Experimental Evaluations of Polymeric Solubility ... - ACS Publications

Subscriber access provided by Caltech Library

Article

Experimental evaluations of polymeric solubility and thickeners for supercritical CO2 at high temperature for enhanced oil recovery Nasser Mohammed Al Hinai, Ali Saeedi, Colin D. Wood, Matthew B Myers, Raul Valdez, Abdul Kareem Sooud, and Ahmed Sari Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03733 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 20, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Experimental evaluations of polymeric solubility

2

and thickeners for supercritical CO2 at high

3

temperature for enhanced oil recovery

4 5

Nasser M. Al Hinai, §† * A. Saeedi, § Colin D. Wood, ψ Matthew Myers, ψ R. Valdez, ɸ

6

A.Kareem Sooud, § Ahmed Sari, §

7

§ Department of Petroleum Engineering, Curtin University, GPO Box U1987, Australia, Perth,

8

W.A. 6151

9

Ψ Commonwealth Scientific and Industrial Rese arch Organization, Australia, Perth, W.A. 6151

10

ɸ Kinder Morgan CO2, 1001 Louisiana St, Suite 1000, Houston, TX 77002, U.S.A.

11

† Petroleum Development Oman L.L.C., P. O. Box 81, Code 100, Muscat, Sultanate of Oman

12

Abstract

13

Supercritical carbon dioxide (scCO2) is considered to be an excellent candidate for miscible gas

14

injection (MGI) as it can reduce oil viscosity, induce in situ swelling of the oil and reduce the

15

IFT of the in situ fluid system. However, the unfavourable mobility associated with scCO2

16

flooding poses a major challenge due to the large viscosity contrast between the crude oil and

ACS Paragon Plus Environment

1

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

1

scCO2 resulting in viscous fingering. An effective approach to overcome this challenge is to

2

increase the viscosity of scCO2 (scCO2 thickening) to effectively control gas mobility and

3

improve the sweep efficiency.

4

The primary focus of this study was on an oilfield (Field A) which is located in the Harweel

5

cluster in southern Oman. In this manuscript, we present results in which the suitability of a

6

library of commercially available polymers capable of thickening scCO2 at a high temperature

7

(377 K). Previous studies have focused on the use of polymers as viscosifiers at much lower

8

temperatures. Out of 26 potential polymers, the four polymers (poly (1-decene) (P-1-D), poly

9

(ethyl vinyl ether) (PVEE), poly (iso-butyl vinyl ether) (Piso-BVE) and poly (dimethyl siloxane)

10

(PDMS)) were found to be completely soluble in scCO2 at 377 K and 55 MPa.

11

Given the relatively low viscosity of oil in Field A (0.23 cP), P-1-D and PVEE could be

12

considered as effective thickeners under the in situ conditions relevant to this field. In addition,

13

Piso-BVE was found to be less effective as it did not change the CO2 viscosity above 358 K (55

14

MPa) when used at a concentration of 1.5 wt%. Furthermore, although it was determined that

15

increasing the side chain length of poly alkyl vinyl ethers would enhance the solubility of this

16

polymer in scCO2, it was determined to be ineffective in noticeably changing the CO2 viscosity.

17

In general, increasing temperature resulted in a decrease in the relative viscosity, while

18

increasing the pressure caused a slight increase in relative viscosity at all temperatures and

19

concentrations.

20

Keywords:

21

Viscosification.

Miscible Gas Injection, Thickened CO2, Parallel gravimetric extraction,


2

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Energy & Fuels

1.

INTRODUCTION

2

Miscible gas injection (MGI) is one of the most effective tertiary recovery methods used in

3

the petroleum industry for improving oil recovery1. MGI is an effective method in which the

4

injected gas (i.e. CO2, associated gas (AG), or natural gas liquids) alters the in situ oil properties

5

such as viscosity and density allowing the otherwise trapped oil to become mobile and easily

6

displaced2, 3. Although, this technique has the potential to improve the microscopic sweep

7

efficiency, it suffers from numerous challenges at the macroscopic level4. An unfavourable

8

mobility ratio where the less viscous gas displaces the more viscous oil in the subsurface is one

9

of the major causes leading to a reduced overall sweep efficiency. In addition, pronounced

10

reservoir heterogeneity makes the situation worse by further promoting gas channelling through

11

high permeability streaks, leading to a low overall oil recovery factor5, 6.

12

Field A is located in the Harweel cluster in southern Oman and is recognised as a viable

13

candidate for miscible gas injection. It is believed that with the implementation of MGI, an

14

estimated 47% of the original oil in place (OIIP) could be recovered.7 In fact, the MGI process is

15

already being implemented where the field’s AG mixture is the main gas source8. The injection

16

process has been carried out by re-injecting the AG mixture into the reservoir at high pressures

17

(up to 55MPa) and due to the high reservoir temperatures (up to 377 K) the gas becomes

18

miscible with the oil. However, even though the oil in Field A is light (42° API), the significant

19

viscosity contrast between the injected gas (0.1-0.03) and oil (0.23 cP) leads to viscous fingering,

20

early breakthrough and high gas oil ratio, which would compromise the macroscopic sweep

21

efficiency of the field. Therefore, MGI presents significant technical challenges in terms of

22

implementation. For example, early gas breakthrough (BT) has resulted in poor volumetric

23

sweep reducing the overall efficiency of the MGI process.


3

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 37

1

To overcome the above mentioned challenges associated with the MGI flooding, serval

2

solutions have been proposed in the literature include water alternating gas flooding (WAG)9-11,

3

foam flooding12-15 and the use of thickening agents to increase the injection gas viscosity16. In

4

field applications, CO2-WAG flooding can result in an incremental oil recovery of 5-10% of

5

OOIP17. However, operational difficulties and challenges (e.g. excessive water production,

6

corrosion, gravity segregation and water blocking) prevent WAG technique from reaching its full

7

potential to improve oil recovery. In addition, the WAG approach would not be applicable to

8

Field A. As a closed system with very low initial water saturation, the surface facilities available

9

in this area have not been designed to handling large quantities of water.

10

An alternate approach to decrease the injection gas mobility is the use of foaming agents.

11

However, this technique suffers from various conceptual, operational and economical challenges

12

that limit its use in the oil fields in the Middle East18,

13

associated with the foam flooding process are controlling the propagation of the foam over long

14

distances in the reservoir and the need for large volumes of the foam20. In addition, surfactant

15

micelle stability (required for foam effectiveness) is difficult to achieve in the harsh reservoir

16

conditions (i.e. high formation brine salinity and high temperatures)18. As such, the effectiveness

17

of foam flooding in Field A is significantly impaired as the formation brine salinity is 275,000

18

ppm and reservoir temperature is 377 K.

