Stable Silica Nanofluids of an Oilfield Polymer for Enhanced CO2

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Environmental and Carbon Dioxide Issues

Stable Silica Nanofluids of an Oilfield Polymer for Enhanced CO2 Absorption for Oilfield Applications Krishna Raghav Chaturvedi, Rakesh Kumar, Japan J Trivedi, James J. Sheng, and Tushar Sharma Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02969 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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Research article

Stable Silica Nanofluids of an Oilfield Polymer for Enhanced CO2 Absorption for Oilfield Applications

Krishna Raghav Chaturvedi†, Rakesh Kumar$, Japan Trivedi#, James J. Sheng£, Tushar Sharma†, *

†Enhanced

Oil Recovery Laboratory, Rajiv Gandhi Institute of Petroleum Technology Jais-

Amethi, UP, India $Green

Separation Lab, Rajiv Gandhi Institute of Petroleum Technology, Jais-Amethi, UP, India

#School £Texas

of Mining and Petroleum Engineering, University of Alberta, Canada

Tech University, Lubbock, TX 79409, USA

*Corresponding

Author.

E-mail address: Tushar Sharma: [email protected], Tel.: +91-7080044156 1 ACS Paragon Plus Environment

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Abstract CO2 injected with water often gives premature breakthrough and reduces its absorption during sequestration and oil recovery applications. Water-soluble polymers are used to increase CO2 absorption via an increase in water viscosity that restricts CO2 movement and thus, its early release. The efficacy of polymer CO2 absorption methods can be further increased in the presence of nanoparticles (NPs) that interacts with polymer chains and creates a steric barrier to improve CO2 absorption. Thus, nanofluid prepared with compatible NPs might be a safe and reliable method to improve CO2 absorption of polymer methods. In this work, a nanofluid prepared with silica NPs (0.1-1.0 wt%) in base fluid of oilfield polymer [(polyacrylamide (PAM) with typical oilfield concentration (1000 ppm)] was tested for CO2 absorption and compared with the one of PAM fluid at different temperature (303 and 353 K). The inclusion of SiO2 in PAM fluid provided stable nanofluids those exhibited good dispersion stability without NP settlement for days. Thus, the efficacy of PAM fluid CO2 absorption significantly increased with nanofluids as reported through microscopic, kinetics, and molality results. The increase in NP concentration and temperature (353 K) showed an inverse relationship with CO2 absorption in nanofluids, mainly due to enhanced NP aggregation; thus the use of nanofluid for CO2 absorption is critical at high temperature and high NP concentration. NP effect on CO2 stabilization and absorption is finally supported through UV-vis measurements. The study highlighted important aspects of CO2 absorption and is a forward step towards the use of nanofluid together with the considerable possibility of enhanced CO2 miscible oil recovery.

Keywords: CO2 Absorption; Nanofluid; Polymer; SiO2 Nanoparticle; Stability; Temperature

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1. Introduction The burning of carbon fossil fuels has increased the concentration of carbon dioxide (CO2) in the atmosphere which consequently has raised the global temperature resulting potential threat of climate change.1,2 The current concentration of CO2 in the atmosphere is around 403 ppm, more than the permissible limit of 350 ppm.3 Thus, the reduction in CO2 is critical and more important these days and as a result, it has brought the attention of every project leading to contributing to CO2 reduction. Several research developments on different scales have been proposed and many are still in the development stage to capture and sequester CO2. Some researches based on the use of amines,4 ionic liquids,5 porous materials viz., polymer6, graphene7, and nanoparticles8 exist. The injection of CO2 in depleted oil and gas reservoirs has also been considered a good practice to sequester CO2 into the geological formations suggesting a sustainable way to maintain ecological balance.9,10 In addition, injecting alternate slugs of water and CO2, called water alternating gas (WAG), is one of the methods to reduce gas channeling. However, water is less viscous fluid and CO2 being lighter gas may move relatively faster showing early breakthrough without fulfilling the purpose. The physical absorption methods are better than other methods and need more attention because of its vital importance in oilfield applications. Compared with conventional methods, the use of NPs to increase CO2 absorption in base fluids has been found widespread attention.11-13 NPs improve the efficiency of the process through increase in water viscosity (called polymer effect)14 and the inclusion of active surface sites of small nanoparticles (NPs) in solution for better absorption of CO2 molecules.15 The trait that makes NP-based formulation suitable for CO2 absorption is also associated with large surface area per unit volume,16 better structuring,17 and improved rheological18 and optical properties.19 Nanofluid can be used as a potential solvent 3 ACS Paragon Plus Environment

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to achieve higher values of CO2 absorption with respect to their base fluid. Pineda et al.20 investigated the influence of silica (SiO2) and aluminum oxide (Al2O3) NPs of 0.05 wt/vol% on absorption rate of CO2 in methanol based nanofluids and concluded that the addition of Al2O3 and SiO2 NPs in methanol enhanced CO2 absorption by 9.4 and 9.7%, respectively. CO2 absorption in TiO2 nanofluids was performed by Zhang et al.21 who found that the particle size plays an important role in determining absorption. The capacity of different NPs (SiO2, Al2O3, carbon nanotubes, and Fe3O4) to increase CO2 absorption in water and amine solutions was tested by Rahmatmand et al.22 They found that SiO2 and Al2O3 were more efficient and increased the CO2 absorption capacity of base fluid by 21% and 18%, respectively. Another study on the use of silver nanoparticles embedded over porous Metal organic frameworks for their use in CO2 fixation was performed and the results showcased that the presence of highly reactive Ag NPs allowed the reaction to procced smoothly at even 1 atm CO2 pressure.23 Recently, Cecilia et al.24 performed the functionalization of hollow silica microspheres by impregnation or grafted of amine groups to boost CO2 absorption. A study on the use of mesoporous MgO nanoparticles for carbon capture was performed by Hiremath et al.25 whose experimental study showed that MgO nanoparticles can be utilized for carbon dioxide (CO2) capture in an integrated gasification combined cycle (IGCC) scheme. The authors have categorically stated that chemisorption played a major role in determining the CO2 uptake in MgO nanomaterials. To confirm, authors recorded CO2 adsorption isotherms which showed that CO2 uptake increased with increasing pressure. Similarly, a significant change in CO2 uptake was observed while changing the temperature from 25 to 50 ºC. The occurrence of chemisorption at high temperature was resulted from the large surface area of NPs as lower surface area and decreased active sites of NPs hindered the chemisorption.


