**2. Direct carbon dioxide thickeners**

### **2.1 Polymeric thickeners**

The use of a polymer thickener is one of the fundamental strategies for increasing CO2 viscosity [2]. The main advantages of this approach are that the thickening process can enhance CO2 viscosity over a wide range of temperatures and improve sweep efficiency across the reservoir formation [6]. Although high molecular weight (Mw) polymers (Mw >106 g.mol<sup>−</sup><sup>1</sup> ) are effective viscosity enhancers at dilute concentrations, it is extremely challenging to dissolve ultrahigh molecular weight in dense CO2 at reservoir conditions [6, 7]. In the literature, several polymers (Mw: 103 –106 g.mol<sup>−</sup><sup>1</sup> ) have been designed and identified that can be dissolved and thicken supercritical CO2 [1]. However, the pressure required for the dissolution of these polymers is very high in the range of 68.95–275.79 MPa, which is significantly higher than the typical reservoir pressures for CO2 flooding (MMP, 10.3–27.6 MPa) [1].

The earliest attempts at viscosity enhancers for dense CO2 were with oil-soluble polymers (e.g. non-polar organic polymers) because CO2 is known to be a nonpolar solvent capable of dissolving certain hydrocarbons and other small molecules quite well [6, 8]. Therefore, it was expected that oil-soluble polymers would be a more likely candidate to dissolve in supercritical CO2 compared to water-soluble polymers. Heller et al. identified 18 hydrocarbon-type polymers that exhibited encouraging solubility (0.22–10 g/litre) in CO2 at pressures of 11.7–21.4 MPa and temperatures of 293–331 K [9–14]. Although several polymers showed a slight increase in CO2 viscosity, none of the studied polymers were capable of enhancing the viscosity of CO2 significantly to a useful level. This is attributed to low solubility in CO2 leading to a random polymer coil structure that is not efficient at significantly increasing viscosity. Furthermore, the molecular weight of the polymers found to be soluble in CO2 was very low. For example, a 1 wt% solution of low molecular weight atactic poly(methyl oxirane) (Mw: 408 g.mol<sup>−</sup><sup>1</sup> ) exhibited slight solubility in CO2 and increased its viscosity by approximately 25% at 301–306 K and 13.7–17.9 MPa [11]. These initial efforts were followed by subsequent attempts to maximise the entropy of mixing between the CO2 and polymers by using disordered polymers with irregularity in multiple components and side chains that varied in chain length [9]. Thereby, focus was put on poly(α-olefins), such as poly(1-hexene), poly(1-pentene), and poly(1-decene) (P-1-D). Although some achievements were made with some of the evaluated polymers, none of these amorphous polymers were considered to be effective thickeners primarily due to their low solubility in CO2. In general, it is concluded that the molecular weight of the polymers had to

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the fracture [1, 3, 4].

solvent molecules [5].

**2.1 Polymeric thickeners**

g.mol<sup>−</sup><sup>1</sup>

weight (Mw) polymers (Mw >106

**2. Direct carbon dioxide thickeners**

that are sensitive to the typical water-based fluids used for fracturing. Increasing the fluid viscosity results in a more effective fracturing fluid [1]. In addition, at high pressures, viscous fluids would be able to propagate wider fractures by improving the transport of proppant particles and reducing the leak-off of gas into the faces of

In previous studies, efforts were centred on identifying thickeners for CO2 and natural gas liquid (NGL) (i.e. ethane, propane, and butane) thickeners. These attempts are based on polymeric and small-molecule candidates as will be reviewed and highlighted in this chapter. The mechanisms behind the thickening of any solvent depend on polymer coil expansion, intermolecular interactions, entanglement, aggregation (affected by the polymer molecular weight distributions), and self-assembly and indirectly through the effect of polymer molecules on nearby

The use of a polymer thickener is one of the fundamental strategies for increasing CO2 viscosity [2]. The main advantages of this approach are that the thickening process can enhance CO2 viscosity over a wide range of temperatures and improve sweep efficiency across the reservoir formation [6]. Although high molecular

) are effective viscosity enhancers at dilute

) exhibited slight

g.mol<sup>−</sup><sup>1</sup>

concentrations, it is extremely challenging to dissolve ultrahigh molecular weight in dense CO2 at reservoir conditions [6, 7]. In the literature, several polymers (Mw:

supercritical CO2 [1]. However, the pressure required for the dissolution of these polymers is very high in the range of 68.95–275.79 MPa, which is significantly higher than the typical reservoir pressures for CO2 flooding (MMP, 10.3–27.6 MPa) [1].

polymers (e.g. non-polar organic polymers) because CO2 is known to be a nonpolar solvent capable of dissolving certain hydrocarbons and other small molecules quite well [6, 8]. Therefore, it was expected that oil-soluble polymers would be a more likely candidate to dissolve in supercritical CO2 compared to water-soluble polymers. Heller et al. identified 18 hydrocarbon-type polymers that exhibited encouraging solubility (0.22–10 g/litre) in CO2 at pressures of 11.7–21.4 MPa and temperatures of 293–331 K [9–14]. Although several polymers showed a slight increase in CO2 viscosity, none of the studied polymers were capable of enhancing the viscosity of CO2 significantly to a useful level. This is attributed to low solubility in CO2 leading to a random polymer coil structure that is not efficient at significantly increasing viscosity. Furthermore, the molecular weight of the polymers found to be soluble in CO2 was very low. For example, a 1 wt% solution of low

molecular weight atactic poly(methyl oxirane) (Mw: 408 g.mol<sup>−</sup><sup>1</sup>