19

. In general, the major challenges

19

A conformance and mobility control technique that does not suffer from the limitations

20

associated with WAG and foam flooding processes is the use of gas thickening agents in the

21

injection gas. Numerical approximations for mixtures generally predict that the overall viscosity

22

will increase with the component viscosity and mole fraction of the more viscous component21.

23

Over the past few decades, many researchers have tested polymer and small molecule additives


4

Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

that can dissolve into CO2, NGL and AG and increase the injection gas viscosity which would

2

lead to improved performance for the MGI process20, 22-28. The major limitation is the limited

3

solubility of the polymer in the injected gas mixtures thus limiting its ability to increase viscosity

4

and ultimately improve oil recovery. In this context, compared with CO2, it is more challenging

5

to identify soluble polymers for hydrocarbon compressed fluids (e.g. AG and NGL). In our

6

previous work, three polymers (PDMS, PMHS and P-1-D) were identified to be soluble in AG of

7

Field A at 55 MPa and 377 K. P-1-D was found to be an effective AG thickener at high

8

concentrations (5-9 wt%), which may make the AG thickening process uneconomical27.

9

Therefore, in order to improve this approach this study is focussed on CO2 whose viscosity

10

enhancement would be easier to achieve for two primary reasons. Firstly, compared to AG, many

11

more low and high molecular weight polymers are soluble in CO2. Secondly, in general a lower

12

polymer concentration is required to enhance the CO2 viscosity to the required level as the initial

13

viscosity of pure CO2 is higher than the AG at the reservoir conditions of interest27. For example,

14

less than 3 wt% of polymer (e.g. P-1-D) in scCO2 is required to reach a target viscosity of 0.13

15

cP, as opposed to 5 wt% needed to achieve the same viscosity for the AG27. A simulation study

16

performed for Field A indicates that if the viscosity of CO2 was increased by 0.1 to 0.16 cP, a

17

high ultimate oil recovery (68 % to 72% OIIP) would be achieved with the thickened CO2

18

flooding compared with 63% recovery achieved from pure CO2 flooding29. A dilute

19

concentration (1.5-3 wt%) of polymer in CO2 is expected to be enough to increase the CO2

20

viscosity to this level.

21

Enick et.al have provided a comprehensive review of the studies that have attempted to

22

thicken CO2 using various chemical additives over the past 40 years20. This review states that a

23

high molecular weight fluoroacrylate−styrene copolymer (polyFAST) identified as the best CO2


5

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 37

1

thickener. A 1.5 wt% of poly FAST in CO2 increased the viscosity by a factor of 19 at 298 K and

2

34.48 MPa. The fluorinated acrylate polymer poly(1,1-dihydroperfluorooctyl acrylate) or PFOA

3

has been shown to slightly increase the viscosity of CO2 by 2.5 fold at 323 K and 31 MPa.

4

Polyperfluoroacrylates have been considered to be among the most CO2-philic material;

5

however, this class of polymers are expensive and are coming under increasing environmental

6

scrutiny. A group of researchers at Chevron and The New Mexico Petroleum Recovery Research

7

Centre attempted to thicken CO2 using a high molecular weight polydimethylsiloxane (PDMS) at

8

a diluted concentration of 0.03 wt%. However, with the limited solubility of PDMS in CO2,

9

there was no significant viscosity change reported at 298 K and 18.96 MPa16. It was also shown

10

that the addition of 20 wt% toluene co-solvent enables up to 4 wt% of PDMS polymer material

11

to dissolve in CO2 resulting in fluid viscosity increase of 30 fold30. However, the high

12

concentration of co-solvent makes the field application of this solution mixture impractical20, 28.

13

In a recent work, aromatic amide based groups were incorporated into the PDMS polymer

14

backbone to promote the formation of supramolecular structures in solution ultimately enhancing

15

the viscosity of scCO222. They observed that amide-terminated-PDMS oligomers with simple

16

aromatic groups and attachments of electron-deficient aromatic groups onto these amides (4-

17

nitrobenzamide, biphenyl-4-carboxamide anthraquinone-2-carboxamide) were not effective

18

thickeners for pure CO2. However, other compounds (4-nitrophenyl, biphenyl, anthraquinone

19

and branched anthraquinone amides) were found to be effective thickeners of hexane. In

20

addition, these compounds were found to be useful thickeners in the presence of substantial

21

amount of hexane as co-solvent into a CO2 solution. For example, at the temperature and

22

pressure of 348 K and 34.5 MPa, respectively, a transparent solution composed of 13.3 %

23

branched anthraquinone amides, 26.7% hexane and 60% CO2, was found to have a viscosity 3


6

Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

times greater than that of a CO2/hexane mixture without a thickener. Furthermore, in another

2

recent study, Doherty et al. examined a series of cyclic amide and urea compounds as small

3

molecule thickeners for light hydrocarbon solvents and scCO223. They found that

4

propyltris(trimethylsiloxy)silane-functionalized benzene trisurea and trisurea compounds

5

functionalized with varying proportions of propyltris(trimethylsiloxy)silane and/or propyl-

6

poly(dimethylsiloxane)-butyl groups were capable of thickening scCO2 (3-300 fold) at

7

remarkably low concentrations (0.5-2 wt%) in the presence of hexane as a co-solvent at high

8

concentrations (18-48 wt%)23. However, due to the high concentration of the required co-solvent,

9

the associated relatively high cost and environmental concerns severely limit the applicability of

10

this approach20, 28, 31.

11

To avoid the aforementioned concerns of either high cost or environment issues associated

12

with silicone or fluorous materials, many researchers have focused on the synthesis and design of

13

alternative CO2 soluble oligomers and polymers. To date, these include polymers such as poly

14

(vinyl acetate) (PVAc), poly (propylene glycol), poly vinyl ethyl ether (PVEE) and poly(1-

15

decene) (P-1-D) have been studied as CO2-philic materials16, 32-35. PVAc is known as the second

16

most CO2 soluble polymer among the non-fluorous polymers with PDMS being the most

17

soluble36. Tapriyal et al. observed no viscosity increase with 1-2 wt % of PVAc (MW 11000) in

18

CO2 at 298 K and 64 MPa36. The dissolution of a high molecular weight PVAc in CO2 required

19

very high pressure to achieve a transparent solution which indicated solubilisation of the

20

polymer37. In many instances, there is a strong correlation between solubility and scCO2 density.

21

A higher density (corresponding to a higher pressure or lower temperature) generally improves

22

solubility. Heller et al.16, 38 found that certain poly α-olefins (PAO) (i.e. P-1-D, poly (1-hexane)


7

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 37

1

and PVEE) to be slightly soluble in CO2 at 298-331 K and 17.1-20 MPa. However, none of these

2

materials increased the viscosity of CO2. A recent study35 examined the solubility of low molecular weight polymers PVEE (MW

3

These compounds had been studied previously16;

4

3800) and P-1-D (MW 910) in scCO2.

