR applications since it can tolerate the high
mechanical forces present during the flooding of a reservoir. In addition,
HPAM is a low cost polymer and is resistant to bacterial attack.9 Although the
HPAM solutions display pseudo-plastic behavior9, 35, 41, 54, 55 (shear thinning) in
simple viscometers, it has been demonstrated that these solutions show
pseudodilatant56, 57 characteristics (shear thickening) in porous media as well
as in viscometers at relatively high shear rates (e.g. values higher than 100
s-1). Research has demonstrated the presence of a critical shear rate at which
the shear thickening behavior arises in viscometers.41, 42, 54, 55, 58, 59 This
critical value depends on the degree of hydrolysis of the HPAM, the solution
concentration, the temperature, the quality of the solvent and also on the
molecular weight of the polymer.42, 54 An increase in the degree of hydrolysis
leads to an onset of shear thickening at lower shear rates.54 By decreasing
the average molecular weight, an increase in the polymer concentration
results in a higher critical shear rate.54, 55
The aforementioned shear thinning of HPAM solutions below a critical
shear rate arises due to uncoiling of polymer chains and the dissociation of
entanglements between separate polymer coils.9 Stiffening of the polymer
backbone has been suggested as a possible approach to control the
dependency of HPAM polymer solutions on the shear.60 A stiff polymer
displays a lower mobility and therefore the entanglements, related to the
solution viscosity, can be conserved as the shear increases.
The shear thickening behavior has been attributed to changes in the
molecular conformation involving the formation of additional links between
two chains.59 The shear thickening behavior is observed both in laboratory
rheometers54 (in pure water and aqueous salt solutions) and in porous media.
According to several studies the shear thickening behavior in porous media
arises due to coil-stretched transitions of the polymer chains. The structure of
a porous medium can be seen as alternating wide openings and confined
throats through which the polymer coils have to navigate. In the wide
openings the polymer chains attain a coil structure. When these coils then
have to pass through a narrow throat the polymer coils are forced to deform
and stretch (elongational strain41, 57, 61) in order to pass. This successive
Chapter 1
23
contraction and expansion of the polymer coils leads to pseudo-dilatant
behavior of the polymer solutions.57, 62, 63 This conformational change of the
macromolecules is reversible since it is commonly explained by formation at
macromolecular level of reversible interactions like hydrogen bonding.
Indeed, it is believed that hydrogen bonding arises for HPAM solutions
between the carboxylic functionalities.64 However, this is contested due to
conflicting data59, 65 on similar polymeric solutions (e.g. for dextran). Instead,
aggregation of hydrophobic bonds has been proposed64, albeit in
poly(methacrylic acid), but this has not been confirmed.66 A schematic
presentation depicting the essential behavior of HPAM solutions in shear flow
has been proposed55 (see Figure 1.7).
Figure 1.7: Schematic presentation of behavior of HPAM coils in shear flow[46]
Another behavior that has been identified for HPAM solutions, which is
important for EOR, is their negative thixotropic (rheopectic) property, i.e. an
increase in viscosity with shear-time at a constant shear rate.41, 67-70
Researchers have identified two different types67 of rheopectic behavior for
HPAM solutions (Figure 1.8), type I and type II.
Figure 1.8: Type I and II of rheopectic behavior of HPAM solutions67
The type I effect is observed at low shearing and consists in a slow viscosity
increase with shear-time up to an asymptotic value. The type II effect is seen
Introduction
24
at high shear rates and is displayed as a steep viscosity increase after a
given shear-time, followed by pronounced viscosity oscillation.67
1.3.3. Xanthan gum
Xanthan gum is a polysaccharide, which is produced through
fermentation of glucose or fructose by different bacteria.71 The most efficient
xanthan gum producer is the Xanthomonas campestris bacterium.71, 72 The
chemical structure of xanthan gum (Figure 1.9) displays the presence of two
glucose units, two mannose units and one glucuronic acid unit with a molar
ratio of 2.8-2.0-2.0.73
Figure 1.9: Chemical structure of xanthan gum
The backbone of xanthan gum is similar to cellulose. The side chains of the
polymer contain charged moieties, i.e. pyruvate groups, and the polymer is
thus a polyelectrolyte. However the classic polyelectrolyte behavior according
to which the solution viscosity decreases with the addition of salt is not
displayed in this case. The thickening capability of xanthan gum is due to its
high molecular weight, which ranges from 2 - 50 · 106 g/mol73, 74 and in the
rigidity of the polymer chains.