The earliest attempts at viscosity enhancers for dense CO2 were with oil-soluble

solubility in CO2 and increased its viscosity by approximately 25% at 301–306 K and 13.7–17.9 MPa [11]. These initial efforts were followed by subsequent attempts to maximise the entropy of mixing between the CO2 and polymers by using disordered polymers with irregularity in multiple components and side chains that varied in chain length [9]. Thereby, focus was put on poly(α-olefins), such as poly(1-hexene), poly(1-pentene), and poly(1-decene) (P-1-D). Although some achievements were made with some of the evaluated polymers, none of these amorphous polymers were considered to be effective thickeners primarily due to their low solubility in CO2. In general, it is concluded that the molecular weight of the polymers had to

) have been designed and identified that can be dissolved and thicken

**66**

103 –106 be fairly low (Mw > 1000 g.mol<sup>−</sup><sup>1</sup> ) and thus less effective at enhancing viscosity to achieve significant solubility in CO2 [6]. Typically, high and ultrahigh molecular weight polymers are used as effective thickeners. In 2012, Zhang et al. reported that less than 1 wt% solutions of two oligomers (i.e. P-1-D and poly(vinyl ethyl ether) (PVEE)) (**Figure 1**) could increase the viscosity of CO2 by 13–14-fold at 329 K [15]. Previous research groups found that neither a 1 wt% mixture of PVEE nor P-1-D was capable of enhancing the viscosity in either toluene or CO2 by more than several percents [2, 6, 8]. Therefore, Zhang et al. findings do not correlate and are inconsistent with the results of other research groups. Most previous studies, reported that for low/high molecular weight polymers, a concentration of 1.5–7 wt% is required to thicken CO2 albeit at very high pressure [6]. In recent studies, [16, 17] P-1-D has been found to have sufficient solubility in both CO2 and associated gas (AG) mixtures (at temperatures above 358 K and pressures of 50–55 MPa) to considerably increase gas viscosity. The viscosity enhancement of P-1-D in an AG mixture (25 mol% CO2) and CO2 was measured in a capillary viscometer at different pressures (50–55 MPa), 377 K, and varying P-1-D concentrations (1.5–9 wt%). For example, at 5 wt% P-1-D, the CO2 viscosity falls in the range of 0.14–0.18 cP (1.2–2.6-fold) over the pressure range of 50–55 MPa, while over the same pressure range, the AG mixture viscosity is in the range of 0.126–0.131 cP (~4-fold).

In 1987, a patent published by Bullen and co-workers [18] claimed that CO2 based fracturing could be improved by adding a small amount of a polycarbonate copolymer (Mw: 20,000–150,000 g.mol<sup>−</sup><sup>1</sup> ) that was formed via low-temperature reaction of CO2 with propylene oxide in a homogeneous catalyst (e.g. diethylzinc and/or acetic acid anhydride). This copolymer exhibits dissolution in CO2 and is capable of increasing its viscosity by threefold at a concentration of 2.5 wt% at 295 K and pressures ranging from 10 to 25 MPa. Furthermore, Sarbu et al. [19] tested the solubility of poly(ether-carbonate) copolymers (derived from propylene oxide and CO2) in CO2. They found that this copolymer (Mw: 16000 g/mol) could only be dissolved at 1 wt% at 295 K and 14 MPa. However, there was no significant increase in CO2 viscosity under these conditions. This calls into question some of the results by Bullen and co-workers as they are much better than later literature [2].

Other researchers have attempted to use entrainers (co-solvents) to improve the solubility of polymers in CO2 [20] and as such increase the CO2 viscosity as well as increase the solubility of crude oil components in the CO2-rich phase [21]. These

entrainers are relatively low molecular weight non-polar or polar organic compounds which include alcohols, glycols, ethoxylated alcohols, and hydrocarbons [21]. Chullick's patent claimed that addition of alcohol and glycol would promote the solubility of polymers in CO2 [20]. The rationale is that polar entrainers improve the polarizability of CO2, which may act in a similar manner to a surfactant in the water/oil system, while non-polar entrainers may function as mutual solvents in a polymer/CO2 system [20]. Therefore, the addition of entrainers to a supercritical fluid (SCF) leads to increase in the solvent power of SCF [21]. Furthermore, a National Institute for Petroleum and Energy Research (NIPER) group evaluated the use of entrainers (without polymer) as CO2 thickeners [21]. They reported substantial increase of CO2 viscosity with high concentration of entrainers in CO2. For example, 13 mol% of isooctane and 44 mol% of 2-ethylhexanol increased the viscosity of CO2 by 243 and 1565%, respectively. However, at dilute concentration of entrainers in CO2, the viscosity enhancement was subtle. For example, 2 mole% of 2-ethylhexanol resulted in 24% of CO2 viscosity enhancement [21]. In another patent, a treating fluid was used to increase the viscosity of CO2 solution. This treating fluid is formed by solubilising a polymer or copolymer of dimethylacrylamide (0.25–2.5 wt%) in the substantially anhydrous liquid which was cross-linked by a metal ion source (0.01–2 wt% of titanium, zirconium, and/or aluminium). The substantially anhydrous fluid/polymer and CO2 solutions formed a single phase and viscosified the fluid (13–30 cP) at temperatures of 338–377 K and pressures of 6.89–12 MPa [22]. Although, there is significant increase in CO2 viscosities, the amount of entrainers added is extremely high [6].