5

however, in this later study an in-house constructed capillary viscometer was also used to

6

measure the viscosity of polymer-thickened CO2 across 14.6-20 MPa pressure at a temperature

7

of 329 K. They reported that at concentrations of less than 1 wt%, these two polymers increased

8

the viscosity of scCO2 by 13 -14 fold. However, many researchers have disputed these results20,

9

28, 39

arguing that to thicken CO2 significantly to the reported levels concentrations in the range of

10

1.5- 7 wt% would be required for high molecular weight polymers (which are more viscous) and

11

that at 1 wt% concentration low MW polymers are not capable of thickening CO2 to the reported

12

levels. In addition, the CO2 thickening ability of low MW P-1-D and PVEE have been studied

13

using the high–pressure falling cylinder or the rolling ball viscometry tests39. They reported that

14

neither of these polymers was capable of increasing scCO2 viscosity by more than 5% at

15

concentrations of 0.5 wt%39. It is worth noting that these results have not been validated yet

16

using a commercially constructed capillary viscometer.

17

As revealed by the literature review presented so far, many studies have attempted to

18

evaluate the suitability of a number of polymer additives to viscosify CO2. However, none of

19

these were conducted at high temperatures (>373K). High temperatures are particularly

20

challenging as the scCO2 density decreases with temperature leading to a lower solvation

21

potential40. At high temperatures, the lower potential to dissolved polymers necessary to increase

22

viscosity is further complicated by the tendency for the viscosity of fluid mixtures to decrease

23

with increasing temperature41. In fact, to our knowledge, all the relevant available data in the


8

Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

literature have been obtained at moderate temperatures of less than 331 K. There are many oil

2

fields around the globe with high in-situ temperatures (e.g. Field A in the southern Oman) and

3

none of the material tested to date may be of use for possible CO2 flooding in those fields42.

4

To cover the above identified gap in the knowledge and help with further recovery from

5

Field A, this work has assessed the properties of a library of low/high molecular weight non-

6

fluorous polymers in scCO2 at high temperatures for their ability to control the gas mobility in

7

miscible CO2-EOR processes. To achieve this objective, firstly, a number of polymers were

8

selected for solubility tests. These particular polymers were selected based on the existing

9

available data in the literature related to solubility and viscosity in scCO2. In total, the solubility

10

of 26 polymers in scCO2 was determined using a high throughput parallel gravimetric extraction

11

method43. This method is very effective and efficient in that it allows for rapid preliminary

12

screening of polymers as solubility in CO2 is an absolute requirement for an effective viscosifier.

13

After selecting the polymers with the highest solubility during the high throughput experiments,

14

a series of cloud point pressure measurements were performed on them at different temperatures.

15

The purpose of these experiments was to determine the compatibility of the polymers with the

16

range of pressures applicable to Field A. In the last stage of the work, polymers displaying low

17

enough cloud point pressures at Field A’s reservoir temperature of 377 K were examined for

18

their viscosity enhancement potential in scCO2 using a commercially manufactured capillary

19

viscometer at different pressure, temperature and polymer concentration conditions.

20

2. EXPERIMENTAL METHODOLOGY

21

2.1. Materials

22

A complete list of the polymers chosen initially to be investigated in this study is provided

23

in Table 1. The polymers were sourced from a number of international commercial suppliers (i.e.


9

Energy & Fuels 1 2 3 1 4 5 2 6 7 3 8 9 10 4 11 12 5 13 14 15 16 17 18Sample 19Number 20 21 22 1 23 24 25 2 26 27 28 293 30 31 324 33 34 355 36 37 386 39 40 417 42 438 44 459 46 4710 48 4911 50 51 5212 53 5413 55 56 57 58 59 60

Page 10 of 37

Jiangsu Yinyang Gumbase Material, Fluka AG, BASF- ICIS, DOW Corning, China Skyrun Industrial Co. and Sigma Aldrich). The CO2 density was calculated using the National Institute of Standards and Technology (NIST) webbook correlation44. The density was needed at different pressure and temperature values for mixture concentration calculations. Table 1. The library of polymers used in parallel gravimetric extraction experiments.

Molecular Polymer, Oligomer and Weight Small Molecule Name (g/mol)

Glass Transition Melting Temperature Temperature Crystallinity (K) (K) Exhibits

Poly(vinyl Acetate )

301-318

250000

333-338

No45, 46 45, 46

Supplier Jiangsu Yinyang Gumbase Material


150000

301-318

333-338

No

Jiangsu Yinyang Gumbase Material


116000

301-318

333-338

No45, 46


45, 46

80000

301-318

333-338

No


Poly ( Vinyl Alcohol) 40% 72000

350-358

433

Yes45, 47

Fluka AG


350-358

445

Yes45, 47

Fluka AG


350-358

453

Yes45, 47

Fluka AG

Poly(Ethylene Glycol)

8000

213

339

Yes48

BASF- ICIS

Poly(Vinyl Pyrrolidine)

10000

403

423

Yes49

BASF- ICIS

Poly(4-Vinyl Pyridine)

60000

415

533

Yes50

BASF- ICIS

Poly(4-vinyl Pyridine)

50000

415

533

Yes50

BASF- ICIS

Poly(Methyl Methacrylate)

15000

378

433

Yes51, 52

Sigma Aldrich

Hydroxyethyl-Cellulose

140000

393

413

Yes53

Sigma Aldrich



10

Page 11 of 37 1 2 3 4 14 5 6 7 8 15 9 1016 11 1217 13 14 1518 16 1719 18 1920 20 2121 22 2322 24 25 2623 27 2824 29 30 3125 32 3326 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Poly (Ethylene Acetate)

Vinyl 55000

233

333

Yes54

Sigma Aldrich

70000

369-434

444-473

Yes55

Sigma Aldrich

Poly(Vinyl Methyl Ether)

60500

242

-

No56, 57

Sigma Aldrich

poly (Vinyl Ethyl Ether)

3800

213

-

No57

Sigma Aldrich

Poly isobutyl vinyl ether

4000

250

438

Yes58, 59

China Skyrun Industrial Co

Poly(dimethylsiloxane)

5180

183

233-243

Yes60-62

Sigma Aldrich

Poly(Butyl Methacrylate)

337000

295-308

-

Yes52

Sigma Aldrich

Poly (Ethylene Succinate)

10000

272

376-379

Yes63-65

Sigma Aldrich

Poly(Isobutylene)

500000

209

460

Yes66-68

Sigma Aldrich

Poly Carbonate)

50000

295-318

423

Yes69

Sigma Aldrich

Methyl-β-Cyclodextrin

1310

317

453-455

-

Sigma Aldrich

Poly(Vinyl Ketone)

500000

301

433

Yes52

Sigma Aldrich

900

208

257

No70

Sigma Aldrich

Cellulose Butyrate

Acetate

(Propylene

Poly (1-Decene)

Methyl

1 2 3

2.2 Experimental setup and Procedure 2.2.1 Polymer solubility measurements

4

2.2.1.1 High throughput Gravimetric Extraction (HTGE)

5

The HTGE screening method has been described in the literature, as a rapid method to use

6

when testing a large library of polymers for their solubility in supercritical solvents (SCF)43, 71.