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Some other advantages associated with the use of NPs are a superior reduction in surface tension of gas-water system to create CO2 bubbles (as CO2-foam) resulting increased in-situ absorption.26 NPs possess high surface activity than conventional methods and thus may provide better reduction in surface forces of CO2-water resulting stable formulation of CO2 foam for applications.27-29 Kim et al.11 performed a comparative study to analyze the absorption of CO2 in a nanofluid of silica NPs (of size 30 nm, 70 nm, and 120 nm) and DI water. The results showed that CO2 bubbles in nanofluid were more in number than DI water. Xue et al.30 investigated the viscosity and the stability of CO2-in-water (C/W) foams stabilized with surfactants and nanoparticles in conjunction with and without polyelectrolytes. From these studies, it can be appreciated that the nanofluid for physical absorption of CO2 holds a great promise and can provide improvement in CO2 absorption capacity of base fluids. However, the synthesis of a nanofluid is critical for any industrial application due to shortcomings such as agglomeration of NPs, reduced dispersion stability, and premature sedimentation under the effect of gravitational forces.18 NPs are solid particulates and in a nanofluid, their agglomeration leads to the formation of NP clusters of large size and relatively denser than the sole NPs. These clusters may settle faster due to gravitational action resulting reduced dispersion stability in a nanofluid and as a result, nanofluid will be ascertained as an unstable colloidal suspension for CO2 absorption. Thus, nanofluid synthesis in a base fluid of less viscosity (such as water) is susceptible to show higher settlement and the water nanofluid will be rendered as unstable. It is thus highly desirable to prepare stable nanofluid which exhibits superior stability so as most of the NPs participate in CO2 absorption phenomenon, which is the main focus of this work. One of the solutions to improve the stability of water nanofluids is to use additives such as high molecular weight polymer such as polyacrylamide (PAM) that provides higher viscosity to 5 ACS Paragon Plus Environment

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water during dissolution. NPs mixed together with PAM is expected to form a complex macromolecular cross-linking structure resulting from the adsorption of PAM chains on the surface of dispersed NPs as proposed by the scheme in Figure 1. Zheng et al.31 prepared SiO2 nanofluid of polyacrylamide (PAM) and hydrolyzed-PAM (HPAM) using in-situ surfacemodification. SiO2 nanofluids (in conjunction with PAM) exhibited better apparent viscosity and storage stability than the ones prepared without PAM. Earlier, we had reported the use of PAM to stabilize SiO2 Pickering emulsions for thermal stability32 and oil recovery applications from a Berea sandstone.33 Recently, we used PAM to synthesize nanofluid of SiO2 NPs for interfacial tension (IFT) reduction, wettability alteration, viscosity improvement, and oil recovery applications.18,34 Despite the vital importance of PAM in the formulation of nanosuspension globally, the study showing PAM based silica nanofluid for CO2 absorption for oilfield applications is fairly limited in the literature. Since PAM fluid is a typical oil recovery method and CO2 in viscous polymer phase is expected to remain trapped for longer period than water, the novelty of the work lies in the use of silica NPs (as nanofluid) to improve the CO2 absorption capacity of polymer methods of CO2 sequestration and EOR projects. Also, since low viscosity and non-wetting nature of CO2 is responsible for an early breakthrough of the injected gas at the surface during an EOR process, nanofluid with enhanced CO2 absorption may solve these issues. CO2 laden nanofluids will carry more CO2 than carbonated water injection allowing engineers to sequester comparatively more CO2 easily and securely in the sub-surface. Thus, we report the synthesis of PAM-based silica nanofluid as a new absorbent to create stable and prolonged CO2 absorption. CO2 absorption capacities in the nanofluid, prepared with varying concentration (0.1, 0.5, and 1.0 wt%) of SiO2 NPs in base fluid of 1000 ppm PAM (a typical oilfield concentration), was investigated at two different temperatures (303 K and 353 K). In addition, the kinetic studies


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of CO2 absorption in each solvent were conducted with first order rate equation and kinetic parameters were evaluated to determine the ease of CO2 miscibility in the solvent. Finally, UV-vis results were discussed to provide more insights into the stability of CO2 absorption in a nanofluid. 2. Experimental Section 2.1. Materials Hydrophilic SiO2 nanoparticles (size 15 nm and 99.5% pure) were obtained from Sisco Research Lab Pvt. Ltd. India and used as received. The water soluble polymer PAM (Pusher 1000, molecular weight = 10 million) was purchased from SNF Floerger, India in powder form. The deionization and filtering of water were carried out using a Millipore® Elix-10 purification apparatus (electrical conductivity of water = 0.0054 mS.cm-1) and DI water was used through the study. A magnetic stirrer (IKA-C-MAG-HS7) was used to dissolve PAM in water at a stirring speed of 600 rpm for 1 h. To prepare nanofluid, SiO2 NPs were mixed in the aqueous phase of PAM using an industrial mixer (Oster Grinder, Model: MCPRO6-WSO) at a stirring speed of 6000 rpm for 0.5 h. A digital ultrasonic cleaner (Rivotek ultrasonic cleaner, Mumbai) at frequency 25 Hz was used to sonicate SiO2 NPs in the nanofluid. To weigh chemicals, an accurate digital weighing balance (Mettler Toledo, ME204/A04) with a repeatability of 0.1 mg was used. A high pressure (capacity 47 liters when full with pressure 98 bars) cylinder filled with CO2 was used to perform the absorption experiments. 2.2. Preparation of nanofluid In this study, two types of SiO2 nanofluids viz., SiO2 mixed in DI water with and without PAM were prepared and used to study CO2 absorption experiments at temperature conditions of 303 K to 353 K. In nanofluid preparation, the concentration of polymer PAM was kept constant 7 ACS Paragon Plus Environment

