Polymer-Stabilized Multi-Walled Carbon Nanotube Dispersions in

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Polymer Stabilized Multi-Walled Carbon Nanotube Dispersions in High Salinity Brines Mohannad J. Kadhum, Daniel Swatske, Javen Weston, Daniel E. Resasco, Benjamin Shiau, and Jeffrey H. Harwell Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00522 • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017

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Polymer Stabilized Multi-Walled Carbon Nanotube Dispersions in High Salinity Brines Mohannad J. Kadhum*1, Daniel Swatske1, Javen Weston1, Daniel E. Resasco1 , Benjamin Shiau2, Jeffrey H. Harwell1 1. School of Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019 2. Mewbourne School of Petroleum and Geological Engineering, University of Oklahoma, Norman, Oklahoma, 73019 *Corresponding author. Email: [email protected]

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Abstract Carbon nanotubes (CNTs) exhibit promising properties for potential applications in oil and gas reservoirs. CNTs can be used as delivery vehicles for contrast agents or catalyst nanoparticles deep inside the reservoir. Dispersing a hundred parts-per-million of CNTs in deionized water is easily achieved by sonication of CNTs using properly selected surfactants or polymers solutions. These surfactants and polymers are non-covalently adsorbed to the nanotube surface inducing dispersion stability. In oil reservoirs, high salinity is the norm; therefore, as the electrostatic double layer is compressed due to the high ionic strength found in a typical reservoir brine, colloid CNT dispersions lose stability and CNTs flocculate and precipitate. To maintain a stable colloidal dispersion of CNTs, a dispersant with functionality providing steric repulsion between the dispersed tubes is needed to prevent aggregation. In this work, suspensions of multi-walled carbon nanotubes (MWNTs) were generated using two polymers, gum arabic (GA) and hydroxyethyl cellulose (HEC-10), in 10% API brine (8 wt. % NaCl and 2 wt. % CaCl2). GA was used as a primary dispersant which is able to de-bundle the tube aggregates. After the first sonication with GA, the secondary dispersant, HEC-10 is added to provide the steric repulsion needed to keep the tubes dispersed in high salinity brines. Polymer adsorption to the nanotube surface was observed using scanning electron microscope (SEM). Focusing the electron beam for extended period of time induced damage to the polymer layer around the individual nanotubes, leaving the tubes intact, as a clear evidence of polymer adsorption. Adsorption experiments showed low to negligible adsorption of MWNTs to crushed Berea sand at 80 oC in both 10% and 20% brines. 2


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Dispersion injection in column and coreflooding tests showed successful propagation of CNT dispersions through porous media with total nanoparticles recovery exceeding 80% in reservoir rock. This work demonstrates the potential of using polymer stabilized carbon nanoparticle dispersions in a range of applications to advance current oilfield technology.

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1. Introduction Nanoparticles have fascinating properties due to their interfacial activity at the water/oil interface and therefore have received significant attention due to potential applications in reservoir development1–9. Nanoparticles can be modified to adsorb selectively resulting in changes of rock wettability which can impact relative permeability of the reservoir10–12. CNTs have the ability to stabilize water/oil Pickering emulsions and change oil phase properties by performing selective catalysis1,13,14. Earlier work investigated the use of nanoparticles as tracers or reporters for reservoir heterogeneity6,15. Rheological studies of different nanoparticles have demonstrated potential uses in hydraulic fracturing technology16–18. Earlier work also demonstrated the ability to stabilize single walled carbon nanotubes dispersion using GA alone in deionized water where electrostatic repulsion dominates between the dispersed CNT strands19. A wide range of surfactants are also capable of dispersing carbon nanotube in deionized aqueous solutions

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Numerous papers have investigated the dispersion chrcteristics of carbon nanotubes in terms of CNTs concentration23, solution pH25, and surface modification of CNTs26. Although it is well understood that inorganic salts cause flocculation of dispersed CNTs27, the effect of high ionic strength (high salinity or high total dissolved solids, TDS) has not been covered extensively. Literature reports flocculation of CNTs in the presence of salt such as NaCl or sodium iodide (NaI) in the solution. CNTs dispersed using n-methyl-2-pyrrolidone flocculated at NaI concentrations as low as 1mM27. An earlier work demonstrated propagation of multi-walled nanotubes(MWNTs) dispersion through a sand pack with minimal particle retention using two dispersing polymers. The 4