7

Figure 1 shows the schematic diagram of the gravimetric extraction setup used in this study. The

8

detailed procedure followed in conducting the experiments is explained in one of our previous

9

publications27 but is covered here again in brief. The polymer samples were accurately weighed

10

(100 mg) into open-ended Borosilicate glass tubes with a length of 60 mm and an ID of 6.6 mm.


11

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

1

Each tube was prepared with a piece of quartz frit in its bottom opening and wrapped with a

2

piece of tissue paper over its top opening. This configuration would allow the CO2 to freely flow

3

through the glass tubes while extracting the polymer samples. Also, any insoluble polymer

4

sample or the unextracted residues would be retained inside the tube for further evaluation. The

5

tubes were loaded into a specially designed holder in parallel and then placed inside the extractor

6

vessel (Figure 1). The CO2 was then passed through the vessel under in-situ pressure (55 MPa)

7

and temperature (377 K) for two hours at the constant flow rate of 80 cc/min. The cell pressure

8

was controlled using a dome-loaded back pressure regulator whose pilot pressure was regulated

9

using a syringe pump. Subsequently, the CO2 injection was stopped and the CO2 inside the

10

extractor was slowly vented at the rate of 0.6 MPa/min. Upon reaching atmospheric pressure,

11

the sample holder removed from the vessel and the glass tubes were carefully removed and

12

individually reweighed to determine any weight loss under the extraction conditions applied. The

13

procedures was repeated twice at the same conditions for every batch of polymers tested. By

14

comparing the results of every two replicate experiments, the weight loss results were found to

15

be reproducible within ±2-4% of the extracted weight.


12

Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1 2

Figure 1. Schematic diagram of gravimetric extraction equipment used for rapid measurement of

3

polymers solubility in scCO2.

4 5

2.2.1.2 Cloud point measurements

6

The cloud point pressure versus temperature curves were determined in a high pressure-high

7

temperature windowed cell (Figure 2, IFT700- Vinci Technologies) using a standard technique

8

set out in the literature involving isothermal compression and then slow decompression of binary

9

mixtures of known compositions72. These measurements were carried out for three polymers

10

with high level of extraction in the HTGE experiments (i.e. PVEE, P iso-BVE and P-1-D) to

11

confirm their solubility in scCO2. Cloud point pressures were determined for range of

12

concentrations for PVEE (0.81 to 2 wt%) and Piso-BVE (0.081 to 3 wt%) in the temperature


13

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

1

range of 329-377 K. For P-1-D, the cloud point pressures were measured for the concentration

2

and temperature ranges of 1 to 5 wt% and 358-377 K, respectively.

3 4

Figure 2. Schematic diagram of experimental setup used for cloud point pressure measurements.

5 6

To ensure the integrity of our experimental results, the high pressure windowed cell and all

7

of its components were thoroughly cleaned with acetone followed by toluene at 323 K before

8

each measurement, to remove any trace of contaminants including residual polymers that may

9

compromise our results. The high pressure cell was then vacuumed for few hours to dry.

10

Knowing the solubility of our polymers in CO2 from the HTGE experiments, for each

11

measurement, a precise amount of a polymer was weighed and placed on a clean glass plate.

12

Doing so allowed us to determine the polymer concentration (wt%) in CO2 in the pressure cell

13

whose internal volume was known (20 cm3). The glass plate was then placed on the turn table

14

inside the pressure cell before it was tightly sealed. The system was then purged with low


14

Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

pressure CO2 to remove any trapped air. Subsequently, we increased the temperature of the

2

system to the desired value and allowed sufficient time for the temperature to stabilise. Then CO2

3

was slowly injected into the cell using a syringe pump to increase the cell pressure at 0.4 MPa

4

increments until a single transparent phase of polymer/CO2 solution was formed. It is worth

5

noting that no stirring was required during the experiment and a high definition camera provided

6

a visual confirmation as whether the polymer was completely dissolved in CO2. Then, to

7

determine the cloud point pressure of the polymer thickened CO2 solution at a given temperature,

8

the pressure of the system was decreased at 0.4 MPa increments until the solution began to

9

appear cloudy in the cell. This process continued until it was no longer possible to see the other

10

side of the cell through the polymer/gas solution (90% reduction in transmitted light intensity) at

11

which point the cell pressure was recorded as the cloud point pressure for the test temperature.

12

For any combination of polymer concentration and temperature, the process of cloud point

13

pressure measurement was repeated three times to ensure the reproducibility of our result. We

14

found the cloud point pressures to be reproducible within 0.2-0.5 MPa.

15

2.2.2 Viscosity Measurements

16

A high pressure-high temperature capillary viscometer (Figure 3, CVL-1000- Core

17

Laboratories Inc.) was used to measure the viscosity of pure CO2

as well as the polymer

18

thickened solution

19

measures the pressure drop created across a long capillary tube when a fluid is flowed through

20

the tube at a known flowrate.

. Operating on the basis of Hagen-Poiseuille Law, the viscometer

21

Prior to each experiment, a polymer thickened CO2 solution was prepared inside a fluid

22

accumulator at a predetermined polymer concentration using the following procedure. First, the

23

accumulator was cleaned with acetone and toluene to remove any residual polymer from the


15

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

1

previous test and then dried with compressed air under a fume cupboard. Then a precise amount

2

of a polymer was weighed (to give a predetermined concentration value) and placed inside the

3

accumulator whose storage volume had already been adjusted to 30 cm3 using its floating piston.

4

The accumulator was then vacuumed to remove any trapped air. The accumulator as well as the

5

flow-lines and fittings/connections carrying the solution into the viscometer were heated to a

6

predetermined temperature using suitable heating tapes and heating jackets (SRH Etched foil

7

jacket and Stretch-To-Length heating tape, Watlow, USA). Subsequently, pure CO2 was injected

8

into the accumulator and pressurized to the desired pressure (above the cloud point pressure).

9

After that, the setup was left for 3 to 4 hours to ensure the polymer would completely dissolved

10

into CO2. Then, the viscometer was also heated to the desired temperature and vacuumed before

11

being filled and pressurized with the polymer thickened CO2 so its viscosity could be measured.