A group of researchers at the University of Wyoming attempted at in situ polymerisation of CO2-soluble alkene monomers, including ethylene, octene, and decene [23]. They found that polymerised monomers could be miscible in CO2 at the tested conditions (306 K and 17.9 MPa). However, the polymers did not form stable solutions and produced solid precipitates over time. As a result, viscosity enhancement was not observed. In an attempt to obtain a very high molecular weight polymeric thickener for CO2, researchers at Chevron [24–26] have assessed candidates that exhibited Hildebrand solubility parameters of less than 7 (cal/cc)0.5 that is closer to the CO2 solubility parameter at reservoir conditions (327 K and 17.2 MPa), which is approximately 6 (cal/cc)0.5 [27]. Furthermore, they found beneficial, if the polymer candidates contained electron donor atoms such as oxygen, nitrogen, and sulphur that are capable of interacting favourably with the carbon atom (i.e. an electron acceptor) within the CO2 molecules. Electron donor functional groups used in this study included ethers, sulphones, siloxanes, thioethers, silyethers, esters, carbonyls, dialkylamides, and tertiary amines. The researchers concluded that such functional groups associated within polymers would enhance the solubility of the polymers to some extent through specific interaction with CO2. However, full dissolution of such polymers in CO2 could not be attained without the addition of toluene as a co-solvent [24].

High molecular weight polydimethylsiloxane (PDMS, Mw: 135,000 g.mol<sup>−</sup><sup>1</sup> ) (**Figure 1**) was initially tested by Heller et al. [11] for solubility in CO2. They found that 0.03 wt% of PDMS could dissolve in CO2 at 298 K and 18.9 MPa. However, at this dilute concentration, PDMS did not appreciably enhance CO2 viscosity. Furthermore, others attempted to increase the PDMS concentrations in CO2; however, the solubility of PDMS in CO2 could not be attained without substantial addition of toluene as a co-solvent. Therefore, it was determined that very high molecular weight PDMS (Mw: 197,000 g.mol<sup>−</sup><sup>1</sup> and 7.3 (cal/cc)0.5) could effectively thicken CO2 only if a toluene co-solvent (10–20%) was added into the solution [26]. For example, addition of 20 wt% toluene co-solvent enabled up to 4 wt% of PDMS to be dissolved in CO2, resulting in a 30-fold of CO2 viscosity enhancement [26].

### *Direct Gas Thickener DOI: http://dx.doi.org/10.5772/intechopen.88083*

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amount of entrainers added is extremely high [6].

addition of toluene as a co-solvent [24].

molecular weight PDMS (Mw: 197,000 g.mol<sup>−</sup><sup>1</sup>

entrainers are relatively low molecular weight non-polar or polar organic compounds which include alcohols, glycols, ethoxylated alcohols, and hydrocarbons [21]. Chullick's patent claimed that addition of alcohol and glycol would promote the solubility of polymers in CO2 [20]. The rationale is that polar entrainers improve the polarizability of CO2, which may act in a similar manner to a surfactant in the water/oil system, while non-polar entrainers may function as mutual solvents in a polymer/CO2 system [20]. Therefore, the addition of entrainers to a supercritical fluid (SCF) leads to increase in the solvent power of SCF [21]. Furthermore, a National Institute for Petroleum and Energy Research (NIPER) group evaluated the use of entrainers (without polymer) as CO2 thickeners [21]. They reported substantial increase of CO2 viscosity with high concentration of entrainers in CO2. For example, 13 mol% of isooctane and 44 mol% of 2-ethylhexanol increased the viscosity of CO2 by 243 and 1565%, respectively. However, at dilute concentration of entrainers in CO2, the viscosity enhancement was subtle. For example, 2 mole% of 2-ethylhexanol resulted in 24% of CO2 viscosity enhancement [21]. In another patent, a treating fluid was used to increase the viscosity of CO2 solution. This treating fluid is formed by solubilising a polymer or copolymer of dimethylacrylamide (0.25–2.5 wt%) in the substantially anhydrous liquid which was cross-linked by a metal ion source (0.01–2 wt% of titanium, zirconium, and/or aluminium). The substantially anhydrous fluid/polymer and CO2 solutions formed a single phase and viscosified the fluid (13–30 cP) at temperatures of 338–377 K and pressures of 6.89–12 MPa [22]. Although, there is significant increase in CO2 viscosities, the

A group of researchers at the University of Wyoming attempted at in situ polymerisation of CO2-soluble alkene monomers, including ethylene, octene, and decene [23]. They found that polymerised monomers could be miscible in CO2 at the tested conditions (306 K and 17.9 MPa). However, the polymers did not form stable solutions and produced solid precipitates over time. As a result, viscosity enhancement was not observed. In an attempt to obtain a very high molecular weight polymeric thickener for CO2, researchers at Chevron [24–26] have assessed candidates that exhibited Hildebrand solubility parameters of less than 7 (cal/cc)0.5 that is closer to the CO2 solubility parameter at reservoir conditions (327 K and 17.2 MPa), which is approximately 6 (cal/cc)0.5 [27]. Furthermore, they found beneficial, if the polymer candidates contained electron donor atoms such as oxygen, nitrogen, and sulphur that are capable of interacting favourably with the carbon atom (i.e. an electron acceptor) within the CO2 molecules. Electron donor functional groups used in this study included ethers, sulphones, siloxanes, thioethers, silyethers, esters, carbonyls, dialkylamides, and tertiary amines. The researchers concluded that such functional groups associated within polymers would enhance the solubility of the polymers to some extent through specific interaction with CO2. However, full dissolution of such polymers in CO2 could not be attained without the