It has been demonstrated that upon addition of salt (mono- or divalent)
the xanthan gum chains undergo a cooperative conformational transition
from a disordered conformation to an ordered and more rigid structure75-78
(Figure 1.10).
The temperature and the ionic strength, i.e. the amount of electrolyte, of
the solution are triggers for the conformational transition. When testing at
Chapter 1
25
low shear, the rheology of the polymer solution is dependent on the
conformation with the disordered conformation displaying higher solution
viscosities.79 Polymeric solutions employing xanthan gum display high
viscosity at low shear rates80 and thus the disordered conformation
predominates at low shear rates. At high shear rates both conformations
display similar rheological behaviors.79 In addition, pseudoplastic behavior is
observed for the polymer solutions.81 Unlike HPAM, xanthan gum displays
good resistance to high temperatures. It was demonstrated that the solution
viscosity of a polymeric solution employing a commercial xanthan gum
remained relatively constant for more than 2 years at 80 °C.82 Loss of
solution viscosity occurs at temperatures above 100 °C.
Figure 1.10: Conformational transition of xanthan gum
Several studies83-86 have investigated the temperature dependence of the
apparent viscosity of xanthan gum solutions. In order to display resistance to
temperatures up to 90 °C, the conventional understanding for xanthan gum
solutions is that the ionic strength of the solution has to be relatively high.
Another positive property of xanthan gum is its ability to withstand high
shear forces. Unlike HPAM the solution viscosity does not decrease at
relatively high shear stresses.56 Especially the ordered structure, i.e. in the
presence of salt, can withstand high shear forces79 (up to a shear rate of
5000 s-1).
A disadvantage of xanthan gum is its susceptibility to bacterial
degradation. It has been demonstrated that salt tolerant aerobic and
anaerobic microorganisms can degrade the xanthan gum chains which leads
to the loss in solution viscosity.87-90 Biocides are used to suppress the growth
Introduction
26
of the xanthan gum degrading microorganisms. In most cases formaldehyde
is the most efficient biocide.89, 90 However, the use of biocides to protect the
xanthan gum renders the low environmental impact of the polymer at least
debatable.
1.4. Possible new polymers for EOR
The limited number of available commercial polymers currently employed
in EOR has been the subject of recent developments aimed at improving their
performance. Indeed, a relatively new concept has been studied in the last
four decades, and involves the association between hydrophobic groups that
are incorporated in the backbone of the polymers.91 Through these
associations a higher thickening capability can be achieved compared to the
traditional polymers.91 Several different types of associating polymers have
been studied. These include the hydrophobically modified polyacrylamide
(HMPAM)92, ethoxylated urethane (HEUR)93, hydroxyethylcellulose (HMHEC)94
and alkali-swellable emulsion (HASE)95. Also combinations of associative
polymers with surfactants have been developed for EOR.96 It has been
demonstrated that the addition of small amounts of surfactants can increase
the viscosity of the aqueous solution containing hydrophobically modified
polymers significantly.94
Other polymers that possess interesting properties, such as high
molecular weight and intrinsic viscosity, have been developed for EOR and
are known as "rigid rod" water-soluble polymers.97 One study compared
hydrophobically modified polyacrylamide (HMPAM) with polyacrylamide (PAM)
in a simple core flood test and demonstrated that the residual resistance
factor (RRF, permanent reduction in the permeability of the formation due to
the adsorption of polymeric chains) after the polymer flood is much higher for
the HMPAM compared to PAM.98 All these modification strategies, together
with new kinds of water-soluble systems, have been extensively reported in
the literature and will be discussed in the next paragraph.
As mentioned earlier, a relatively new class of water-soluble polymers is
the one constituted by hydrophobically associative polymers.91 The first
hydrophobically associative polymers were synthesized almost fifty years
ago99, 100, albeit for a different purpose than EOR. The research on these
polymers has been primarily fueled by the coating industry91, where
improvement in the rheology of the coating systems was required. During the
1980’s, when the oil crisis hit, a lot of research was performed on EOR. From
the many patents101-106 that have been filed during those years, it is evident
Chapter 1
27
that this accelerated the development of hydrophobically associative
polymers for use in EOR applications.