12

In order to ensure the reproducibility of our results, for every pressure-temperature-concentration

13

combination, the viscosity measurement was repeated three times. In doing so, we found our

14

measurements to be reproducible within

.

15

It is worth noting that cleaning the viscometer after each experimental run was an area of

16

concern, as there was a possibility that flushing the extremely narrow capillary tubes (ID = 0.18

17

mm) with solvent (toluene) might not clean residual polymer from the system , which would

18

impact on the viscosity result of the next measurement. To address this concern, we used a new

19

capillary tube each time and the instrument was recalibrated.


16

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1 2

Figure 3. Schematic of the capillary viscometer used for viscosity measurements (50-55 MPa

3

and 329-377 K).

4

3. RESULTS AND DISCUSSIONS

5

3.1 Polymers Solubility in the CO2

6

3.1.1 High Throughput Gravimetric Extraction (HTGE) Screening Method

7

Figure 4 shows the percentage of the original weight extracted by CO2 for each of the 26

8

different polymers during the HTGE experiments measured at 377 K for three different pressures

9

(i.e. 41 MP, 48 MPa and 55 MPa). Larger extraction indicates a higher solubility in scCO2. As

10

pointed out before, all 26 polymers examined were commercially available and their solubility in

11

pure CO2 at lower temperature conditions had already been confirmed by other researchers.

12

Based on the solubility data presented in Figure 4, the polymers are divided into four categories

13

in Table 2.


17

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Page 18 of 37

Table 2. Classification of polymers based on extraction ability in scCO2. Categ ory 1 Categ ory 2 Categ ory 3

high

level

of

96 )

extraction ( moderate

Piso-BVE (4000 g/mol) and PDMS (5180 g/mol)

level

of

level

extraction (5

2

PVAc (Mw 80000), PVAc (Mw 116000), PVME and Poly (ethylene succinate)*

extraction (45-85%) low

P-1-D (900 g/mol), PVEE (3800 g/mol),

of

Poly(Vinyl

Pyrrolidone),

Methyl-β-Cyclodextrin

)

and

P4V

Pyrdine,

Hydroxyethyl-

Cellulose Categ ory 4

negligible extraction

(

5

high molecular weight polymers containing carboxyl or hydroxyl functional groups such as poly vinyl acetate (PVAc) and poly vinyl alcohol (PVA)

2

*These polymers require more pressure and/or more volume of the gas passed through the

3

extraction vessel so they may completely dissolve in scCO2.

4

5 6

Figure 4. Extracted weight % in scCO2 for the library of the 26 polymers at 41, 48 and 55 MPa

7

and 337 K. The name of the polymers whose numbers are presented on the horizontal axis are

8

defined provided in Table 1.


18

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Figure 4 also reveals the effect of pressure on CO2 solubility of the polymers with appreciable

2

amount of weight extraction. For category 1 polymers including PVEE and PDMS, the pressure

3

change has a minor effect on the solubility which is consistent with previous studies at lower

4

temperatures 16, 35, 38. For polymers, the solubility decreases slightly at 48 MPa and sharply at 41

5

MPa. This is consistent with a lower solvation ability at lower scCO2 densities40. Notably, at 41

6

MPa, the carboxyl functionalized polymer (i.e. PVAc) did not indicate any weight extraction

7

because this polymer requires very high pressures (43– 69 MPa) to become soluble in scCO273.

8

Furthermore, it can be seen from Table 1 that the non-crystalline polymers or polymers with a

9

low melting point are soluble in scCO2, whereas the crystalline polymers with high melting point

10

are more difficult to dissolve in scCO2. This is in line with previous reports where the dissolution

11

of crystalline polymer with weak polymer-solvent interactions was found to be impossible below

12

the melting temperature.74

13

temperature must be surpassed. In this work, only the polymers with melting points below 377

14

K were soluble in scCO2 and above that were non-soluble. As a major conclusion drawn from

15

Figure 4, based on their solubility only four polymers of PVEE, P-isoBVE, PDMS and P-1-D

16

can be considered as viable candidates for application in Field A. The rest of the polymers are

17

rejected on the basis of insufficient solubility under the pressure/temperature conditions used in

18

this study.

19

3.1.2 Cloud Point Pressure Determinations

For the dissolution of the crystalline compounds, the melting

20

The solubility of those polymers with high extraction percentages (i.e. category 1 and 2 from

21

Table 2) were further verified using the conventional cloud point pressure measurements

22

conducted over a range of temperatures and polymer concentrations. Cloud point pressure

23

experiments confirmed that the PVAc and PVME included in Category 2 only partially dissolved


19

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

1

in scCO2 at pressures of 41 to 55 MPa and temperature of 377 K. This observation is consistent

2

with the results obtained from the HTGE measurements. Due to their only partial dissolution in

3

scCO2 under the study conditions, these were not considered for viscosity measurement studies.

4

PDMS was excluded from the cloud point pressure measurements because the phase behaviour

5

in scCO2 at high temperatures has already been well studied16, 30, 75-80. Furthremore, PDMS was

6

shown to be an ineffective CO2 thickener at high temperatures. PDMS has also been tested for its

7

solubility in NGL and shown to be an ineffective NGL thickener25. However, in this work,

8

PDMS was used in the extraction tests to validate our methods as PDMS has proved to be more

9

CO2-philic than other hydrocarbon based polymers20.

10

The phase behaviour of polymers in scCO2 containing functional groups with oxygen within

11

the polymer backbone has been assessed by Kilic et al32. These researchers found that the

12

presence of ether oxygen in a polymer exhibits induces a lower miscibility pressure in scCO2 due

13

to the strong interaction forces between CO2 and the ether group (Lewis acid and Lewis base

14

interactions) as well as their high chain flexibility (low glass transition temperature and low

15

surface tension)32. Figure 5 shows the measured cloud point pressures for ether containing

16

polymers, which include PVEE and Piso-BVE, over the temperature range of 329-377 K and

17

concentrations of 0.81-3 wt%. These results show that both polymers exhibit LCST (lower

18

critical solution temperature) behaviour in CO2 where cloud point pressure increases with

19

temperature. This is caused by the reduction in the solubility power of CO2 towards PVEE and

20

Piso-BVE due to a decrease in fluid density with increasing temperature leading to an entropic

21

effect (i.e. a reduction in the free volume difference between CO2 and polymers). Compared to

22

Piso-BVE, the cloud point curves for PVEE are more sensitive to changes in temperature at all

23

polymer weight loadings. In general, the LCST curve behaviour is controlled by the free volume


20

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

difference between the solvent and polymer in addition to the polymer expansion coefficient81, 82.