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necessity of using two polymers was justified by the ability of the first (primary) polymer to de-bundle the nanotubes clumps into individual tubes while the second polymer was responsible for providing the steric repulsion needed to reduce nanotube agglomeration at high salinity6. Polyvinyl pyrrolidone (PVP) was used as dispersing agent primarily followed by hydroxyethyl cellulose (HEC-10) as a salt tolerant polysaccharide to provide steric repulsion3. In this work, gum arabic (GA) was used as a primary dispersant. The ability to propagate nanoparticles successfully using GA as a primary dispersant rather than using polyvinyl pyrrolidone (PVP) is justified by the adsorption of PVP on sand particles due to the strong interaction between the pyrene rings of PVP and silanol groups on sand surfaces28. Earlier work demonstrated the high adsorption on an alumina support in sand packs of multiwalled carbon nanotubes dispersed using PVP polymer29. The retention of PVP-dispersed MWNT-Al2O3 in sand packs was severe and resulted in filter cake formation at the sand face. GA which is a mixture of polysaccharides consisting mainly of arabinoglactin with a molecular weight of 300 KD and 10% of arabinoglactinprotein complex with a molecular weight of 1,000 KD has the advantage of generating better carbon nanotubes dispersions in DI water conditions19,30. In this work, dispersions were prepared sequentially by first dispersing MWNTs in a solution containing GA by sonication and then adding the secondary dispersant (HEC-10) and sonicating again. The hypothesis is that the lower molecular weight components of GA are effective in disaggregating the tubes into individual ones while the salt resistant, reasonably higher molecular weight polymer (HEC-10) provides adequate steric repulsion which keeps the tubes well separated in the presence of high electrolyte 5


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concentrations. Earlier work suggested that a longer polymer chain is not an optimum dispesant as it may adsorb between adjacent nanotubes, reducing the stability of the dispersion3.

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2. Experiments 2.1 Materials CNTs nanohybrids are amphiphilic materials with tailored hydrophilic/hydrophobic balance. The starting material consisting of multiwalled carbon nanotubes was provided by SouthWest Nanotechnologies Inc. (SWeNT), Norman, OK. Nanotube growth is controlled to the desired length (~ 1 micron) and number of walls (~ 10) by adjusting the synthesis conditions. The alumina support and metal catalysts used in the growth process are later dissolved by an acid attack leaving a purified MWNTs product with > 98% carbon content. To adjust the interfacial activity of the CNT nanohybrids, the hydrophilicity of these nanotubes can be changed by oxidation creating hydrophilic carboxylic groups on the nanotube surface. DI Water was purified and deionized using ion exchange and reverse osmosis units. Gum arabic was purchased from Acros Organics and hydroxyethyl cellulose (HEC-10) was provided by Dow Chemicals. HEC-510K was purchased from American Polymer Standards Corporation. Hydroxyethyl cellulose of molecular weight of 250 KD and sodium nitrate were purchased from Sigma-Aldrich, and HPLC grade water was purchased from Fisher Scientific. Berea cores were crushed with a ceramic mortar and sieved through a set of standard sieves (Sieves designations: #60/ 250 µm, #200/ 75 µm) and used in the range between 75 µm to 250 µm with a d50 of about 150 µum. Sodium chloride and calcium chloride were purchased from Sigma-Aldrich. The column used in this study was low-pressure glass Chromaflex, purchased from Kimble Chase Co. 7


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2.2. Experimental Methods 2.2.1 Dispersion Preparation. MWNTs were dispersed in brine or DI water with GA at the desired concentrations (indicated later) by sonication using a 600 W, 20 KHz hornsonicator. HEC-10 stock solution was prepared according to guidelines31and was added to the dispersed solution of MWNTs to set the HEC-10:GA ratio to 8:1. Subsequently, the solution was sonicated again and then centrifuged for one hour at 2000 rpm to settle any large, non-dispersed aggregates of MWNTs. 2.2.2. Adsorption Experiments. Experiments were performed by adding 10 mL of MWNTs dispersion into vials containing 2 g of crushed Berea sandstone. A stirring bar was added and the vials were sealed and placed on a stirrer for 24 hours. The concentration of all suspensions were measured on an UV-Vis spectrometer and compared to calibrated standards of known concentrations, as described elsewhere32,33. The reproducibility of UV-Vis measurements were confirmed by measuring the concentration of a 100 ppm CNT sample 14 times over a period of 5 days. The standard deviation of the measurements was found to be 0.05ppm. The difference between initial and final MWNTs concentrations reflects the amount adsorbed or retained by the sand in each adsorption test. The experiments were repeated at a range of MWNTs concentrations from 20 up to 200ppm. The salinity through all experiments was 10% by weight, keeping a constant NaCl:CaCl2 weight ratio of 4:1 in all experiments. 2.2.3 Thermal stabilities of polymers. HEC-10 and GA were investigated by preparing vials of 20 mL of 2000 ppm HEC-10 and 5000 ppm of GA and treating every vial for a 8