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as 1000 ppm throughout the study while the amount of SiO2 NPs was varied from 0.05 to 2 wt% at 303 K. The nanofluid preparation in water without PAM was unsuccessful due to premature sedimentation of dispersed SiO2 NPs; NPs settled within 8-10 h after preparation.18 Hence, PAM with concertation of 1000 ppm was used before mixing of SiO2 NPs. 1000 ppm is a typical oilfield concentration of PAM,35 often used in nanofluid synthesis.18,36 SiO2 NPs were added slowly in batches to avoid particle agglomeration in PAM solution followed by extensive mixing using a mixer to evenly disperse them. Finally, nanofluids were sonicated in a sonication bath for 2 h at 25 Hz frequency to ensure homogenization of dispersed NPs.37 The compositional details and nomenclature for prepared nanofluids are given in Table 1. 2.3. Experimental setup for CO2 absorption CO2 absorption is referred to the encapsulation of CO2 bubbles in nanofluid interstices. The detailed experimental setup has been reported in our previous work.38 However, brief details are provided for the sake of brevity in Figure 2. In general, the experimental set-up consists of a high pressure stainless steel equilibrium cell (capacity = 25 ml). The temperature of the cell is maintained by thermal jacket attached to a circulator (HRC2, IKA). The pressure inside the cell was measured using a pressure transducer (DiGi gauge TX-430, 0-65 bars, accuracy = 0.25%). Constant stirring of 300 rpm was provided to equilibrium cell using a magnetic stirrer to ensure the uniformity of gas in the solvent. The solubility of CO2 in nanofluid was determined using a volumetric method as suggested in previous research articles.39 First, a known amount of 5 ml of nanofluid was chosen and loaded into the equilibrium cell for CO2 solubility measurements. The cell is now degassed by a vacuum pump and the desired temperature across the cell was maintained using the hot plate. CO2 gas of known volume was then introduced into the reservoir and brought to a constant temperature in order to calculate the initial number of moles of CO2 before injecting 8 ACS Paragon Plus Environment

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into the equilibrium cell. The initial pressure of the reservoir was measured and an initial number of moles of CO2 were calculated using Eq. 1.

𝑛𝑖𝐶𝑂2 =

𝑃𝑖 (𝑉𝑟𝑒𝑠)

(1)

𝑍𝑖𝐶𝑂2 𝑅 𝑇𝑒𝑞

where, 𝑛𝑖𝐶𝑂2 is the number of moles available in gas reservoir, Pi is the initial pressure, Vres is the volume of the gas reservoir, R is gas constant, Teq is the measured temperature, and 𝑍𝑖𝐶𝑂2 is the compressibility factor CO2 was then injected in equilibrium cell using a valve connecting the reservoir and equilibrium cell. As the absorption of CO2 in nanofluid starts, the pressure inside the cell reduces in a continuous manner which was recorded periodically. Nanofluid and CO2 mixture was constantly stirred (at 300 rpm) during the experiment till the system reaches to an equilibrium under pressure. At equilibrium, the pressure was measured and the moles of CO2 left in the cell were calculated using Eq. 2

𝑛𝑒𝑞 𝐶𝑂2 =

𝑃𝑒𝑞 (𝑉𝑐𝑒𝑙𝑙 ― 𝑉𝑁𝐹)

(2)

𝑍𝑓𝐶𝑂2 𝑅 𝑇𝑒𝑞

where, 𝑛𝑒𝑞 𝐶𝑂2 is the number of moles remaining in the equilibrium cell, VNF is the volume of nanofluid in the equillibrium cell, Peq is the equilibrium pressure, and 𝑍𝑓𝐶𝑂2 is the compressibility factor at equilibrium conditions. Thus, the number of moles of absorbed CO2 in nanofluid is given by Eq. 3 𝑖 𝑒𝑞 𝑛𝑎𝑏𝑠 𝐶𝑂2 = 𝑛𝐶𝑂2 ― 𝑛𝐶𝑂2

(3)


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2.4. Microscopic characterization The microscopic characterization of samples after absorption of CO2 was carried out using an optical microscope (Motic® microscope, Hong Kong) with inbuilt imaging software (Motic Images Plus 2) under ambient conditions. The stability of CO2 bubbles in nanofluid was monitored and captured using a digital camera (Moticam-10) attached with an optical microscope. 2.5. Scanning electron microscope (SEM) analysis SEM analysis was performed to visualize the morphological structure of NPs and NP adsorption on CO2 bubbles in the nanofluid. An instrument called Nova NanoSEM (450, ThermoFisher® USA) was used for this purpose. A drop of nanofluid stabilized CO2 bubbles was poured on a tested aluminum stab of the equipment and dried in an oven to remove moisture from the layer of nanofluid and CO2 bubbles. The drying left the footprints of CO2 bubbles on the stab. The dried layer was then gold coated and analyzed to record the images in SEM equipment. 2.6. UV-Vis spectroscopy experiments To determine the role of NPs in CO2 absorption, UV-vis spectroscopy experiments on SiO2 nanofluids before and after solubility measurements were conducted using UV-vis equipment (3200, LabIndia). UV-vis experiments were conducted to determine the amount of UV absorption on remaining NPs in remainder nanofluid which did not participate in the process of CO2 absorption. Therefore, to avoid CO2 inclusion, a small volume from the continuous phase of remainder nanofluid was firmly imbibed with the help of a syringe and analyzed in UV apparatus. Nanofluid samples were examined for UV-vis experiments at room temperature with a 1 nm/s scan rate over the wavelength range of 190 nm to 800 nm. Before each measurement, the cuvette was carefully cleaned using DI water to avoid contamination with the samples. 10 ACS Paragon Plus Environment