Hydrophobically associative polymers contain, in most cases, a small
number of hydrophobic groups, i.e. 8-18 carbon atoms moieties107-110,
distributed along the main backbone111-113. These hydrophobic groups can be
distributed randomly or block-like 92, 95, 107, 112, 114-125, and coupled at one or
both ends 108, 126-136. Above a given polymer concentration (dependent on the
molecular structure) the hydrophobic groups associate, when the polymer is
dissolved in water, to form hydrophobic micro-domains (intra or
intermolecular liaisons).92, 93, 108, 109, 112, 137-140 These lead to an increase in
hydrodynamic volume, which in turn yields a polymer with a much better
thickening (higher viscosity112) capability compared to its non-associative
analogue.92 Depending on the concentration, intra- or intermolecular
associations as schematically illustrated in Figure 1.11, is detected.
Figure 1.11: Intra- and intermolecular associations137
When the hydrophobic elements are distributed in a block-like fashion along
the backbone of a water-soluble copolymer, the intramolecular associations
are stronger compared to randomly or discretely distributed hydrophobic
groups.108, 126
The temperature dependence of the solution viscosity is an interesting
property of hydrophobically modified polymers for EOR applications. It has
long been accepted that increasing the temperature of the polymer solution
will lead to a reduction in viscosity, probably due to the fact that an increase
in temperature implies a decrease of the association strength of the
hydrophobes.107, 138, 141-145 Increasing the temperature of the solution leads to
Introduction
28
a reduction of the solvent viscosity and hence an increase in the mobility of
the polymer chains while the solubility of the polymer will increase with
temperature. However, many different aqueous systems have been
demonstrated to display an increase in viscosity upon increasing the
temperature.146-159 Indeed, a temperature increase results in a decrease of
the solubility of one of the components (Lower Critical Solution Temperature
[LCST]-groups) of the polymers. These less soluble components self-
aggregate with the hydrophobic groups of the polymers, which leads to an
increase in viscosity.124 Several researchers have proposed a concept for
thermo-associative polymers based on the switch, i.e. the transition between
low and high temperature, of the polymers characterized by a lower critical
solution temperature.145, 156, 157 The concept involves a highly water-soluble
polymer containing blocks or side chains of LCST groups. Upon heating of the
polymer solution, these LCST groups will segregate. A schematic illustration
of this behavior has been presented by Hourdet and coworkers156 and is
depicted in Figure 1.12.
Figure 1.12: Thermal induced microdomains156
Above the critical overlap chain concentration this transition will lead to an
increase in the viscosity of the solution through intermolecular associations.
Fundamental research on different polymers, in binary (polymer-water)
and ternary (polymer-water-surfactant) systems, has been performed using
different techniques which include 13C-NMR160-163 (solution or solid-state), 1H-
NMR114, 23Na-NMR164, 165, 19F-NMR166, NMR self-diffusion131, 167-170
potentiometry171-173, Static and Dynamic Laser Light Scattering133, 135, 136, 151,
168, 172-177, UV-Spectroscopy for polymers bearing chromophores92, 115, 116, 178-
Chapter 1
29
186, Small-Angle Neutron Scattering (SANS)187, Non-Radiative Energy
Transfer (NRET) studies183, 188-190, Size Exclusion Chromatography (SEC)175,
191-194 and surface tension136, 138, 167, 168, 170. Several different associative
hydrophobically modified polymers have been developed which include
polyacrylamides (HMPAM), ethoxylated urethanes (HEUR), alkali swellable
emulsions (HASE), and polysaccharides (HM-polysaccharides). Their
synthesis, rheological behavior and adsorption on surfaces has been
thoroughly discussed and the reader is kindly reverted to a recent review
paper195 covering these aspects.