2

Therefore, Piso-BVE gains more free volume than PVEE due to the steric bulkiness of the iso-

3

butyl functional group and the increase in the side chain length of Piso-BVE. It becomes easier

4

to dissolve the polymers in CO2 if the free volume of a polymer is increased83. Previous studies

5

have found that at low temperature (295 K), PVEE has better miscibility than PVME because the

6

increase in the side chain length enhances the free volume, which results in CO2 becoming more

7

accessible to the polymer with the longer side-chain branches for better solute–solvent

8

interactions

9

CO2 at high temperatures. Piso-BVE has a lower cloud point pressure curve despite having a

10

slightly higher molecular weight compared to PVEE and appears to be more CO2-philic than

11

PVEE. However, at moderate temperatures (< 346 K), PVEE has a lower miscibility pressure

12

than Piso-BVE. It is likely that the thermal expansion of free volume in the PVEE structure is

13

higher than Piso-BVE below this temperature. With increasing temperature, the polymer chain

14

mobility is greatly increased, providing the motional space corresponding to an increase in

15

polymer free volume 33. Hence, as the temperature increases, the cloud point pressure difference

16

between both polymers in CO2 becomes larger.

32

. In this work, we observed similar effects for Piso-BVE and PVEE solubility in

17

The temperature dependency of the solubility of P-1-D in CO2 was also studied and the

18

results are presented in Figure 6. In contrast to PVEE and Piso-BVE, the cloud point pressure

19

decreases with temperature increase implying that the P-1-D solubility in CO2 increases with

20

temperature. It is evident then that P-1-D exhibits UCST (upper critical solution temperature)

21

phase behaviour in scCO2. However, its solubility is relatively insensitive to temperature change

22

compared to PVEE and Piso-BVE. This is due to the fact that the enthalpic interaction between

23

CO2 and P-1-D in the solution increases slightly with increasing temperature. Previous work


21

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

1

showed that P-1-D exhibited solubility value of 10.3 g/L at 19.9 MPa and 298 K 16, 38. Terry et.al

2

also found P-1-D to be partially soluble in CO2 at 11.7 MPa and 344 K 84. In a recent study, P-1-

3

D had a solubility 0.081 wt% in CO2 at 20.1 MPa and 329 K 35. However, in our work we found

4

this polymer completely dissolves up to 5 wt% in CO2 only at temperatures above 358 K. Below

5

this temperature, P-1-D was only partially soluble even at the pressure of 55 MPa. Although, the

6

solubility increased significantly with temperature. This characteristic enables P-1-D to be used

7

at concentrations higher than 1 wt% to increase CO2 viscosity at high temperatures, indicating

8

that P-1-D could be an effective thickener for deep reservoirs such as Field A.

9 10

Figure 5. Compare cloud point pressures between PVEE and Piso-BVE at different

11

concentrations and temperatures.


22

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1 2

Figure 6. Measured cloud point pressures for P-1-D at different concentrations and temperatures.

3 4

Based on the cloud point pressure measurement results presented above, the three polymers

5

of PVEE, Piso-BVE and P-1-D have high or adequate level of solubility in scCO2 under the in-

6

situ conditions encountered in Field A . P-1-D has a lower cloud point than PVEE and Piso-BVE

7

at comparable concentrations. This could be due to the difference in molecular weight between

8

the polymers and different solubility phase behaviours (LCST vs. UCST). The lower cloud point

9

pressure of P-1-D in scCO2 makes it possible to increase its concentration in CO2 to values

10

higher than 5 wt% to further increase the CO2 viscosity. At 5 wt% of P-1-D in CO2, the

11

minimum required pressure to have it fully soluble is 45.3 MPa at 377 K which is still below the

12

reservoir pressure of 55 MPa. In comparison, the cloud point pressures for both PVEE and Piso-

13

BVE are considerably higher at high temperatures. As a result, at 377 K, the concentration of

14

PVEE and Piso-BVE cannot increase to more than 2 wt% and 3 wt%, respectively, because

15

above these concentrations the cloud point pressure would exceed the reservoir pressure of 55


23

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

1

MPa. However, at the above concentrations, both polymers can be considered adequately soluble

2

in CO2 at the Field A’s reservoir conditions.

3

3.2 CO2 Viscosity Enhancement Measurements

4

The viscosity of solutions prepared by dissolving the three different polymers (either P-1-D,

5

PVEE or Piso-BVE) separately into scCO2 at different pressures (50, 53 and 55 MPa) and

6

temperatures (329-377 K). For the measurements, the polymer concentrations varied between

7

0.81 to 5 wt% for P-1-D, 1.2 to 2 wt% for PVEE and 1.5 to 3 wt% for Piso-VBE. The measured

8

results are presented in figures 7, 8 and 9 in the form of relative viscosity (i.e.

9

in Table 3 in the form of actual measured viscosity values for all three polymers at 377 K. As

10

evident from the figures and the table, for all three polymers, there were a considerable increase

11

in the viscosity of CO2 can be achieved at a range of temperatures and pressures. Although, as

12

expected, the viscosity of the polymer-thickened CO2 exhibits a decreasing trend as the

13

temperature increases41. There is also a slight increase in relative viscosity with increasing

14

pressure at all temperatures and concentrations due to the polymer coil expansion caused by an

15

increase in solvent strength with increasing density. The same effect has also been observed with

16

PDMS and DRA polymer in NGL25.

17

3.2.1 PVEE and Piso-BVE results

, and

18

Figure 7 shows the thickening ability of PVEE in CO2 for the concentrations of 1.2 to 2 wt%

19

over a range of temperatures (329-377 K) and two pressures of 53 and 55 MPa. As can be seen,

20

for all temperatures, the viscosity of the thickened CO2 increases significantly with increase in

21

the PVEE concentration. Also, the data follow a similar trend across all temperatures examined

22

as the pressure and polymer concentration change. The relative viscosity decreases almost

23

linearly with increase in temperature. However, the reduction rate is slightly higher above 348 K


24

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

because the intramolecular associations between the polymer and CO2 molecules are reduced as

2

the temperature increases. For example, at 53 MPa, the relative viscosity of 1.5 wt% PVEE in

3

CO2 decreases by 1-5% over 329-348 K, whereas at the same concentration, the relative

4

viscosity decrease by 6.8-10% over 348-377 K. Not surprising, the enhancement in CO2

5

viscosity by PVEE is the lowest at Field A’s high in situ temperature of 377 K. For example, as

6

revealed by Figure 7, at 2 wt% concentration, 55MPa and 377 K, the relative viscosity is close to

7

1.7 while it is about 2.1 at 329 K for the same concentration and pressure. Furthermore, as

8

revealed by the cloud point pressure measurements, 2 wt% is the highest achievable

9

concentration of this polymer under the in-situ conditions of Field A.