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2.7. Zeta (ζ) potential measurements Zeta (ζ) potential of nanofluid was determined using a particle size analyzer (SZ-100, Nanopartica, Horiba Scientific®, Singapore) which involves the principle of dynamic light scattering (DLS) technique. The instrument uses a 173o detector scattering angle for the measurements and all the measurements were conducted at 25 °C. These measurements were carried out just after sonication on nanofluid to avoid any possibility of NP settlement. 3. Results and Discussion In this section, first, the stability of the prepared nanofluids with and without PAM is presented followed by the discussion on the characterization of CO2 absorption in various fluids. Next, determination of Henry’s constant and first-order absorption kinetics for the various fluids is demonstrated. Finally, UV-Vis results are provided to propose a method of determining NP participation in CO2 absorption. 3.1. Stability of nanofluids The stability of nanofluid is a major concern while proposing their use in any industrial process. The stability of a nanofluid can be defined as its ability to avoid settlement of suspended NPs which for a nanofluid regarded as dispersion stability. However, NPs are solid particulates and their settlement due to gravitational action is apparent.18 Therefore, a nanofluid exhibiting extended dispersion stability may be regarded as stable nanofluid as compared to the one gets an early settlement. For example, Haghtalab et al.39 noted that the nanofluid of 0.1 wt% SiO2 prepared in a base of DI water was stable only for a week. Figure 3 shows the nanofluid prepared with varying concentration of SiO2 NP (0.1, 0.5, and 1.0 wt%) in the aqueous phase of water and 1000 ppm PAM (see Table 1 for nomenclature). The dispersion stability for the nanofluids was 11 ACS Paragon Plus Environment

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examined visually and therefore, nanofluid immediately after the preparation was transferred to transparent vessels and left undisturbed to examine the changes in nanofluid appearance. All of the prepared nanofluids were milky in color at the time of preparation but showed gradual settlement with the course of time. It was observed that nanofluid prepared with 0.1 wt% SiO2 and DI water (NF-1) started to show the first sign of settlement within first 2 h, while the nanofluids prepared with 1000 ppm PAM did not show any sign of settlement for days. Nanofluid prepared with 0.1 wt% SiO2 in PAM (NF-2) was least dense in color and did not show any NP settlement till 30 days (Table 1). PAM was found to increase the water viscosity that reduced the extent of the downward settling of NPs and therefore, dispersion stability of nanofluid increased. The results of dispersion stability in formulated nanofluids are also provided in Table 1. For NF-2, the color of appearance almost disappears after 44 days. In NF-3, the first NP settlement was observed after 26 days while NF-4 exhibited settlement after 24 days (Table 1). Thus, it is to be noted here that nanofluid with a higher concentration of SiO2 NP showed faster settlement in PAM solution. Since the concentration of PAM and size of NPs were constant in the preparation of each nanofluid, the reason of varying rate of settlement is probably linked with that fact that NP size changed and its concentration increased with increasing amount of NPs. It is possible as NPs may combine to minimize the surface energy and formed clusters of higher sizes with increasing concentration of NPs.18 To confirm this, DLS measurements for nanofluids were conducted and the results are provided in Table 1. DLS measurements provided information on both stability of a colloidal dispersion and size of dispersed NPs in a nanofluid. ζ potential for a colloidal dispersion should be in the range of ± 30 mV.40 NF-1 showed good stability with ζ potential value of -36.22 mV (at the time of preparation) which steeply decreased in next 2 h and NF-1 became unstable (ζ potential = -11.85 12 ACS Paragon Plus Environment

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mV). For NF-2, ζ potential was measured as -37.14 mV which remained significantly higher even after 40 days (-26.41 mV) as given in Table 1. ζ potential for NF-3 and NF-4 nanofluids was higher due to high amount on NPs in nanofluid and therefore, ζ potential for them was measured as -51.87 mV and -52 mV, respectively.18 However, ζ potential for NF-3 and NF-4 decreased relatively faster than NF-2 and these nanofluids became unstable after 40 and 38 days, respectively. It is expected that the reason of unstable behavior of these nanofluids is a faster settlement of NPs than NF-2 nanofluid. The Faster settlement is possible with the higher size of NPs as confirmed by size measurements. It was observed that the size of the original NPs (15 nm) increased after mixing into nanofluid. NF-3 was found to consist of NP clusters of 0.85 µm while NF-4 exhibited an average cluster size of 2.8 µm. The average size of NP clusters in NF-2 was measured as 0.41 µm which is lesser than the size associated with NF-3 and NF-4. The occurrence of aggregation resulting in the formation of NP aggregates of higher sizes is confirmed by SEM analysis and the results are shown in Figure 4 for NF-2 nanofluid. The image clearly indicates that NP size increased in nanofluid phase which is only possible if NPs aggregate and formed clusters of higher sizes than 15 nm. Thus, the results indicate that the extent of NP settlement in NF-3 and NF-4 was higher than NF-2 which also supports the formation of NP aggregates of greater sizes (Table 1) and therefore, ζ potential decreased relatively faster in these nanofluids consistent with dispersion stability results. 3.2. Characterization of CO2 absorption in fluids Microscopic analysis was used to characterize the absorption of CO2 in base water, PAM fluid, and nanofluids (NF-1 to NF-4) at different temperature of 303 K and 353 K. After CO2 absorption experiments, the sample was immediately transferred to a microscopic slide and examined to visualize the presence of CO2. It was observed that microscopic analysis for water 13 ACS Paragon Plus Environment

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did not show the presence of CO2. For PAM fluid, CO2 was distributed in form of bubbles in the aqueous phase of PAM as shown in Figure 5. However, the distribution progressively reduced due to eventual CO2 seepage from the surface which indicates that the absorption of CO2 bubbles in PAM was not stable. This correlates well with the fact that CO2 absorption in water soluble polymers is predominately a physical process (which is reversible in nature) where CO2 is trapped into the elastic network of polymer chains under high pressure.41,42 Moreover, CO2 bubbles remain in an encapsulated state inside the fluid under confining pressure (evident in microscopic image, Figure 5) and on removing the pressure, CO2 started to escape back into the atmosphere which is hardly possible when chemisorption takes place in polymer networks as chemisorption is an example of mono-layer absorption and the whole action is mostly irreversible.43-45 PAM increases the viscosity of the water which restricts the gravitational action on CO2 bubbles46 as a result, PAM exhibited CO2 bubbles to remain trapped for a total time period of 24 h as confirmed by microscopic analysis at 303 K (see Table 2). The efficiency of CO2 absorption further reduced at a higher temperature of 353 K as a result, the time of CO2 absorption reduced to only 19 h (see Table 2). This behavior may be credited to the effect of high temperature that reduced the viscosity of PAM fluid and consequently, the network keeping CO2 bubbles trapped became weakened.47 The decrement in PAM elasticity increased the extent of CO2 movement towards the surface and therefore, CO2 escaped early from the aqueous phase as confirmed by microscopic analysis. The use of PAM as polymer method for oil recovery applications is very attractive and the results show that PAM can restrict CO2 movement by creating the elastic network of chains. As a result, this may shift CO2 into other pore openings where crude oil is trapped, providing significant miscibility to recover oil. However, this absorption was not superior and stable, therefore, PAM use for CO2