High molecular weight polydimethylsiloxane (PDMS, Mw: 135,000 g.mol<sup>−</sup><sup>1</sup>

(**Figure 1**) was initially tested by Heller et al. [11] for solubility in CO2. They found that 0.03 wt% of PDMS could dissolve in CO2 at 298 K and 18.9 MPa. However, at this dilute concentration, PDMS did not appreciably enhance CO2 viscosity. Furthermore, others attempted to increase the PDMS concentrations in CO2; however, the solubility of PDMS in CO2 could not be attained without substantial addition of toluene as a co-solvent. Therefore, it was determined that very high

thicken CO2 only if a toluene co-solvent (10–20%) was added into the solution [26]. For example, addition of 20 wt% toluene co-solvent enabled up to 4 wt% of PDMS to be dissolved in CO2, resulting in a 30-fold of CO2 viscosity enhancement [26].

)

and 7.3 (cal/cc)0.5) could effectively

**68**

This viscosity enhancement was compared only with pure CO2 viscosity, but it was not compared against toluene/CO2 viscosity, because it was expected that toluene addition into CO2 may not contribute directly to the CO2 viscosity enhancement and it only improves the solubility of polymer in CO2. However, their core flooding experiment results showed that CO2/toluene flood gives higher oil recovery than pure CO2 flood. This attributes to that toluene is a strong solvent, which causes a higher oil swelling and oil viscosity reduction. It was also found the viscous solution (20 wt% toluene, 4 wt% of PDMS, and 76 wt% of CO2) can significantly reduce gas mobility, increase oil recovery (10–20%), and delay the breakthrough in porous media. In another study a group of researchers from Texas A&M University added an organic co-solvent (e.g. toluene) into CO2-philic polymeric thickeners (PDMS and polyvinyl acetate (PVAc)) during core flooding experiments in order to enhance the solubility in CO2 [28, 29]. They prepared solutions of PDMS (5 wt% with a Mw of 260,000 g.mol<sup>−</sup><sup>1</sup> ) and PVAc (5 wt% with a Mw of 170,000 g.mol<sup>−</sup><sup>1</sup> ) with a range of toluene concentrations from 10 to 20 wt% added as a co-solvent. In addition, PVEE was used at the concentration of 0.8 wt% in CO2 without the addition of a co-solvent, as this polymer has the ability to dissolve in CO2 without a co-solvent [29]. Their results proved that PDMS and PVAc with the addition of toluene could improve the gas mobility, accelerate the oil recovery (5–10% with PDMS and 4–9% with PVAc), and delay CO2 breakthrough. These results were consistent with the finding of Chevron researchers that 4 wt% PDMS was soluble and could thicken CO2 in the presence of a co-solvent. In other words, both groups found that PDMS and PVAc are both to be CO2-philic and effective thickeners with the use of substantial amounts of a co-solvent. PVEE (Mw: 3800 g.mol<sup>−</sup><sup>1</sup> ) at low concentration (0.8 wt%) did not show any increase in viscosity or improvement in CO2 mobility and oil recovery [29]. This means that the PVEE may not be able to increase the CO2 viscosity at this concentration which further confirms that the thickening level reported by Zhang et.al is higher than expected.

A group of researchers at the New Mexico Petroleum Recovery Research Center (PRRC) proposed another route towards high-performance thickeners by introducing self-interacting functional groups at each end of the polymer chains [14]. These polymers with terminal ionic groups (linear, difunctional, and telechelic ionomers) were thought to be effective thickeners in non-polar solvents as the ionic groups in each chain could aggregate into multiple ion pairs causing interaction between separate chains. They incorporated sulphonate groups onto the ends of polyisobutylene, and although this polymer is soluble in CO2 at a concentration of 0.4 wt%, the sulphonated ionomer was only partially dissolved in CO2, which is a weak solvent for this ionic group. In a recent publication, O'Brien et.al [30] used a route similar to the approach proposed by the PRRC group. The difference between the two studies is that the PRRC group used functional groups in the polymer chain that experiences intermolecular interactions with CO2, while O'Brien proposes using functional groups that form self-assembly and intermolecular interactions via hydrogen bonding (donor-acceptor), π-π stacking, and dipole-dipole interactions. Therefore, the strategy used by the O'Brien's group could be used to explore oligomeric molecules. So, they synthesised a series of aromatic-amide terminated PDMS oligomers (i.e. low molecular weight polymers) to maximise the entropic characteristics for oligomeric species interacting with CO2 by choosing a solute with high free volume and flexible chains. In addition, the aromatic moieties promote the formation of supramolecular structures among the low molecular weight oligomers. Amide and aromatic amide further enhance this interaction and induce self-assembly through strong hydrogen bond donor-acceptor interactions. The researchers found that amide-terminated-PDMS oligomers with simple aromatic groups through the incorporation of electron-deficient aromatic groups onto these