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different amount of time at 90oC. For HEC-10 samples, viscosity measurements were done using a Brookfield viscometer to study degradation. Since GA solutions were low viscosity, only visual observation was necessary to ascertain when the sample degraded. 2.2.4. Gel permeation chromatography(GPC).

GPC was conducted using a system

consisting of a model 515 HPLC pump, 717 Plus Autosampler, Ultrahydrogel 1000, and 486 Tunable Absorbance Deterctor, all purchased from Waters Co. The carrying fluid was HPLC grade water with 0.1M sodium nitrate to reduce the interaction between polymer and the column packing34. 2.2.5. Sand pack testing. Propagation studies were done using glass columns described earlier. The glass columns were dry packed with crushed Berea sandstone. Figure S1 shows the experimental sand pack setup used in this work. Different liquid suspensions were injected using a peristaltic pump connected to an injection line with pressure gauges that can measure pressure drops across the column. A sample collector was used to collect effluent from the columns. The Berea sand packs consisted of columns which were 6 inches long with adjustable bed support and 1 inch in diameter. After the columns were packed, the porous media were characterized by measuring porosity and permeability. Porosity was measured by injecting water at 0.3 mL/min until no air bubbles were detected in the effluent. The pore volume was measured from the difference between the total amount of the injected water and the amount of recovered water in the effluent plus the water remaining in the lines. Permeability was estimated using Darcy’s law. The pressure drop through the sand packing was measured at different flow rates 9


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ranging between 0.30 and 20 mL/min. The measured values for porosity and permeability were 35% and 4.1 D respectively, using the protocol described. Experiments were run in 1 inch (L) x 1 inch (D) sand packs unless stated otherwise. The tests were run by injecting 5 pore volumes (PV) of the particle dispersion followed by brine injection until no particles were detected in the effluents; usually less than 5 PV of brine flush were needed to achieve an undetectable concentration of nanoparticles. 2.2.6. Core flood experiments. Coreflooding experiments were conducted using a coreflood setup (Figure S2) consisting of a syringe pump filled with mineral oil connected to four accumulators to inject test fluids. The core holder is situated inside a heating oven controlled by a temperature controller. Pressure transducers are connected to a computer to record pressure changes through all experiment. The effluent stream of the core holder is connected to a sample collector. Samples were collected and analyzed using UV-Vis. The rock samples used are Berea sandstone cores. The permeability was measured independently prior to the test and was confirmed later by the pressure difference using Darcy’s law. The permeabilities of the two core samples were 200 and 250md. 2.2.7. Scanning electron microscopy (SEM). SEM imaging was conducted using a Zeiss NEON High Resolution SEM, by diluting a sample of the carbon nanotube dispersion by a factor of 100 in deionized water. Dilution helps to minimize imaging artifacts due to drying, e.g. aggregation due to capillary forces during drying, large salt crystals from API brine, etc. The diluted dispersions were then placed in an air-brush spray gun and sprayed 10


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using compressed nitrogen onto a substrate. The substrates were prepared by sputter coating glass microscope cover slips with a 2-4nm layer of iridium in order to provide a conductive, but atomically smooth surface for imaging. The spray coated substrates were then attached to stainless steel pin stubs using conductive carbon tape and placed in the SEM for imaging.

3. Results and discussions 3.1 Adsorption Studies MWNTs samples of a number of concentrations were dispersed in GA by sonication for two hours and then secondary dispersant polymer HEC-10 was added and the suspensions were sonicated again for another 30 minutes. The suspended concentration of MWNTs in the final dispersions ranged between 20 and 200 ppm, with 200 ppm of GA and 1600 ppm of HEC-10. The dispersion was then centrifuged at 2000 rpm for one hour. All dispersions, unless otherwise stated, were prepared in 10% API brine with a sodium chloride to calcium chloride ratio of 4:1. Adsorption experiments were done by adding 10ml of dispersion to 2g of crushed Berea sand and stirring for 24hrs. The mass of MWNTs adsorbing to the sand was quantified using UV-Vis spectrophotometry. Figure 1a and 1b, below, shows a comparison between the adsorption of MWNTs using GA and PVP 40 as primary dispersants at temperatures of 22 oC and 50 oC where the concentrations(x-axis) represents equilibrium concentrations of MWNTs. Appreciable adsorption has been observed using PVP40 as the primary dispersant in our earlier work3. From adsorption values estimated in Figure 1, it can be concluded that a dispersion made 11