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oil recovery projects may face challenges and provide less increment in miscible oil recovery. Thus, a promising solvent suggesting enhanced CO2 absorption in PAM fluid is required. Next, SiO2 NPs of different concentration (0.1-1.0 wt%) were added to PAM fluid resulting into silica nanofluids which were examined for CO2 absorption and compare with the results of PAM fluid. Nanofluid NF-1, that does not contain PAM, exhibited premature NP settlement and therefore, it was not tested for CO2 absorption. CO2 absorption in nanofluid of PAM and SiO2 NPs was better and CO2 bubbles in the nanofluid were appeared to be greater in number and uniformly distributed as evident for NF-2 in Figure 6. In addition, the size of CO2 bubbles in nanofluid was lesser than the size of bubbles in PAM fluid (see Figure 5 and 6). The credit is attributed to Brownian motion of NPs in suspension; NPs tend to move continuously in the system. As a result, these particles collide repetitively with the gas-liquid interfaces spread throughout the fluid body and their constant motion causes the breaking of larger CO2 bubbles into smaller CO2 bubbles.11,48 The mechanism of enhanced CO2 absorption in nanofluids with respect to water and PAM fluid can be explained in the reference of (a) surface adsorption of CO2 by dispersed NPs and (b) surface renewal effect induced by the particle adhesion to the gas surface.11,49 According to these principles, nanofluid exhibiting stable NP suspension is capable of providing additional sites for gas adsorption in solution resulting increased area for gas bubbles to adsorb and CO2 adsorption on NP surface leads to enhanced loading of CO2 in fluid phase.11 Similar results for the interaction of NP and CO2 have been reported in previous studies;50,51 the enhanced CO2 absorption comes from the CO2 attachment to the surface of dispersed NP which increases with increasing surface area of NP in suspension. Thus, stable nanofluid with a larger number of independent surface sites of NPs is suitable for improved CO2 absorption. This is possible as CO2 absorption in NF-3 and NF-4 was short only for 6 and 4.5 days, respectively, at 303 K (see Table 2). For NF-2, the 15 ACS Paragon Plus Environment

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presence of CO2 bubbles was observed for 9 days as confirmed by microscopic analysis which indicates that CO2 seepage from the surface of NF-2 was lesser. This indicates an increasing concentration of NP in nanofluid reduced the extent of CO2 absorption, in line with a previous study.39 It is attributed to the smaller size of NP aggregates in NF-2 which provided a higher surface area for CO2 absorption, consistent with dispersion stability and DLS results. In case of NF-3 and NF-4, the size of NP aggregates was greater which indicates that the surface Gibbs energy and number of surface sites of NPs reduces due to aggregation and consequently, the tendency of CO2 absorption on NPs surface becomes weaker. The effect of high temperature on the CO2 absorption capacity of nanofluids was also measured for 353 K. It was observed that the CO2 absorption for nanofluid reduces at high temperature.39 This is expected to occur due to temperature-induced reduction in nanofluid viscosity consequently, the elasticity of SiO2-PAM network keeping CO2 bubbles trapped slightly relaxed resulting release of CO2 bubbles. Thus, released CO2 bubbles comparatively risen up faster towards the surface of nanofluid and nanofluid exhibited absorption for lesser days. At 353 K, the number of days till CO2 remained absorbed in suspension was higher for NF-2; the presence of CO2 bubbles perish after 7.8 days (see Table 2). The results show that the efficacy of PAM fluid for CO2 absorption significantly increased in the presence of SiO2 NPs and the obtained nanofluids were reflected as better solvents to be applicable in oilfield applications for CO2 storage and enhanced oil recovery. This is now expected that CO2 retention is improved due to the synergistic effect of PAM and SiO2 NPs in a nanofluid. SiO2 NPs absorption on CO2 bubbles is further studied through SEM analysis. Nanofluid stabilized CO2 absorption was stable and did not collapse when the sample was maintained at room temperature of 303 K. However, the liquid sample cannot be processed for SEM analysis hence, layer from the sample was dried considering CO2/SiO2 system 16 ACS Paragon Plus Environment

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behaved similarly in the dry state. SEM image of the dry layer of nanofluid NF-3 is shown in Figure 7. Dry nanofluid layer resembles as porous material in which interstitial space is contained by CO2 bubbles surrounded by irregular clusters of SiO2 NPs (Figure 7). The images showed that NPs were present and associate themselves with a network to stabilize CO2 against deformation. Thus, SEM studies support the network stability of CO2 in presence of SiO2 NPs. However, the exact identification of CO2 and SiO2 nanocomposite is difficult owing to the 3D nature of dry nanofluid layer. 3.3. Determination of the Henry’s constant The following reactions take place when CO2 is injected in water. CO2 (g) ↔ CO2(l) Here, l and g denote the liquid and gas phases, respectively. The final equilibrium is established between the dissolved CO2 and H2CO3, also referred as carbonic acid (a weak acid formed by the interaction of CO2 and water). CO2 + H2O ↔ H2CO3

(1)

H2CO3 ↔ HCO3- + H+

(2)

HCO3 ↔ CO3= + H+

(3)

However, these reactions involving the formation of carbonic acid are kinetically very slow and it was reported that only insignificant amount of dissolved CO2 undergoes change to H2CO3 and most of the CO2 remains in solution as solvated molecular CO2.52 Henry's law is a gas law which states that the amount of dissolved gas is proportional to its partial pressure in the gas phase. The


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proportionality factor is referred to as simply Henry’s constant. Assuming CO2 as a simple gas, Henry’s law can be applied to this system. Using Henry’s law: pCO2 = K*XCO2

(4)