amides (i.e. 4-nitrobenzamide, biphenyl-4-carboxamide, and anthraquinone-2-carboxamide) did not show any significant impact on CO2 viscosity at a concentration of 1 wt% due to the poor interaction of functional groups in these compounds with CO2. However, they achieved promising results with anthraquinone-2-carboxamide (AQCA) as an end group that was found to be a gelator of hexane at a concentration of 15 wt%, and at concentrations ranging from 5 to 10 wt% in hexane, a significant increase in viscosity was observed. However, this behaviour did not extend to other similar compounds based on either biphenyl-4-carboxamide or 4-nitrobenzamide end groups. Therefore, they attempted to improve the intermolecular interaction with the AQCA end group by utilising branched anthraquinone amides, where the number of AQCA end groups per molecule can be increased. It was found that branched anthraquinone amides were insoluble in CO2 at concentrations of 1 wt%. However, it was soluble in CO2 when hexane as co-solvent was added. Hence, this branched compound can be a useful thickener in the presence of substantial amount of hexane as co-solvent. For example, at a temperature and pressure of 348 K and 34.5 MPa, respectively, a transparent solution composed of 13.3% branched anthraquinone-amide functionalised oligomers, 26.7% hexane, and 60% CO2 was found to have a viscosity three times greater than that of a CO2/hexane mixture without a thickener. Given the low-viscosity enhancement (threefold) and high concentration of this compound and the co-solvent required, this compound was not considered to be economical and practical for CO2 flooding. These compounds that have considerable intermolecular interactions with CO2 are further discussed in the next section of this chapter. Overall, all studies found that high/low molecular weight PDMS polymers were more CO2-philic than hydrocarbon-based polymers [31], although they were not capable of viscosifying CO2 without the use of substantial amounts of a co-solvent. However, the high cost of PDMS polymer/ oligomer and high concentration of co-solvent required make the field application for this polymer impractical [2, 6, 7].

DeSimone's research group [32] was arguably among the first to report on a high molecular weight polymer-based CO2 thickener capable of increasing the viscosity without the need of a co-solvent. They found that 3.4 wt/vol% of either poly(1,1-dihydroperfluorooctyl acrylate) (PFOA) (**Figure 1**) or polyfluoroacylate (PFA, Mw: 1,400,000 g.mol<sup>−</sup><sup>1</sup> ) could be dissolved in CO2 increasing the viscosity of CO2 by a factor of 2.5 at a pressure of 31 MPa and temperature of 323 K. **Figure 2** shows the increase in CO2 viscosity resulted from dissolving 3.7 and 6.7 wt/v% of PFOA at 323 K. This is the first example of high Mw polymers that can be dissolved in CO2 and significantly thicken CO2 in the absence of a co-solvent. To date, PFOA is still recognised as the most soluble polymer in CO2 and among the most effective thickeners of CO2. Unfortunately, PFOA is a fluoropolymer type, which makes it relatively expensive. Furthermore, fluorinated polymers possess environmental concerns as they are suspected as carcinogen [33]. Therefore, if the cost and environmental constraints are considered, PFOA is not practical for field application in CO2 flooding [2].

To limit these negative aspects of fluorinated polymers and potentially make them viable, Enick and Beckman and other co-workers at the University of Pittsburgh have tried to reduce the amount of fluorinated polymers needed without affecting its performance [4, 34, 35]. They prepared a copolymer based on a perfluoropolyacrylate and a functional group, which engages strongly in intermolecular interactions, in order to promote an increase in CO2 viscosity. This copolymer is composed of 71–79 mol% of fluoroacrylate monomer (1,1,2,2-ttatrahydro heptaecfluorodecylacrylate) and 21–29 mol% of styrene group (polyfluoroacrylate styrene or polyFAST) (**Figure 1**). The fluoroacrylate monomer is highly CO2-philic and facilitates polyFAST solubility in CO2. The associating styrene

### *Direct Gas Thickener DOI: http://dx.doi.org/10.5772/intechopen.88083*

### **Figure 2.**

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for this polymer impractical [2, 6, 7].

(PFA, Mw: 1,400,000 g.mol<sup>−</sup><sup>1</sup>

CO2 flooding [2].

amides (i.e. 4-nitrobenzamide, biphenyl-4-carboxamide, and anthraquinone-2-carboxamide) did not show any significant impact on CO2 viscosity at a concentration of 1 wt% due to the poor interaction of functional groups in these compounds with CO2. However, they achieved promising results with anthraquinone-2-carboxamide (AQCA) as an end group that was found to be a gelator of hexane at a concentration of 15 wt%, and at concentrations ranging from 5 to 10 wt% in hexane, a significant increase in viscosity was observed. However, this behaviour did not extend to other similar compounds based on either biphenyl-4-carboxamide or 4-nitrobenzamide end groups. Therefore, they attempted to improve the intermolecular interaction with the AQCA end group by utilising branched anthraquinone amides, where the number of AQCA end groups per molecule can be increased. It was found that branched anthraquinone amides were insoluble in CO2 at concentrations of 1 wt%. However, it was soluble in CO2 when hexane as co-solvent was added. Hence, this branched compound can be a useful thickener in the presence of substantial amount of hexane as co-solvent. For example, at a temperature and pressure of 348 K and 34.5 MPa, respectively, a transparent solution composed of 13.3% branched anthraquinone-amide functionalised oligomers, 26.7% hexane, and 60% CO2 was found to have a viscosity three times greater than that of a CO2/hexane mixture without a thickener. Given the low-viscosity enhancement (threefold) and high concentration of this compound and the co-solvent required, this compound was not considered to be economical and practical for CO2 flooding. These compounds that have considerable intermolecular interactions with CO2 are further discussed in the next section of this chapter. Overall, all studies found that high/low molecular weight PDMS polymers were more CO2-philic than hydrocarbon-based polymers [31], although they were not capable of viscosifying CO2 without the use of substantial amounts of a co-solvent. However, the high cost of PDMS polymer/ oligomer and high concentration of co-solvent required make the field application