10 11

Figure 7. Relative viscosity

12

pressures.

at different PVEE concentrations, temperatures and

13 14

The level of viscosity enhancement attained by dissolution of Piso-BVE in scCO2 at the two

15

concentrations of 1.5 wt% and 3 wt% and pressures of 50, 53 and 55 MPa over temperatures


25

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

1

ranging 329 to 377 K is given in Figure 8. As can be seen, compared to PVEE, the relative

2

viscosities for Piso-BVE are substantially less under the same or similar conditions. For

3

example, over the temperature range of 329-377 K, 1.5 wt% of Piso-BVE, resulted in 1 to 1.23

4

fold increase in the CO2 viscosity, while the same concentration of PVEE improved the viscosity

5

by 1.36 to 1.9 fold over the same temperature range. This indicates that the CO2 viscosity

6

enhancement capacity of alkyl vinyl ether polymers decreases with increase in their backbone

7

length. At the lower Piso-BVE concentration of 1.5 wt%, the CO2 viscosity enhancement is

8

negligible relative to just supercritical CO2 at temperatures above 368 K and over the pressure

9

range 50 to 55 MPa. The viscosity increases modestly with an increase in the polymer

10

concentration to 3 wt%. For example, at 377 K, 3 wt% of Piso-BE produces a relative viscosity

11

of 1.2. Overall, the poly alkyl vinyl ether may result in subtle viscosity enhancement when

12

dissolved in CO2 at elevated temperatures. Also, the steric effect and increase in the alkyl arm

13

length have a negative effect on their ability to increase viscosity.

14


26

Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1


2

pressures.

3

3.2.2 P-1-D result

at different Piso-BVE concentrations, temperatures and

4

The CO2 viscosity enhancement capacity of P-1-D over the concentration, pressure and

5

temperature ranges of 0.81 to 5 wt%, 50 to 55 MPa and 358 to 377 K, respectively, is illustrated

6

in Figure 9. As can be seen, despite P-1-D having a low molecular weight, a considerable

7

increase in the CO2 viscosity (1.2-2.77 fold) could be achieved at concentrations of 1.5 wt% and

8

above. In comparison, Zheng et al35. found that the same polymer, initially tested by Heller

9

et.al16, could thicken CO2 by 15 fold at the concentration of 0.81 wt%. Our results significantly

10

differ from those reported by Zheng et al. In our first measurements, we found the polymers to

11

precipitate inside the capillary tube of our viscometer upon depressurisation. Therefore, to avoid

12

any undesirable effects, as mentioned in our experimental procedure section, we decided to

13

replace the capillary tube from one measurement to the next. We found hot toluene to be

14

insufficient at completely dissolving and removing the residual polymer deposited in such a

15

small diameter tube.

16

Table 3. The measured viscosities for pure CO2 and CO2 thickened using different

17

concentrations of PVEE, Piso-BVE and P-1-D at different pressures (T= at 377 K).

Polymer

P-1-D

Polymer/Thickened viscosity, cP

CO2

Concn

Pure CO2 viscosity, cP

wt%

55 MPa

53 MPa

50 MPa

55 MPa

53 MPa

50 MPa

0.81

0.077

0.076

0.074

0.094

0.09

0.088

1.5

0.077

0.076

0.074

0.125

0.119

0.102

3

0.077

0.076

0.074

0.147

0.142

0.133


27

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5

0.077

0.076

0.074

0.183

0.175

0.16

0.077

0.076

0.074

0.078

0.076

0.074

3

0.077

0.076

0.074

0.094

0.09

0.088

1.2

0.077

0.076

0.074

0.094

0.087

-

1.5

0.077

0.076

0.074

0.112

0.104

-

4

2

0.077

0.076

0.074

0.133

-

-

5

Piso-VBE 1.5

PVEE

Page 28 of 37

1 2 3

6 7

8 9 10


at different P-1-D concentrations, temperatures and

pressures.

11 12

After testing all three polymers, despite the significant difference in their molecular weights,

13

P-1-D was found to be a promising CO2 thickening agent under the high temperature conditions


28

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

of Field A (i.e. 377 K). Other research work have also demonstrated the thermal stability of P-1-

2

D at high temperature27. For example, at 377 K, 55 MPa and 1.5 wt% concentration, P-1-D

3

improved the CO2 viscosity by 1.7 fold while under the same conditions, PVEE and Piso-BVE

4

yielded 1.39 and 1 fold increase in the viscosity, respectively.

5

4. CONCLUSIONS

6

The solubility of 26 commercial polymers in scCO2 at reservoir conditions was examined

7

using a fast and efficient gravimetric extraction method. Then, a series of cloud point pressure

8

measurements at 377 K was used to validate the solubility of polymer candidates with high

9

extraction weights. Three polymers of PVEE, Piso-BVE and P-1-D were found to be adequately

10

soluble in scCO2 under the Field A’s in-situ reservoir conditions. Subsequently, these polymers

11

were assessed at diluted concentrations (0.81-5 wt%) for their CO2 viscosification capacity at

12

different pressures and high temperatures. In general, it is known that increasing the temperature

13

results in a decrease in the viscosification capacity of polymers while increasing pressure causes

14

a slight improvement at all temperatures and concentrations.

15

Our results show that P-1-D and PVEE could be considered as effective CO2 thickeners at

16

Field A’s conditions. P-1-D increases the CO2 viscosity by 1.2-2.77 fold over the concentration

17

and temperature ranges of 0.81-5 wt% and 358 -377 K, respectively, while 1.2-2 wt% of PVEE

18

improves CO2 viscosity by 1.2-2.1 fold over the temperature range of 329-377 K. The

19

enhancements in CO2 viscosities reported above are significantly less than those (13-14 fold)

20

reported by Zhang et.al at comparable compositions and temperatures.

21

Our results indicate that change in the alkyl arm and steric effect on the alkyl vinyl ether have

22

great influence on the solubility of polymers in CO2, but are ineffective in changing the CO2

23

viscosity. Piso-BVE exhibits a higher solubility in CO2 than PVEE at high temperatures, but its


29

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

1

viscosity enhancement capacity is lower than PVEE at comparable concentrations and molecular

2

weights. For example, PVEE increases the viscosity of scCO2 by 43% at the concentration of 1.5

3

wt% and temperature of 377 K, while the same concentration of Piso-BVE cannot increase CO2

4

viscosity noticeably at 377 K. Piso-BVE is required at high concentrations to change the CO2

5

viscosity at high temperatures. This research concludes that P-1-D has a better CO2 viscosity

6

enhancement ability than PVEE and Piso-BE, making it a suitable candidate for improving gas

7

mobility in Field A, and under high temperatures in general, during CO2 flooding.

8

AUTHOR INFORMATION

9

Corresponding Author

10

* E-mail: [email protected] or [email protected]

11

Tel: +61452589291

12 13

Author Contributions

14

All authors contributed to the manuscript; and all authors approved the final version of the

15

manuscript.