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using GA adsorbs to crushed Berea sand to a lesser extent than previously reported with PVP40. This can be explained by the high tendency of PVP40 to adsorb to crushed Berea sand due to hydrogen bonding between the carbonyl group of the PVP40 pyrene ring and the silanol groups on the sand particles28,35.

(a)

(b)

Figure 1. Adsorption and retention of MWNTs by crushed Berea sandstone using two different primary dispersants at (a) 22oC (b) 50oC. Adsorption experiments performed using GA as primary dispersant at 80oC were successful in producing stable dispersion in contact with sand; however, the adsorption was still significant at this temperature (>0.1mg/g sand). An additional filtration step with 1 µm filter paper was included following centrifugation to remove any large particles that may initiate filtration of the dispersion. Figure 2 shows the adsorption measurements at 80 and 90oC using pre-filtered MWNTs dispersions. The data series represented with triangles indicate no adsorption at 80oC, and the data series represented with squares showed very low adsorption at 90oC, which corresponds to around 30% of total MWNTs 12


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concentration. It is worth mentioning here that a stable dispersion could not be produced using PVP40 in contact with the sand at 80oC as the dispersion completely separated which demonstrate the high stability of the filtered dispersions.

Figure 2. Adsorption of gum arabic coated MWNTs using pre-filtered dispersions at high temperature. Using the most stable dispersion, adsorption experiments were repeated using 20% salinity(twice the API salinity) to check for the effect of higher ionic strength. It was found that this system of dispersing polymer is able to remain stable at 20% salinity and 80oC. The adsorption was found to be less than 0.01mg/g sand at these conditions. The boundary between stable dispersion at 80oC and unstable dispersion at 90oC in contact with the sand where not fully explored. Further investigation of polymer stability was needed to explain instability at 90oC as explained later.

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3.2 Thermal Stability of Polymers and Gel Permeation Chromatography From these adsorption experiments it was observed that no adsorption or retention occurs for MWNTs at 80oC and limited adsorption at 90oC. The reasons behind the losses at 90oC were not clear and therefore it was decided to investigate the thermal stability of GA and HEC-10 at 90oC. Stock solutions of 2000 ppm of HEC-10 and 5000 ppm of GA both in 10% brine were treated at 90oC over a period of 1 week and compared with fresh untreated polymer samples both aerobically and anaerobically. Figure S3 shows viscosity measurements for HEC-10 in aerobic and anaerobic environments respectively. In these two figures we observe the trend of decreasing viscosity with increased treatment time which indicates significant polymer degradation; however, there are no significant differences between the aerobic and anaerobic conditions. The drop in viscosity of HEC10 is then due to thermal degradation above 80oC, and has been reported elsewhere36,37. Similar experiments were repeated for 5000 ppm GA polymer solutions and since no reliable viscosity measurements could be obtained using GA due to the low viscosity at 5,000 ppm, only visual observation was conducted. From visual observations (Figure S4a and S4b), it was concluded that aerobic degradation of GA takes place at 90oC indicated by the formation of yellowish, lower, coacervate phase. This was evident by the color change in vials treated at aerobic conditions only. As a continuous effort to understand the dual effect of polymer on stability, gel permeation chromatography was performed on HEC-10 to identify its molecular weight and the significance of its molecular weight on dispersion stabilization and any possible molecular weight changes that can take place 14