Where pCO2 is the partial pressure of the gas in bulk atmosphere (Pa), K is a constant (Pa) and XCO2 is the equilibrium mole fraction of solute in the liquid phase.38 To model the solubility of CO2 in water, a modified form of Henry’s law has been proposed53 to determine Henry’s constant as-

𝐻 ≡ lim 𝑥2→0

𝑓𝑙2

()

(5)

𝑥2

Where H is Henry’s constant in l.bar/mol, x2 is the mole fraction of CO2 dissolved in the nanofluid, 𝑓𝑙2 is the fugacity of liquid in the vapor phase. The equation can be further simplified and presented as(6)

𝐻 ≈ 𝑃2/𝑥2

P2 is the observed pressure reading at the time of estimating x2. This method can be used to easily calculate the value of Henry’s constant for a solvent. It is clear that Henry’s constant is inversely proportional to the solubility of CO2 in solvent and thus, a higher value of Henry constant will suggest lesser solubility of CO2. Henry constant for each fluid can be determined by Eq. 6 using a second-degree polynomial to obtain the slope between pressure and mole fraction data. The slope of the line is typically referred as Henry’s constant and its value calculated for CO2 absorption in fluid systems are given in Table 3. It was observed that Henry’s constant for water was 32.4 bar at 303 K which accords with the reported value.54 Its value slightly decreases with the inclusion of NPs for nanofluids. This is credited to the fact that the rate of pressure increases with the solubility of a gas in solution and once the solubility tends to achieve saturation, the pressure increasing rate 18 ACS Paragon Plus Environment

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starts to limit as demonstrated55 resulting decrease in solubility and thus, Henry constant increases. In case of NF-3 and NF-4, the higher absorption performance of CO2 probably exhibits pressure increasing rate to rise comparatively slowly than NF-2 causing a reduction in CO2 solubility and therefore, Henry constant was found to slightly increase than NF-2 (Table 3). In addition, the value of Henry constant significantly increased for all fluids at elevated temperature (353 K) as shown in Table 3. At high temperature, it was observed that CO2 solubility of fluid systems decreases which consequently increases the value of Henry constant. 3.4. CO2 absorption kinetics The driving force, behind CO2 absorption at any time t (min), is the difference between the concentration of gas at equilibrium (ne) and that of bulk gas concentration (nt).56 Thus, the rate of physical absorption in terms of moles can be written as 𝑑𝑛 𝑑𝑡

(7)

= 𝑘(𝑛𝑒 ― 𝑛𝑡)

Integrating this equation, ( 𝑛𝑡 ― 𝑛𝑒)

(8)

ln (𝑛𝑜 ― 𝑛𝑒) = ― 𝑘𝑡

Where, nt is the number of moles of gas at an observed time t, no is the number of moles of gas initially present in equilibrium cell, ne number of moles of gas remaining at equilibrium pressure and k is the apparent absorption rate constant. The value of k can be determined from the slope of ( 𝑛𝑡 ― 𝑛𝑒)

the line plotted between ln (𝑛𝑜 ― 𝑛𝑒) and t. Figure 8 shows absorption kinetic results obtained from experimental data [CO2 moles absorbed (n) vs. time] at 303 K. The method utilized to plot the graphs was chosen as demonstrated.56 The


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effect of high temperature (353 K) on absorption kinetics is investigated and a slight decrease in k value was determined for each fluid as shown in Table 3 and these graphs are not provided here. The kinetic results show that NF-2, as compared to PAM fluid, exhibits higher values of k which indicates greater affinity to uptake CO2. However, nanofluids NF-3 and NF-4 exhibit lower values of k than NF-2 at both temperatures which is in accordance with Henry’s constant results. 3.5. Molality results for CO2 absorption The experimental data for the amount of absorbed CO2 in PAM fluid and nanofluids were presented in terms of equilibrium pressure (bar) of CO2 versus its molality as a function of different temperature (303 and 353 K). Molality measures the concentration of gas in a solution in terms of a number of moles in 1 kg of solvent. Figure 9 presents molality results (mole of CO2/Kg of PAM fluid) for PAM fluid of 1000 ppm and nanofluids at 303 and 353 K and compare with the ones of pure DI water. It is clear from the images, the molality value for PAM fluid is slightly higher than the molality for DI water at each equilibrium pressure and temperature. In addition, molality increases with increasing equilibrium pressure which indicates that the higher partial pressure of CO2 is more favorable for CO2 absorption. The molality results for nanofluids are also provided in Figure 9. It was observed that the molality values for nanofluids were higher than PAM fluid which confirms that the inclusion of NPs has intensified the absorption capacity of PAM fluid by a significant margin; the average increase varies between 8% to 30% for various concentrations (0.1, 0.5, and 1.0 wt% SiO2) and at different pressure and temperature. Since absorbed CO2 was distributed in smaller bubbles, the higher interfacial area increases mass transfer from CO2 to base nanofluid. Thus, the enhanced CO2 absorption capacity of nanofluids is primarily the favorable effects of mixing and breaking of gas bubbles due to the disordered motion of NPs. A similar phenomenon was studied in earlier work.11 It is to be noted here that the effective CO2 absorption 20 ACS Paragon Plus Environment

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decreases with increasing NP concentration and NF-4 exhibited lesser molality values than NF-3 and NF-2. It indicates that the maximum absorption is possible with low NP concentration (0.1 wt%). The advantage of high absorption resulting from interfacial effects, mass transfer, and breaking of gas bubbles is free Brownian motion which typically becomes restricted if the particle loading increases because inter-particle interaction hinders the motion.39 Thus, inter-particle interaction leads to homo-aggregation as particles always tend to minimize the surface energies and the aggregation leaves the less interfacial area for absorption.36 Next, it is expected that aggregation results in high size clusters which relatively tend to stay in nanofluid phase rather than participating in the Brownian action to break gas bubbles. Therefore, the molality for NF-2 progressively decreases with increasing SiO2 concentration from 0.1 wt% to 0.5 wt% (NF-3) and further 1.0 wt% (NF-4). One can now say that increasing NP concentration beyond a threshold value (0.1 wt% for the current study), will cause CO2 solubility to decrease. Another important inference which can be drawn from our results is that CO2 absorption increases with an increase in pressure but reduces with an increase in temperature (353 K).39 Any water-soluble gas becomes more soluble at lower temperatures. This is due to the thermodynamics of the reaction: GAS (l) ↔ GAS (g) and entropy change, ∆S, of this reaction are positive. Therefore, increasing temperature causes an increase in kinetic energy resulting in more motion in gas molecules and break their intermolecular bonds to escape from solution. Hence, CO2 loading in nanofluids has been found to reduce with an increasing temperature and this effect becomes more severe on molality at high pressures which might be due to more CO2 absorption at high pressure. 3.6. UV-vis results of CO2 absorption