As evident from the discussion195 a lot of research has been performed on
different water-soluble polymers capable of enhancing the viscosity of the
subsequent polymer solution. Although many breakthroughs have been
accomplished in the application for personal care products and the paint
industry, the application of water-soluble polymers for EOR techniques is
limited. With the correct specification of the required product properties for
EOR applications the optimal water-soluble polymer can in principle be
designed. However in order to be successful a toolbox is needed that
correlates the molecular design of the polymer, i.e. its topology and chemical
composition, to the properties that are subsequently obtained and to the
response to external stimuli (i.e. pH, ionic strength, temperature, salt and
surfactant). For EOR applications there are several parameters that can affect
the rheological properties of the polymeric solutions employed: solution pH,
ionic strength, temperature, electrolyte concentration, shear and the
presence of bacteria. Depending on the polymeric system, these parameters
can have either a positive or negative effect on the rheological properties as
discussed in the following.
Polyelectrolytes, bearing only one charge, loose their solution viscosity as
the concentration of electrolytes, temperature and shear increase. However
they are resistant to changes in the pH of the solution. Zwitterionic polymers
(polyampholytes) are pH(ionic strength)-responsive polymers. The rheology
of these polymers can be tuned to the desired rheological properties by
changing the pH/ionic strength of the solution. In addition the temperature
dependence of the solution viscosity is minimal. Furthermore these polymers
are capable of enhancing the solution viscosity in salt environments up to
relevant concentrations (e.g. values higher than 100000 ppm TDS), which is
common in many oil reservoirs. At high shear though, a loss in solution
viscosity is observed.
Another category of polymers that are resistant to high temperatures are
the LCST polymers. However, these polymers are susceptible to phase
separation, which leads to a significant drop in the solution viscosity.
Introduction
30
Amphiphilic polymers do not bear charges and are therefore resistant to the
presence of electrolytes. Increase in the temperature leads to a loss in the
solution viscosity. The rheology of amphiphilic polymers is dependent on the
applied shear. At high shear rates, the solution viscosity is low and at low
shear the solution viscosity is high. This behavior is reversible, which for EOR
applications should be a beneficial property. Nevertheless, the distribution of
the hydrophobic groups is crucial in obtaining the required properties. If the
hydrophobic moieties are block-like distributed a stronger enhancement of
the solution viscosity is obtained compared to when the situation where the
distribution is random. The hydrophobicity of the groups also affects the
thickening capability of the polymer with higher hydrophobicity groups having
higher thickening capacity.
Combinations of the properties can also be achieved with amphiphilic
polyelectrolytes or polyampholytes. These polymers bear one or two
(different) charges and hydrophobes. The rheological properties of these
polymeric systems can be tailored by careful molecular design, i.e. the ratio
of the different monomers, of the polymers. Dependent on the molecular
design polymers can be obtained, which are pH-responsive, temperature
insensitive or salt resistant. For certain conditions, i.e. low pH and high
electrolyte concentration, carboxylic groups lead to better responsive
polymers compared to when sulphonic groups are used.
By using the above mentioned properties of the different polymeric
systems, a general trend can be identified where different systems can be
used for different applications. All the polymers have advantages and
disadvantages when compared to each other. These are presented in Table
1.1.
As can be observed in Table 1.1 many of the different polymeric systems
have one or two parameters, which affect their properties in a negative
manner. Therefore, it is crucial to correctly formulate the required properties
of a given application in order to design the correct polymeric system.
It is clear that the successful design of new water-soluble polymers for a
given application requires an integral multiscale and multidisciplinary
approach. Proper definition of the required product properties is in this case
crucial. Knowledge of polymer chemical architecture (and thus of the
synthetic methods used) must be conceptually linked to the desired product
application requirements. In this case viscosity measurements under different
shear conditions are of paramount importance and should be ideally
correlated with the “nature” (i.e. architecture and overall chemical
composition) of the corresponding water solution.
Chapter 1
31
Table 1.1: Advantages and disadvantages of the different polymeric systems
Parameters
Polymer type
High
shear pH
Ionic
strength
High
temperature
High electrolyte
concentration Bacteria
Polyelectrolyte - + + - -- ++
Polyampholyte - ++ + + ++ ++
LCST polymer - + ++ ++
Amphiphilic +- + - ++ ++
Amphiphilic polyelectrolyte +- + + - ++ ++
Amphiphilic polyampholyte +- ++ + +- ++ ++
Backbone type
Acrylamide (AM) - +- +- +- +- ++
Ethylene oxide (EO) +- - + ++
MMA-MA-EO +- +- +- +- ++
Cellulosic ++ +- +- +- +-/++ --
The influence of external parameters (e.g. pH, temperature etc.) on the
rheological behavior must be coupled with an in depth knowledge of the
relationship between the chemical structure and architecture of the polymer
and the rheological behavior. In this respect, an overall correlation cannot be
defined only as a function of the chemical/molecular structure. Rheological
properties will be affected by a combination of external parameters and the
chemical nature and molecular structure of the polymer. For instance, the
rheological properties of an aqueous solution of an amphiphilic polyelectrolyte
are similar to those of an unmodified analogue without amphiphilic moieties.