16 17

ACKNOWLEDGMENT

18

The Authors would like to acknowledge the support provided by the Petroleum Development

19

Oman (PDO) and Ministry of Oil and Gas (MOG) in Oman. They provided reservoir and fluid

20

data for Field A and the scholarship and financial support for PhD student Nasser Al Hinai. We

21

are also very grateful to Mr. Bob Webb (Laboratory Technical Officer), and Mohsen (PhD


30

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

student) for their generous support including laboratory management and procuring the materials

2

used in this research.

3

ABBREVIATIONS

4

EOR, Enhance Oil Recovery; MGI, Miscible Gas Injection; AG, Associated Gas; P-1-D, Poly

5

(1-Decane); PVEE, Poly (ethyl vinyl ether); (Piso-BVE), Poly (iso-butyl vinyl ether) PDMS,

6

Poly (Dimethyl Siloxane); PVAc, Poly Vinyl Acetate; PVA, Poly Vinyl Alcohol; OOIP, Original

7

Oil In Place; PAO, Poly Alfa-Olefin; HTGE, High Throughput Gravimetric Extraction; WAG,

8

Water Alternative Gas; SCF, Supercritical Fluid; scCO2, Supercritical CO2.

9

REFERENCES

10 11 12 13

!

14 15

$

,+ . "

16 17

3 4

18 19 20

!

/

)

% &#

0 $1, & 2

' ()

#

$! #

"*

+

,- ' (

+

4 &

-

+ , 5 .. #" # ,

(3 ( 4 ! +

21 22

(

23 24 25

:

26 27

" #

"

6

$

% &#

'' )

#

$! #

"*

,- '' 7 +

#

& 8 1 2 ,# # , ,# * *

)

# >

; 1) 4 $! # *

) %

"* A &

# # *, & !" ! #

&

2

+ #

, ,

.

6/ $ * $

+

+

&

+

*. .

'

+ 8) ?" 4 /$$,

% +

* 6

3" $

,

" () /$$,

) +

=)6

) #

)=

&

>)

"*

? =

## (:' ( (

"+

0

,$

* & #

$ 7 @ 2

%.

#

$ 6/ $

(

,

$

3

>)1 , " ) $"#" . ,. #, 1) /, 4 ."

+

"*

. $ * $0 # # # + , "*& $7 @ , # # . :

-

"* # ) ) 1 # 0 ) * 7 # ", + 6/ 2 *- 2 .. # # 1 %*B / # ? 6 & # , !" '

#

"*

(

!

6

+ ! #

$ ! #

-

. , , >

( 7

#

#

(

( >

: 6

*

#0

** ? ? ) 1 # 4) E , A A ! +, 3# # # & ! 9 / B&" + '' ) '' ) . ' )?" 4 ? &

3)

+0 A ! " # - $$ # $ 7 @ # .# +

? %. , #

$

A AE , .# $2 *2 * + "$ # 2 *,- 2" * # , .. # , ( 3 : ' (

.

3 F" F ) # . *

"*

+0 A ) # * ! # "*

#

A)1

1 # ,

>+

25 26

! #

$

18 19

32 33 34

,#

#

6

30 31

#

$, ) #

15 16 17

27 28 29

#

< # A) ,# 9 # # D" , $ $ 9 # + , *

9 10 11 12 13 14

&" = & ,3

Page 32 of 37

1) #

= 4 ) ** A ) . # ,, % *. , "* )

! ? A 2 ( " #

7 $ $! #

"

* ,, ,#

) 56 7 "* +

+2 *! * # , $ 1 & # $ * >+ *. #" > +

6/

$

/ -

0 $

.#

+

3 !

,-


32

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Energy & Fuels

E 1) E #" @ , 9 #

)7 $

23 24 25

#

?

)> * # # L +" . , & & , . * , +

3" ,- ' :

!

, $

1 & #

*.

#

#

*. , "*

6

#

= #

:

4 / ) "* = ) ,". # "

" $

#

1)A , A 4 ) 6 ,# 0 # A ) / % # 3(# # / # * ,# 6/ , "& % + # 3( :

(

, /*

$"

* ,

'

)? *. " ,

A)= (

$ .

$.

1 " ,, , # $ ! ##,&" +

#

@ 1 6 # $ " , ! # - .. # ' 3

#" ,

# = ,+

$

D" , +

$ 6/

9 #

#

$ # , "& # $ , # * #

0

%

! # #

# & #

(: :

/$$, $! #

1 "*

+

,# 6 & # ,- 1", #

(

3 < 6 ?) & , ,".

24 25

33 ! A ? ,# * E"*& (' -

26 27

3 A , $ *, ( 9 9

28 29

3( 2 +

30 31

3:

# # # # #

$

#

' ? A * ,, # # H

, &

" # &

+ 8) # # .

+ =)> 6 * ,#

15 16 17

22 23

=)7 ", . (

= / $

A ! "

8 ) @" 8 !

= = ,+ $ ! ##,&" +

. !

* .

* (

)

. ,#

H

6)

6) , 6 3 3 3 ' 33

3 4 5

( $. 3:

) # =)4 # ," # 3

6 7

(3 ; " ) $ *# * .

8 9

# :'

( ;" ) &"# ,"

10 11

(( 1

12 13

(: 1 . *

14 15

( # #

0 !

.

$ .

#

6 ,#

) E, #

#

, 6 )

0, )1 , &"#

%

B # 33

( "

&

?) =

, %

$ ,# # , ", + ." ,

* , $ . , &"# . >

&

#

.

K

0,

) A

1

#"

" # ,

,"

,#

&

#

, $.

( #

.

, 3

, &"#

* & ' (

, $ #

6 ,#

# " #"

$

3

: =* A)< , A ) $

:' 4

1 ?) 6 "& #

< # * # , % ': 3

A) 4 )1 , . * ,# # #

;) 2 + 1) A ,# 4 !) 7 $> * . * , 6 . * , : (: :'

)?" 4 . # =

#

11 12 13

A) =

)4 ,".

*, & #

. , &

1, & # . , , . # & %

A)

1

)4 , 6) < + ".

&

, 6 &

#

*

!) *# 6 =) 4 , = % * !

#

0# .

"* #

$

16 17

! ## , (: (::

18 19

=

20 21 22

3 ) ) + , 6 ) 7 #* = ? ! 6/ K/ 1 & # # , ( * ( + ,# % , ' :

1 >"+ 1 ) 7 -E 08 -

#

* !"

* #

) 33

A ?)

, %

'

6 2 ) 1 >"+ 1 '' ( ( = 2

)

Experimental Evaluations of Polymeric Solubility ... - ACS Publications

Recommend Documents