DeSimone's research group [32] was arguably among the first to report on a high molecular weight polymer-based CO2 thickener capable of increasing the viscosity without the need of a co-solvent. They found that 3.4 wt/vol% of either poly(1,1-dihydroperfluorooctyl acrylate) (PFOA) (**Figure 1**) or polyfluoroacylate

of CO2 by a factor of 2.5 at a pressure of 31 MPa and temperature of 323 K. **Figure 2** shows the increase in CO2 viscosity resulted from dissolving 3.7 and 6.7 wt/v% of PFOA at 323 K. This is the first example of high Mw polymers that can be dissolved in CO2 and significantly thicken CO2 in the absence of a co-solvent. To date, PFOA is still recognised as the most soluble polymer in CO2 and among the most effective thickeners of CO2. Unfortunately, PFOA is a fluoropolymer type, which makes it relatively expensive. Furthermore, fluorinated polymers possess environmental concerns as they are suspected as carcinogen [33]. Therefore, if the cost and environmental constraints are considered, PFOA is not practical for field application in

To limit these negative aspects of fluorinated polymers and potentially make them viable, Enick and Beckman and other co-workers at the University of Pittsburgh have tried to reduce the amount of fluorinated polymers needed without affecting its performance [4, 34, 35]. They prepared a copolymer based on a perfluoropolyacrylate and a functional group, which engages strongly in intermolecular interactions, in order to promote an increase in CO2 viscosity. This copolymer is composed of 71–79 mol% of fluoroacrylate monomer (1,1,2,2-ttatrahydro heptaecfluorodecylacrylate) and 21–29 mol% of styrene group (polyfluoroacrylate styrene or polyFAST) (**Figure 1**). The fluoroacrylate monomer is highly CO2-philic and facilitates polyFAST solubility in CO2. The associating styrene

) could be dissolved in CO2 increasing the viscosity

**70**

*The viscosity enhancements of PFOA in CO2 at different pressures and concentrations and temperature of 323 K [32].*

**Figure 3.** *The effect of temperature on the relative viscosity of polyFAST in CO2 solution at 34 MPa [35].*

group is a mildly CO2 phobic monomer that promotes intermolecular interactions and improves viscosity enhancement through supramolecular interactions. This copolymer was found to be soluble in CO2 at pressure and temperature conditions close to those used in CO2-EOR [34]. However, the solubility was found to decrease with an increase in the styrene content [4]. For instance, the cloud point pressure of 1 wt% of 29 mol% styrene-71 mol% fluoroacrylate copolymer and 35 mol% styrene-71 mol% fluoroacrylate copolymer in CO2 at 297 K is 12 MPa and 16.2 MPa, respectively. Furthermore, it was also found to significantly increase CO2 viscosity at dilute concentrations of polyFAST. As it can be seen in **Figure 3**, 0.5 and 1 wt% of polyFAST in CO2 at 298 K are able to increase the viscosity of CO2 by 1.5- and 2.3-fold, respectively [34]. However, polyFAST is the most effective polymeric thickener for CO2 at dilute concentration in the absence of a co-solvent. Comparing to PFOA results, this copolymer was successful to reduce concentration by 73% to achieve the same viscosity improvement (2.3-fold) at 323 K and 34 MPa. However, it was not practical to be used for CO2-EOR application due to the cost of the copolymer (roughly \$132 per kg) and lack of its availability in large quantities [2]. In addition, this copolymer contains a large amount of fluorine, which is environmentally and biologically persistent [29].