However, in the presence of salt a markedly different behavior is observed.
The solution viscosity of the unmodified polymer decreases with increasing
salt concentration, whereas the solution viscosity of the aqueous solution
containing the amphiphilic polyelectrolyte is not affected. Another good
example is the thermal performance of amphiphilic polymers: the rheological
properties of an aqueous solution containing the amphiphilic polymer or its
unmodified analogue, e.g. without the NIPAM monomer, are quite similar.
However, when exposing both solutions to higher temperatures significant
differences arise. The effect of temperature on the solution viscosity of an
aqueous solution containing the amphiphilic polymer is limited, whereas the
viscosity of the unmodified analogue changes significantly.
Although there are many different water-soluble polymers capable of
enhancing the solution viscosity, it is important to understand their
differences and analogies. Different polymers display in general differences in
the agglomeration principles governing their water behavior. On a molecular
level the basic principle is indeed quite general: the presence of relatively
weak inter(macro)molecular interactions (e.g. hydrophobic association,
Introduction
32
hydrogen bonding etc.) factually, albeit “virtually”, increases the molecular
weight of the polymer coils. As a consequence the solution viscosity
increases. However, a careful balance must be observed here since
predominantly weak interactions (both in terms of strength and number
thereof) do not result in observable rheological differences while excessively
strong ones might compromise the solubility of the system by leading, for
example, to gel formation.
1.5. Thickening capabilities
As mentioned before, the main purpose of water-soluble polymers is to
control the rheological properties of the solution. In all cases an increase in
the solution viscosity is required. The thickening capability of polymers is the
ability to increase the solution viscosity by the addition of a determined
amount of polymer. Polymers with a high thickening capability can increase
the solution viscosity significantly even at relatively low concentration
(typically in the order of few ppm). The mechanism behind the enhancement
of the solution viscosity can be conceptually divided into several different
possibilities (Figure 1.13).
Figure 1.13: Different methods to increase the solution viscosity of aqueous solutions.
Traditionally the two main types of viscosity enhancement using polymers
were to either increase the concentration (Fig. 1.13, A) or the molecular
weight (Fig. 1.13, B) of the polymer. The increase in concentration leads to
Chapter 1
33
more entanglements and thus a higher viscosity. Higher molecular weights
could be obtained when taking into account the continuous improvements in
the different polymerization techniques. With higher molecular weight
polymers less material is required to reach a predetermined viscosity given
the higher hydrodynamic radius of the polymer coils in water. This is testified
by the well-known Mark-Houwink36, 196 (M-H) equation relating the intrinsic
viscosity ( ) to the molecular weight ( ).
(1.5)
where and are constants characteristic for a given polymer-solvent
system at a specific temperature.
Another way of improving the thickening capability of a polymer is to
introduce charged moieties along the backbone of the polymer (Fig. 1.13, C).
The charged nature of the backbone will lead to electrostatic repulsions thus
increasing the hydrodynamic volume of the polymer coil which is synonymous
to a higher solution viscosity. The persistence length36 ( ) of a polymer chain
(closely related to the radius of gyration, ) is a function of the persistence
length without electrostatic interactions ( ) plus a contribution (
) related to
the electrostatic repulsions (equation 1.6).
(1.6)
where = the charge, = relative dielectric permittivity, = electrical
permittivity of vacuum, = Boltzmann’s constant, = temperature, = the
Debye screening length, and = the distance between the charges.
Increasing the amount of charges will lead to a lower distance between the
charges ( ) and thus a higher value. The persistence length ( ) is then
higher leading to a higher thickening capability.