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due to the effect of sonication. Figure S5 shows the differential refractometer measurements done on two samples of polymer, HEC-10 and a standard polymer of HEC with a molecular weight of 510 KD. Both HEC-10 and HEC-510KD has the same retention time indicating a molecular weight of HEC-10 of about 500kD. The effect of sonication on HEC-10 was also investigated by sonicating a 100 ml solution containing 2000 ppm of HEC-10 for different times ranging from 30 minutes up to 2 hrs and conducting GPC on samples. It was found that possible shear degradation could take place; however, this degradation does not appear to be severe enough to affect the molecular weight as indicated in Figure S6. From this Figure we can expect the degradation after two hours to reduce the polymer weight after two hours down to around 400-450 KD. This was not expected to significantly change dispersion stability as concluded from the experiments. 3.3 Column Studies Propagation experiments were performed using column described earlier. Adsorption studies can depict the particle retention that may be experienced by the MWNTs dispersions in transport studies. However, factors like particle filtration and deviations from plug-flow in porous media contribute to the propagation of these particles. Sand pack tests were performed to compare MWNTs propagation of two binary dispersant systems: HEC-10 and PVP40; HEC-10 and GA. The HEC-10 concentration in both cases were 1600 ppm. The concentration of GA and PVP40 was 200 ppm for their respective dispersion. Column studies were performed at 25 and 50oC for each system with 10% 15


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API brine in both cases. Each of these dispersions was pre-filtered through a 1µm filter prior to the dispersion injection, such that physical straining does not play a major role during propagation experiments38. The results comparing both of the dispersions at 25oC are presented in Figure 3.

Figure 3. Propagation of MWNTs through crushed Berea sand packs at 25oC using two binary dispersant systems: HEC-10 and PVP40, and HEC-10 and GA. Both systems exhibited excellent propagation with negligible differences in particle recovery (92% and 91%). Adsorption experiments have shown that both of these systems experience low adsorption at this temperature and this is in agreement with our previous results reported earlier3. The system with GA reached a higher normalized concentration in the 3rd PV, while the normalized concentration in the 2nd PV was lower than that of the system with PVP40. The variation in the particle breakthrough resulted in negligible differences in particle propagation overall. For the application of this technology, these dispersions must be able to propagate through the porous media while maintaining particle stability and reducing particle-rock 16


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interaction. Figure 4 shows the results from the propagation of both binary dispersant systems at 50oC.

Figure 4. Propagation of MWNTs through crushed Berea sand packs at 50oC using two binary dispersant systems: HEC-10 and PVP40, and HEC-10 and GA.

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The particle propagation with the HEC-10/PVP40 binary dispersant system clearly suffered from the elevated temperature. This effect was not observed before in our previous adsorption experiments where PVP was used as the primary dispersant. Particle recovery decreased 5%, and the nanoparticle breakthrough was delayed. In the fourth PV at 25oC the normalized concentration of this system has reached approximately the injected concentration, while at 50oC the normalized concentration is about 10% less than that. Furthermore, the overall shape of the breakthrough curve of the HEC-10/PVP40 system suggests that the particles are eluting from the column later at elevated temperatures. In contrast, The HEC-10/GA dispersant system had an insignificant response to the elevated temperature, indicating that particle propagation was not hindered. These results agree with the adsorption experiments: elevated temperatures increase the adsorption of the particles onto collectors in the Berea sand when dispersed using the HEC-10 and PVP40 binary dispersant system. Consequently, the propagation of this dispersion at higher temperatures yields lower particle recovery. In addition, in adsorption experiments, at a temperature of 50oC, no significant retention of the particles was observed when dispersed with the HEC-10 and GA binary system 3.4 Propagation through Core Samples A MWNT sample was dispersed in GA stock solution by sonication for two hours and then HEC-10 solution was added to prepare the dispersion used in experiments as described earlier. The final dispersion is 100 ppm of MWNTs, 200 ppm of GA and 1600 ppm of HEC-10 in 10% API brine. The solution was centrifuged for 1 hour at 2000 rpm 18


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and filtered using a 1mm glass filter (grade B). Two cores of Berea sandstone were tested with permeabilities of 200 and 250 mD. The injection temperature was 50oC and the overburden pressure was 1000 psi. The back pressure was set to 500 psi. The breakthrough of a 100ppm dispersion of nanotubes is shown in Figure 5 below. The cores tested were 1” in diameter. For the first test shown in Figure 5a, five pore volumes of dispersion were injected at 50oC and 5 pore volumes of brine flush. The second coreflood test was done with a 200 mD core and the core length was 2”. The testing temperature was higher at 65oC. The shaded area correspond to PV of MWNTs dispersion injected.