Another promising strategy to obtain effective CO2 thickeners was introduced to avoid the aforementioned environmental and economic concerns associated with fluorous and silicone-based polymers. Several researchers have focused on the synthesis and design of non-fluorinated oligomers and polymers. Tapriyal et al. [36] found that PVAc is the second most CO2 soluble polymer among non-fluorous polymers with PDMS being the most soluble. However, the dissolution of high Mw PVAc in CO2 requires a very high pressure. In addition, no measureable viscosity increase was observed with 1–2 wt% of PVAc (Mw: 11000 g.mol<sup>−</sup><sup>1</sup> ) in CO2 at 298 K and 64 MPa. Furthermore, Enick and co-workers [36] designed a non-fluorinated version of PFOA in the hope of finding a thickener candidate that is cheap, environmentally friendly, and capable of increasing CO2 viscosity at low concentration. Therefore, they developed new copolymers based on an oxygenated hydrocarbon polymer (making it CO2-philic) and a self-interacting group (or CO2-phobic) to enhance viscosity. Some of the attractive oxygenated hydrocarbon monomers include vinyl acetate, alkyl vinyl ether, carbonyl, and sugar acetate functional groups [37–43]. Oxygenated hydrocarbon monomers containing functional groups with one or more oxygen atoms can induce thermodynamic interactions with CO2. These oxygen atoms are electropositive, while the carbon atoms in CO2 are electronegative, which facilities Lewis acid-base interactions. In addition, the hydrogen bond in the polymer backbone or side chain having increased the positive charge (H…O) acts as Lewis acids towards electron the oxygen atoms in CO2 [41]. As mentioned above, PVAc is among the most CO2-philic high MW oxygenated hydrocarbon polymers [43]. Therefore, Enick and co-workers replaced the fluoroacrylates in polyFAST with vinyl acetate monomers in order to reduce the cost of the polymer. They designed non-fluorous copolymers for CO2 solubility, while the styrene group was replaced with a benzoyl group for intermolecular association. This approach simplifies the copolymer synthesis as styrene cannot be polymerised with the vinyl acetate monomer due to the large reactivity ratio difference [36]. They synthesised a 5% benzoyl-95% vinyl acetate copolymer or polyBOVA (Mw: 7840 g.mol<sup>−</sup><sup>1</sup> ). A modest increase in CO2 viscosity of 40–80% at a concentration of a 1 and 2 wt% was observed; however, high pressure was required (64 MPa) to attain the dissolution of this copolymer in CO2 at 298 K.

### **2.2 Small-molecule self-associating thickeners**

An alternate strategy to increase the viscosity of CO2 is to employ self-interacting low molecular weight compounds as thickeners (**Table 1**). In order to differentiate between co-solvents and this class of thickeners, these compounds interact with each other and the CO2 resulting in a self-assembled structure that contains a solventphilic group and a solvent-phobic segment, while a co-solvent is nonassociating and consists of solely a solvent-philic segment [7]. These small molecules do not have the required molecular weight to substantially increase the gas viscosity; however, these compounds self-associate forming a supramolecular network that enhances the CO2 viscosity [2]. In general, these compounds contain functional groups with both CO2 philic segments that promote dissolution, and the CO2-phobic moieties induce intermolecular association [44]. Therefore, the various associations between neighbouring molecules within the CO2 matrix lead to viscosity enhancement [2]. Furthermore, the self-assembly of these molecules in solution can be characterised via a dramatic viscosity change or small-angle neutron scattering (SANS), FT-IR, circular dichroism, X-ray diffraction, electron microscopy, or differential scanning calorimetry [45–53]. To date, small associative molecules have yielded little success in thickening CO2, because CO2 is a poor solvent for these ionic and polar associating groups [8]. The following section describes the different types of small molecules used to thicken CO2.

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copolymer or polyBOVA (Mw: 7840 g.mol<sup>−</sup><sup>1</sup>

**2.2 Small-molecule self-associating thickeners**

was observed with 1–2 wt% of PVAc (Mw: 11000 g.mol<sup>−</sup><sup>1</sup>

Another promising strategy to obtain effective CO2 thickeners was introduced to avoid the aforementioned environmental and economic concerns associated with fluorous and silicone-based polymers. Several researchers have focused on the synthesis and design of non-fluorinated oligomers and polymers. Tapriyal et al. [36] found that PVAc is the second most CO2 soluble polymer among non-fluorous polymers with PDMS being the most soluble. However, the dissolution of high Mw PVAc in CO2 requires a very high pressure. In addition, no measureable viscosity increase

64 MPa. Furthermore, Enick and co-workers [36] designed a non-fluorinated version of PFOA in the hope of finding a thickener candidate that is cheap, environmentally friendly, and capable of increasing CO2 viscosity at low concentration. Therefore, they developed new copolymers based on an oxygenated hydrocarbon polymer (making it CO2-philic) and a self-interacting group (or CO2-phobic) to enhance viscosity. Some of the attractive oxygenated hydrocarbon monomers include vinyl acetate, alkyl vinyl ether, carbonyl, and sugar acetate functional groups [37–43]. Oxygenated hydrocarbon monomers containing functional groups with one or more oxygen atoms can induce thermodynamic interactions with CO2. These oxygen atoms are electropositive, while the carbon atoms in CO2 are electronegative, which facilities Lewis acid-base interactions. In addition, the hydrogen bond in the polymer backbone or side chain having increased the positive charge (H…O) acts as Lewis acids towards electron the oxygen atoms in CO2 [41]. As mentioned above, PVAc is among the most CO2-philic high MW oxygenated hydrocarbon polymers [43]. Therefore, Enick and co-workers replaced the fluoroacrylates in polyFAST with vinyl acetate monomers in order to reduce the cost of the polymer. They designed non-fluorous copolymers for CO2 solubility, while the styrene group was replaced with a benzoyl group for intermolecular association. This approach simplifies the copolymer synthesis as styrene cannot be polymerised with the vinyl acetate monomer due to the large reactivity ratio difference [36]. They synthesised a 5% benzoyl-95% vinyl acetate

40–80% at a concentration of a 1 and 2 wt% was observed; however, high pressure was required (64 MPa) to attain the dissolution of this copolymer in CO2 at 298 K.