As mentioned before, a relatively new approach is the introduction of
hydrophobic groups (Fig. 1.13, D), either randomly distributed or block
copolymers. In a water solution these hydrophobic groups have the tendency
to aggregate with each other. Depending on the concentration either intra or
intermolecular hydrophobic associations arise (vida supra).
To illustrate the differences between the polymers studied so far, a
comparison between the thickening ability of the polymers at a fixed
concentration (1 wt.%) is presented in Figure 1.14. The thickening ability of
polyacrylamide based polymers lies in either the high molecular weight or
electrostatic repulsions between charged moieties.195 The same applies for
Introduction
34
biopolymers; however chain rigidity also plays an important role, especially in
the case of xanthan gum.195 Hydrophobically modified polymers though,
achieve high solution viscosity due to intermolecular hydrophobic
interactions.195 The molecular weights of these polymers are usually much
lower than that of PAM based polymers and biopolymers.195
9
5
2,1
0,0350,2 0,08
0,56
2
HPAMPAM
HMPAM
HEURHASE
HMPAA
HM-C
ellulo
sic
Xanthan g
um
0
2
4
6
8
10 Viscosity
Vis
cosity (
Pa
.s)
0
2
4
6
8
10
Molecular weight
Mo
lecu
lar
we
igh
t (x
10
6 g
/mo
l)
Figure 1.14: Thickening abilities of different polymers, the solution viscosity (at =
10 s-1) of the polymer solution (1 wt.%) with corresponding molecular weight.
Another method, as will be presented by the research in this thesis, is the
introduction of branches, as long as the corresponding solutions are not in
the dilute regime. It has been demonstrated that in the melt state, branched
polymers display a higher zero shear rate viscosity (0) compared to their
linear analogues.197-215 This behavior has also been demonstrated in
concentrated solutions.216 Polymer solutions can be classified into three
different states. In the dilute regime the polymer concentration is so low that
no overlap between the polymeric chains is present. In the semi-dilute
regime overlap starts to arise and in the entangled (concentrated solutions)
state no separate polymeric coils can be distinguished. In dilute solutions it
has been demonstrated that the hydrodynamic volume of branched polymers
is lower than their linear analogues.217 Therefore, if no entanglements are
present a lower solution viscosity is observed. It is worth noting that polymer
solutions used for EOR are, in most cases, semi-dilute solutions. Up to date
and to the best of the author’s knowledge, the effect of the macromolecular
Chapter 1
35
architecture (i.e. presence of branches) on the thickening ability of PAM-
based polymers in semi-dilute solutions has not yet been reported and
constitutes therefore a relevant novelty aspect of the present work. One
major hurdle that must be overcome in order to demonstrate this is the
controlled polymerization of acrylamide. This is necessary in order to be able
to correctly specify the macromolecular architecture.
1.6. End-use requirements for polymers in EOR
Although many different water soluble polymers have been identified as
possible new chemical agents for polymer flooding it remains crucial that
certain requirements are met by these polymers for them to be successfully
applied in EOR. Indeed, some are trivial such as solubility in water and
capability of increasing the solution viscosity at low concentrations. However,
other requirements are less obvious but yet as important as the obvious
ones. The following list of requirements (arbitrary order) for polymers to be
considered for EOR has been established in cooperation with Shell:
- Ability to withstand high salt concentration, >20000 ppm total
dissolved solids (TDS)
- Applicable at high temperature, 80 – 120 °C
- Hydrolysis resistant
- Applicable for oil viscosity values of 1 – 200 cP
- Usable in low permeable reservoirs, 1 – 50 mD
- Stability of the polymer solution (in terms of solution viscosity) for a
few years
The majority of the oil reservoirs in the world are injected with brine to
increase the recovery rate of the field. However, given the anionic character
of the currently used HPAM, a significant reduction in solution viscosity is
observed when the polymer is dissolved in salt water. Therefore, new
polymers for EOR should be able to resist the presence of salt without a
significant reduction in the solution viscosity. When taking into account
applications where the TDS reaches values above 20000 ppm, a complete
screening of the charges on the anionic HPAM is observed. The solution
viscosity drops significantly, by almost two orders of magnitude.218, 219
Significant progress has been booked in identifying polymers that display
resistance to the presence of salts such as polyampholytes220-227 (bearing
both positive and negative charges), polyelectrolyte amphiphilic120, 161, 180, 181,
188, 228-237, zwitterionic amphiphilics238-241 or amphiphilics107, 108, 114, 137, 138, 143,
Introduction
36
242-254. Polymers specifically prepared for the use in high salinity oil reservoirs
have also been developed.255, 256 Nevertheless, other limitations arise with
the use of such polymers. Therefore, progress in new polymers capable of
coping with the presence of salt without introducing new limitations is still
required.