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UV-vis analysis is one of the important studies that can measure nanofluid performance in CO2 stabilization via absorbance of light to the proportion of passing material. The absorbance level depends on the amount of light absorbed by suspended NPs in nanofluid and high or lower absorbance will be recorded. The study used these measurements to determine the amount of NPs participated in CO2 stabilization in terms of absorbance level; it is expected that some NPs from the bulk nanofluid phase preferably goes to the interfacial area of CO2-nanofluid resulting remaining NPs to stay in nanofluid phase as shown by the schematic in Figure 10. Thus, nanofluid before and after CO2 absorption will show different absorbance levels. Nanofluids immediately after the preparation was used to perform UV-vis experiments to avoid the possibility of NP settlement. Before CO2 absorption, all NPs are subjected to absorb light and maximum peak on absorbance level will be observed while after absorption (Figure 10), the nanofluid phase is expected to consist of a reduced number of NPs to absorb light.36 As a result, the level of absorbance peak must decrease which indirectly confirms NP participation on CO2 stabilization. Therefore, after CO2 absorption, a small amount from remnant nanofluid is carefully extracted using a needle syringe with the assurance that CO2 did not come in with nanofluid. To ensure that no CO2 bubble was the part of UV analysis, nanofluids were examined under a microscope before using in UV equipment. For nanofluids, UV-vis results, conducted before and after CO2 absorption, are provided in Figure 11 at 303 K. From the results, it was observed that absorbance peak before CO2 absorption was higher for NF-2 than NF-3 and NF-4; 3.55 for NF-2, 3.12 for NF3, and 3.01 for NF-4 at the wavelength 220 nm. It indicates that a greater number of NPs participated in absorbance when the concentration of SiO2 was lower (NF-2) and NPs with high concentration (NF-3/NF-4) aggregate leaving less surface area to absorb light. It also shows that the density of distribution in nanofluid decreases and consequently, some rays pass without


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striking NPs in NF-3 and NF-4 and gives less absorbance (Figure 11). UV results after CO2 absorption are also provided in Figure 11. For NF-2, the peak absorbance was recorded lowest value of 0.82 nm while its value for NF-3 and NF-4 was recorded as 1.44 nm and 1.31 nm, respectively. Thus, the absorbance peak for NF-3 and NF-4 does not change much, which is comparatively much lesser than NF-2. The percentage change in absorbance before and after CO2 absorption experiments for nanofluids is also shown in Figure 11. The percentage change for NF-3 and NF-4 was not more than 60% while its value for NF-2 was maximum at 80%. The occurrence of higher absorbance peak shows that strength of NPs after CO2 absorption did not change much which indicates that most of the NPs remained in bulk phase of nanofluid and does not move to the interfacial area of CO2-nanofluid. It is likely because movement becomes difficult with an increase in size than NF-2 and NPs preferred to stay in nanofluid phase which correlates well with the fact that aggregation reduced the rate of CO2 absorption. For NF-2, peak absorbance reduced significantly after CO2 absorbance which indicates most of the NPs moved towards the interfacial area and NF-2 remained with less NPs to absorb light. Thus, nanofluid exhibiting polymer and NPs of lower sizes can provide promising improvements in polymer methods of CO2 absorption in oilfield applications. 4. Conclusion In this work, stable nanofluid of SiO2 NPs (0.1-1.0 wt%, size 15 nm) and high molecular weight PAM (1000 ppm) were prepared for CO2 absorption and compared with conventional PAM fluid at different temperature (303 and 353 K). Nanofluids prepared without PAM (NF-1) were not stable against sedimentation and nanofluids with PAM exhibited moderate dispersion stability 6 weeks. The inclusion of silica NP influenced CO2 absorption capacity of PAM fluid differently;

nanofluid prepared with 0.1 wt% SiO2 and PAM (NF-2) had CO2 absorption till 9 days while CO2 23 ACS Paragon Plus Environment

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absorption in NF-3 (0.5 wt% SiO2) and NF-4 (1.0 wt% SiO2) lasted only for 6 and 4 days, respectively. Adding NP is better than PAM itself, but increasing NP concentration reduces absorption. From the microscopic, kinetics, and molality results, it was confirmed that increasing NP concentration decreases CO2 absorption capacity of nanofluids. This decrease in CO2 absorption on increasing nanoparticle is due to increase in the homo-aggregation between NPs (confirmed by DLS results). Molality results also confirmed that CO2 absorption capacity of nanofluids (NF 2-4) was higher than PAM. The increase in test temperature reduced CO2 absorption capacity of nanofluids. The CO2 absorption was found to increase on increasing the confining pressure (for all solvents). Finally, UV measurements were presented to find the percentage change in absorption capacity of different nanofluids; NF-2 exhibited a maximum change of 80% while NF-3 and NF-4 persisted with 58% and 52%, respectively. It can be concluded from our work that while increasing NP concentration and temperature reduce CO2 absorption capacity, an inverse of this occurs on increasing pressure. NF-2 nanofluid which has the lowest concentration of silica NPs (0.1 wt%) has the highest desired property (maximum absorption of CO2). Thus, the proposed use of SiO2 is noteworthy for CO2 absorption capacity to be applicable in situations where conventional PAM fluid face limitations. Acknowledgment Department of Science and Technology, India (Grant: SB/S3/CE/057/2015) is gratefully acknowledged for providing financial assistance for the work. Also, as a part of the University of Alberta’s future energy systems research initiative, this research was made possible in part thanks to funding from the Canada first research excellence fund. References