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Figure 5 Core flood experiments [Top] 250mD, 1” length core at 50o[below] 200mD, 2” core at 65oC. The total cumulative recovery was 79% for the 250 mD 1” core and 89% for the 200 mD 2” core. The transport of particles showed little to no retention at the sand face. No blockage of rock sample was observed during both tests, as indicated by no increase in

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pressure across the core. The total adsorption was found to be 8.6 and 9.8 µg/g for the 1” and 2” coreflood respectively. The PVP:HEC-10 dispersion system did not result in successful propagation through the 250 mD core, despite the stability of the dispersion under these conditions. This is clear evidence for the robustness of using the GA:HEC-10 dispersant system and an illustration of the importance of low adsorption for both the primary and secondary polymers. 3.5 Scanning Electron Microscopy While imaging the dispersion samples, several clumps of nanotubes were observed and several of the nanotubes appeared to have polymer/salt crystals attached to them. In order to determine whether these attached aggregates were polymer, the electron beam was focused on a small region of the sample and allowed to interact with the sample for a time period of 3-5 minutes. Polymer degradation during interactions with the beam of accelerated electrons present in an electron microscope is a well-understood phenomenon39,40. A combination of primary interactions with the electron beam, beaminduced x-rays, and charging effects destabilize a variety of bonds within organic polymers causing them to lose integrity and react to form gaseous compounds or a cokelike solid contaminant on the sample surface. The beam conditions used were selected to be likely to cause polymer degradation, but lack the intensity required to damage the carbon nanotubes. Figure 6a and 6b shows a single nanotube surrounded by smaller polymer/salt aggregates before and after the focused beam/polymer degradation experiment. In the first image, it can be seen that the nanotube (an irregular cork-screw 21


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tube) is covered with material that was hypothesized to be polymer, specifically the gum arabic or HEC. When the beam was focused onto the tube in particular all of this material was removed, leaving the bare nanotube seen in the second image. Figure 6c shows the same nanotube at higher magnification. This qualitative experiment appears to confirm our hypothesized “polymer wrapping” mechanism for dispersion stabilization and confirms that the material adhering to the nanotubes in Figure 6a is polymer and not salt crystals.

a

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b

c

Figure 6. Scanning Electron Microscopy of CNT showing polymer wrap(a) before focusing the beam(b) after focusing the beam and inducing polymer damage(c) magnified CNT after treatment.

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4. Conclusions Stable MWNTs dispersions using two dispersing polymers GA and HEC-10 have shown outstanding tolerance at reservoir conditions of up to 20% salinity and a temperature up to 80oC. GA is able to debundle the MWNTs while HEC-10 is able to provide the steric repulsion needed to prevent MWNTs aggregation. The adsorption of MWNTs to the crushed Berea sand was in the order of 0.01 mg/g of sand at 80oC. Thermal stability studies of polymers at 90oC show the degradation of HEC-10 at that temperature as expected while GA visual observation suggests that this polymer degrades aerobically at high temperature which explains the loss in particles to the sand observed at 90oC. Gel permeation chromatography of HEC-10 allowed for the identification of molecular weight and molecular weight changes as a result of sonication. The effect of sonication is expected to have small effect on the molecular weight of HEC-10. Column studies in accordance with adsorption isotherm have shown the beneficial effect of using GA instead of PVP40 as a primary dispersant, before stabilizing the dispersion with HEC-10, in reducing retention of MWNTs at a temperature of 50oC or higher. The higher plateau value of C/Co and the earlier breakthrough of MWNTs dispersed with GA suggest more interaction between PVP coated MWNTs and Berea sand, which explains the higher particle retention observed by using PVP as primary dispersant. Coreflooding performed earlier with sample dispersed using PVP/HEC-10 polymer systems were not successful. Using GA/HEC-10 on the other hand resulted in successful

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propagation of nanoparticles through reservoir-like rock samples with more than 80% of the particles successfully propagated. SEM imaging shows evidence for the physical adsorption of polymer layer onto nanotubes which agrees with previous assumptions of polymer strands physically wrapped around the individual nanotubes. Supporting Information The supporting information is available free of charge on the ACS publications website. Figure S1, Sand pack experiment. Figure S2, Core flooding unit. Figure S3, HEC-10 viscosity measurements with different treatment times at 90oC at (a) aerobic conditions (b) anaerobic conditions. Figure S4, Visual observation of GA treated at 90oC at (a) aerobic conditions (b) anaerobic conditions. Figure S5, Differential refractometer measurements of HEC-10 and HEC-510K. Figure S6, Effect of Sonication on HEC-10 molecular weight in comparison with HEC250K.

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Acknowledgments This work was supported by the Advanced Energy Consortium (http://www.beg.utexas.edu/aec/). Member companies include Repsol, Statoil, Shell, and Total. References

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