Another important parameter of many of the world’s oil reservoirs is the
high temperature (in many cases > 50 °C). The general reduction in solution
viscosity with temperature, due to the increased mobility of the polymeric
chains in solution, is also observed for the currently used EOR polymers.
Nowadays in general, oil reservoirs with a temperature higher than 50 °C are
not considered for polymer flooding. Given the high polymer concentration
required to match the viscosity of the oil (at high temperatures), applying
polymer flooding would not be economically beneficial. Nonetheless, a safe
limit (without chemical degradation) of ≤75 °C has been defined for
polyacrylamides257 and ≤80 °C for xanthan gum84. Significant progress has
been accomplished on thermoresponsive water-soluble polymers.258 A couple
of different monomers, displaying a LCST behavior, have been copolymerized
with acrylamide for EOR applications.255, 259, 260 However, systematic studies
on the effect of the “size” of the LCST on the rheological properties remains
elusive given the difficulties in controlled synthesis of acrylamide based
polymers.
The application of acrylamide based water-soluble polymers in oil
reservoirs with temperatures above 50 °C (and in the presence of a base)
creates a new problem. Hydrolysis (Scheme 1.1) of polyacrylamide is
extensive at elevated temperatures. Due to the more ionic character of the
polymer, this leads initially to an increase in solution viscosity. However, in
the presence of divalent ions, such as Ca2+ and/or Mg2+, precipitation of the
polymer is observed with a significant loss in solution viscosity and
injectivity257.
Scheme 1.1: Alkaline hydrolysis of polyacrylamide
Copolymerization of acrylamide with hydrolysis resistant monomers has been
extensively investigated. Homopolymers of sodium-2-acrylamido-2-
Chapter 1
37
methylpropane sulfonate (NaAMPS) are resistant to hydrolysis at
temperatures up to 120 °C.257 However, copolymers of acrylamide and
NaAMPS hydrolyze readily, depending on the molar ratio between acrylamide
and NaAMPS, at these temperatures leading to precipitation in the presence
of divalent ions.257 Another monomer that has been extensively studied to
improve the hydrolysis resistance of the acrylamide-based polymers is
vinylpyrrolidone.34, 261-263 The homopolymer of vinylpyrrolidone is resistant to
hydrolysis, but, more importantly and unlike NaAMPS, the vinylpyrrolidone
moieties also seem to “protect” the acrylamide units from hydrolysis in the
corresponding copolymers.261-263 Nevertheless, one of the major advantages
of acrylamide can still not be overcome by these monomers and that is its
low price. The introduction of hydrolysis resistant moieties increases the price
of the polymer and ultimately changes the whole economic picture of polymer
based flooding.
The application of polymer flooding depends, among other parameters,
on the viscosity of the oil in the reservoir. The viscosity of oil varies
significantly, from water like consistency up to bitumen (tar sands). It is
desirable to be able to apply polymer flooding for oil viscosities up to 200 cP.
The higher the oil viscosity the more polymer is required to match the
viscosity of the displacing fluid (water). The higher the required polymer
concentration, the less attractive (higher polymer costs) the oil reservoir is
for the application of polymer-based floods. Therefore extensive research has
been done195, and is ongoing, to improve the thickening capabilities of water
soluble polymers.
The permeability of a reservoir is, as mentioned before, the ability of a
fluid to pass through the porous media. A great number of oil reservoirs
around the world have porous media whose permeabilities are lower than 50
mD, the so-called carbonate reservoirs. Currently used polymers are high
molecular weight polymers and will block the pores of such low permeable
reservoirs. Adsorption to the rock surface24 and bridging264-266 by the polymer
chains lead to injectivity loss. Lab core flood testing confirms this by showing
a significant increase of the pressure over the core sample increases as more
and more polymer solution is flowed through.265-267
1.7. Aim and scope of this thesis
The challenges facing the supply of energy are briefly discussed and oil
recovery is introduced along with enhanced oil recovery in Chapter 1.