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(37) Al-Anssari, S.; Barifcani, A.; Wang, S.; Maxim, L.; Iglauer, S. Wettability alteration of oil-wet carbonate by silica nanofluid. J. Colloid Interface Sci. 2016, 461, 435-442. (38) Haider, M. B.; Hussain, Z.; Kumar, R. CO2 absorption and kinetic study in ionic liquid amine blends. J. Molecular Liquids 2016, 224, 1025-1031. (39) Haghtalab, A.; Mohammadi, M.; Fakhroueian, Z. Absorption and solubility measurement of CO2 in water-based ZnO and SiO2 nanofluids. Fluid Phase Equili. 2015, 392, 33-42. (40) El-sayed, G. M.; Kamel, M. M.; Morsy, N. S.; Taher, F. A. Encapsulation of nano disperse Red 60 via modified miniemulsion polymerization I. Preparation and characterization, J. Applied Poly. Sci. 2012, 125, 1318-1329. (41) Park, S.; Choi, B.; Lee, J. Effect of polyacrylamide on absorption rate of carbon dioxide in aqueous polyacrylamide solution containing monoethanolamine. J. Ind. Eng. Chem. 2007, 13, 7-13. (42) Palomar, J.; Gonzalez-Miquel, M.; Polo, A.; Rodriguez, F. Understanding the physical absorption of CO2 in ionic liquids using the COSMO-RS method. Ind. Eng. Chem. Res. 2011, 50, 3452–3463. (43) Arab, P.; Rabbani, M. G.; Sekizkardes, A. K.; İslamoğlu, T.; El-Kaderi, H. M. Copper(I)-catalyzed synthesis of nanoporous azo-linked polymers: Impact of textural properties on gas storage and selective carbon dioxide capture. Chem. Mat. 2014, 26(3), 1385–1392. (44) Dani, A.; Crocellà, V.; Magistris, C.; Santoro, V.; Yuan, J.; Bordiga, S. Click-based porous cationic polymers for enhanced carbon dioxide capture. J. Mat. Chem. A 2016, 5(1), 372–383. (45) Taniguchi, I.; Kinugasa, K.; Toyoda, M.; Minezaki, K. Effect of amine structure on CO2 capture by polymeric membranes. Sci. Tech. Adv. Mat. 2017, 18(1), 950–958. (46) Jung, J.; Jungyeon, J.; Ahn, J. Characterization of a polyacrylamide solution used for remediation of petroleum contaminated soils. Materials 2016, 9, 16.


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1 2 3 4 5 6 7 8 Tables 9 10 11 12 Table 1. Compositional details, nomenclature, stability, and DLS results for various fluid systems 13 14 ζ potential (mV) First sign 15 T Dispersion Concentration Nomenclature of NP 16 Fluid type After 40 (K) stability Initial 17 settlement days 18 19 Aqueous 20 polymer ------------1000 ppm PAM fluid 21 PAM 22 solution 23 1.0 wt% SiO2 24 NF-1 2h 3h -36.22 -11.85 + DI water 25 26 303 0.1 wt% SiO2 27 NF-2 30 days 44 days -37.14 -26.41 + PAM fluid 28 Nanofluids 29 0.5 wt% SiO2 NF-3 26 days 40 days -51.87 -18.74 30 + PAM fluid 31 1.0 wt% SiO2 32 NF-4 24 days 36 days -52 -16.54 33 + PAM fluid 34 *T = Temperature 35 36 Table 2. Duration of CO2 absorption in various fluid systems at 303 and 353 K 37 38 Duration of microscopic 39 Fluid type T (K) stability (days) 40 41 1 (24 h) 303 42 PAM fluid 43 0.8 (19 h) 353 44 45 ---303 46 NF-1 47 ---353 48 9 49 303 NF-2 50 7.8 353 51 52 6 303 53 NF-3 54 4.2 353 55 56 57 58 31 59 ACS Paragon Plus Environment 60

Average size (µm)

----

---0.41 0.85 2.8

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NF-4

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303

4.5

353

3.5

Table 3. Henry constant and absorption kinetics for the fluid systems at 303 and 353 K Fluid type Water PAM fluid NF-1 NF-2 NF-3 NF-4

T (K)

Henry constant (l.bar/mol)

k (min-1)

303

32.4

0.069

353

36.5

0.059

303

32.6

0.103

353

36.3

0.092

303

32.5

0.063

353

36.8

0.049

303

31.4

0.128

353

35.8

0.113

303

31.7

0.115

353

36.0

0.096

303

31.9

0.108

353

36.2

0.094


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Figures

Figure 1. Schematic showing CO2 molecule stabilization by a complex created from the adsorption of PAM on the surface of NP.


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Figure 2. Schematic of the experimental set-up used for CO2 absorption experiments in fluid systems.


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Figure 3. Nanofluids prepared with the varying concentration of SiO2 NP (0.1, 0.5, and 1.0 wt%) in the aqueous phase of water and PAM (1000 ppm). The changes in nanofluid appearance with time is also evident from the image.

Figure 4. SEM image showing NP aggregates of higher sizes than the actual size of SiO2 NP (15 nm) for NF-2 nanofluid at 303 K.


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Figure 5. Microscopic image of CO2 bubbles stabilized in aqueous phase of PAM fluid at 303 K.

Figure 6. Microscopic image of CO2 bubbles stabilized in aqueous phase of NF-2 nanofluid at 303 K.

Figure 7. SEM image of a dried CO2-nanofluid layer showing the distribution of CO2 bubbles into the matrix of aggregated SiO2 NPs at 303 K.


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Figure 8. CO2 absorption kinetic results for DI water, PAM fluid, and different nanofluids (NF-2, NF-3, and NF-4) at 303 K.


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Figure 9. CO2 molality results for DI water, PAM fluid, and different nanofluids (NF-2, NF-3, and NF-4); (a) 303 K: (b) 353 K.

Figure 10. Proposed schematic showing CO2 stabilization by NPs in nanofluid and nanofluid left with some NPs (preferred to stay in bulk phase) without exhibiting adsorption on the CO2nanofluid interfacial area.


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Figure 11. UV-vis results (absorbance and % change), conducted before and after CO2 absorption, for different nanofluids (NF-2, NF-3, and NF-4) at 303 K.


Stable Silica Nanofluids of an Oilfield Polymer for Enhanced CO2

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