An alternate strategy to increase the viscosity of CO2 is to employ self-interacting low molecular weight compounds as thickeners (**Table 1**). In order to differentiate between co-solvents and this class of thickeners, these compounds interact with each other and the CO2 resulting in a self-assembled structure that contains a solventphilic group and a solvent-phobic segment, while a co-solvent is nonassociating and consists of solely a solvent-philic segment [7]. These small molecules do not have the required molecular weight to substantially increase the gas viscosity; however, these compounds self-associate forming a supramolecular network that enhances the CO2 viscosity [2]. In general, these compounds contain functional groups with both CO2 philic segments that promote dissolution, and the CO2-phobic moieties induce intermolecular association [44]. Therefore, the various associations between neighbouring molecules within the CO2 matrix lead to viscosity enhancement [2]. Furthermore, the self-assembly of these molecules in solution can be characterised via a dramatic viscosity change or small-angle neutron scattering (SANS), FT-IR, circular dichroism, X-ray diffraction, electron microscopy, or differential scanning calorimetry [45–53]. To date, small associative molecules have yielded little success in thickening CO2, because CO2 is a poor solvent for these ionic and polar associating groups [8]. The following section describes the different types of small molecules used to thicken CO2.

) in CO2 at 298 K and

). A modest increase in CO2 viscosity of

**72**


**Table 1.**

*Outline of the solubility in CO2 and CO2 thickening capability of small-molecule compound.*

## *2.2.1 Trialkyltin fluorides and semi-fluorinated trialkyltin fluorides*

Heller and co-workers studied a series of trialkyltin fluoride compounds as light alkane and CO2 thickeners [12, 13, 44]. These compounds show a moderate increase in CO2 viscosity via the formation of intermolecular associations between the tin and fluorine atoms in the solution. **Figure 4** shows the association of tributyltin fluoride molecules. Trialkyltin fluoride forms a long linear transient polymeric chain through intermolecular association between the tin atom and the fluorine

**Figure 4.** *Association mechanism of tributyltin fluoride [54].*

atom of neighbouring molecules. In fact, the tin atom is slightly electropositive which interacts with the electronegative fluorine atom to form an intermolecular Sn-F association, as can be seen in **Figure 4**, while the hydrocarbon arms branching from the tin atom enhance the free volume which facilitates the solubility in CO2 [55]. Apparently, these molecular structures form linear and associating structures in which the alkyl arms stabilise the aggregation, while the tin atoms in each molecule associate with the fluorine atoms in adjacent neighbour molecules [55]. Although there was some success with tributyltin fluoride or other trialkyltin fluorides in thickening light alkane components, these compounds were insoluble in CO2 and ineffective as thickeners, even with the addition of pentane as a cosolvent [56, 57]. Later on, Shi et al. [55] synthesised semi-fluorinated trialkyltin fluorides and fluorinated telechelic ionomers to prepare a solution containing both CO2-philic fluorinated groups to enhance the solubility and CO2-phobic associating group to promote intramolecular association for viscosity enhancement. Both ionomers were soluble in CO2 at 2–4 wt% without requiring the addition of a cosolvent. Their results indicated that both ionomers were capable of increasing the viscosity of CO2 by 2–3-fold over a concentration range of 2–4 wt%. For example, at 4 wt% of tri(2-perfluorobutyl ethyl) tin fluoride in CO2, the viscosity increased three times at 298 K and 16.5 MPa. This viscosity increase was found to be much less than expected because the side chain fluorine atoms on the Sn-F associations were disrupted. This is attributed to the fluorine atom at the end alky arms competing with the fluorine atom attached to the tin atom caused by the electronegativity differences between these chain-end fluorines and those adjacent to the tin. Hence, the disruption of the fluorinated alkyl chains is responsible for the viscosity increase [55]. Overall, given the necessary high concentrations of the ionomers required and their high costs, these fluorinate oligomers are not considered viable thickeners for field applications [2, 6, 55].

### *2.2.2 Fluorinated and non-fluorous hydroxyaluminum disoaps*

Hydroxyaluminum disoaps were developed to thicken gasoline which was used to make napalm, the infamous weapon type used in World War II [58–60]. These molecules are an aluminium-based soap with two carboxylic acid groups linked to the aluminium atom [61]. A small amount of hydroxyaluminum disoap added to low-viscosity gasoline transforms it to a thick and extremely viscous fluid referred as napalm. In an analogous manner, these compounds were studied to determine their solubility in CO2 and quantify their ability to thicken CO2. Enick and co-workers synthesised a series of hydroxyaluminum disoaps [62]. Unfortunately, none of the hydroxyaluminum disoaps were soluble in CO2. Similar to the results with trialkyl tin compounds summarised above, unpublished results by Enick showed that the solubility of some of these compounds in CO2 could be enhanced either by fluorinating the alkyl arms or using highly branched alkyl chains [2]. However, this trial has not been successful in fully dissolving the hydroxyaluminum disoaps in CO2 [2]. Another attempt to thicken CO2 was done by heating a mixture of CO2 and

### *Direct Gas Thickener DOI: http://dx.doi.org/10.5772/intechopen.88083*

metallic stearate powders [63]. Metal stearates are salts that are produced from the reaction of stearic acid and metal oxide, which are dissolved in hydrocarbon-based oils usually with the assistance of heat to break up strong intermolecular forces. The viscosity of the hydrocarbon-based oils is enhanced when the solution cools down. This same approach was attempted with CO2; however, this was unsuccessful as they are insoluble even with the assistance of heat.