Currently used polymers are discussed along with their benefits and
limitations. The state of the art of polymers for enhanced oil recovery is
Introduction
38
thoroughly reviewed and new possibilities for other (new) polymers that can
improve on the limitations of currently used polymers are discussed. In
addition, the product specifications are identified and serve as a guideline for
the design of new polymers.
Chapter 2 reports the first successful atomic transfer radical
polymerization (ATRP) of acrylamide in water at room temperature.
Polyacrylamide with molecular weights higher than 150 000 g/mol with
dispersities as low as 1.39 can be prepared. Evidence for the “living”
character of the synthetic method is provided by; good concordance between
the theoretical molecular weight and the actual molecular weight, low
dispersities, linear increase in molecular weight with conversion, and
successful chain extension.
The successful ATRP of acrylamide in water is further expanded to
prepare branched polyacrylamide in Chapter 3. Star (4-arm) polyacrylamides
are prepared through the use of a commercial tetra-functional initiator. The
preparation of comb-like (12-arm) polyacrylamides is also discussed where
novel macroinitiators based on alternating aliphatic polyketones are used.
Evidence for the controlled preparation is provided along with preliminary
results on the rheological properties of aqueous solutions containing the
architectural different polyacrylamides.
Architectural different polyacrylamides are prepared in Chapter 4. The
dependence of the rheological properties on the molecular architecture of the
polymer is investigated. The solution viscosity of an aqueous solution is
heavily dependent on the architecture of the polyacrylamide above the critical
overlap concentration. Both an increase and a decrease in the solution
viscosity can be achieved by the introduction of branches, depending on the
number of arms. In addition, the visco-elastic response of an aqueous
solution containing the polyacrylamides can also be manipulated by
controlling the number of branches.
In Chapter 5 the synthesis of block copolymers of AM and NIPAM is
reported. PAM-b-PNIPAM block copolymers were prepared through ATRP in
water at room temperature. The block lengths of both moieties were varied in
order to obtain polymers with varying hydrophilic-lyophilic balances (HLB).
The solution properties, i.e. CMC and solution viscosity as a function of
temperature, of these polymers was correlated to the solubility parameter
).
The synthesis of different, both chemically and architecturally, thermo-
responsive polymers based on acrylamide is discussed in Chapter 6. The
solution properties of the random and block copolymers are investigated. The
importance of chemical structure on the thermo-responsiveness of the
Chapter 1
39
polymers is demonstrated. In addition, the effect of the chemical structure
(block or random) and molecular architecture on the surface tensions is
discussed. In general, the block copolymers tend to precipitate from the
solution upon heating while the random copolymers stayed in solution. The
strength of the hydrophobic interactions plays a crucial role in the observed
behavior.
In Chapter 7 the flow properties (through porous media) of the different
polymers is evaluated. Initial filter tests are performed in order to predict the
polymers’ ability to permeate sandstone or carbonate core samples. The ease
of passage through filters of different pore size is independent of the number
of branches. In addition, the recovery of oil out of core samples was
evaluated for the different polymers. The efficiency of the oil recovery
depends on both the chemical structure and the architecture of the polymers.
In addition the recovery of residual oil in a two dimensional flow cell was also
investigated. It is demonstrated that both the chemical structure and the
architecture of the polymer employed affects the amount of residual oil that
can be recovered through the use of polymers in EOR.
To conclude the thesis, a thorough discussion on the problems that have
been tackled and (partially) solved through the polymers developed in this
project is presented in Chapter 8. In addition, polymers applied in EOR often
are injected in solutions containing alkali, leading to chemical degradation of
polyacrylamide. Hydrolysis resistant polymers are synthesized through ATRP.
It is demonstrated that branched polymers can be prepared using hydrolysis
resistant moieties. In addition, the resistance of branched polyacrylamide
against alkaline hydrolysis is improved compared to that of linear
polyacrylamide. Finally an overview of biopolymers that might have potential
for application in EOR is presented.
1.8. Acknowledgement
This work is part of the Research Programme of the Dutch Polymer
Institute DPI, Eindhoven, the Netherlands, projectnr. #716.
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Introduction
40
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