Improving the physical properties of nano-cellulose through chemical grafting for potential use in enhancing oil recovery

The performance of nano-cellulose fluid as a "green" flooding agent in enhancing oil recovery was evaluated in our previous study. Expanding upon our prior findings, in this study the physical properties of nano-cellulose were further improved through chemical grafting with 2-acrylamido-2-methylpropane sulfonic acid monomer (AMPS) and alkyl chain. Scanning electron microscopy (SEM) observation indicated that the morphology of the nano-cellulose maintained fibrillar and was not altered after the chemical modification. The thermal stability of the AMPS and alkyl chain grafted nano-cellulose was investigated through thermogravimetric analysis (TGA). A similar thermal response behavior was observed for the three evaluated samples. Compared to the non-grafted nano-cellulose, the grafted nano-cellulose remained homogenous in an electrolyte solution against storage time, suggesting a superior sanity-tolerance. Rheological analysis also proved the advanced viscoelastic properties of the nano-cellulose dispersion.


INTRODUCTION
The initial pressure of an oil reservoir is usually not sufficient to extract all the oil.At the end of the natural depletion stage, a common practice in commercial oil production is to inject water to displace the residue oil, which is known as "waterflooding".However, due to the drastic viscosity contrast and geological heterogeneity, the injected water will easily finger or channel through the formation, leaving a large volume of oil untouched.Given this issue, in the past several decades, numerous methods have been studied at industrial and academic levels aiming to further improve the oil recovery from the water flooded reservoirs.Among these methods, polymer flooding was proven to be one of the most efficient ways for enhancing oil recovery through the mechanism of correcting the unfavorable mobility ratio. 1,2Currently, hydrolyzed polyacrylamide is still the most extensively used polymer to date due to its availability in large quantities with customizable properties (molecular weight, hydrolysis degree, etc.) and low manufacturing cost.However, this type of polymer usually experiences severe susceptibility to chemical, mechanical, thermal, and microbial degradations. 35][6] There is a need to develop stable and eco-friendly alternatives to hydrolyzed polyacrylamide.
Nanofluids, which are obtained through dispersing nanoparticles (nanofibers, nanotubes, or nanodrops) in the base fluids, are attracting more and more attentions in this area of research.As reported by Choi, 7 these fluids usually exhibit high thermal conductivities, which is significant to the development of energy, and have been widely used in drug delivery solar cells, lipase immobilization, soil remediation, lubrication, and hydraulic fracturing of gas and oil.][10][11][12][13][14][15][16][17] Much work has been done to study the EOR performance of nanofluids composed of different nanoparticles.Hendraningrat et al. carried out a coreflood experiment at laboratory scale to evaluate the displacement efficiency of the nanofluid.They found that the nanofluid could produce about 7%~14.3% of additional oil, and the optimum concentration was 0.05 wt% for water-wet core. 18,19The stability of nanofluids was considered as an important factor for enhancing oil recovery, which is closely associated with the success of nanofluid flooding. 20Roustaei et al. demonstrated that the modified silica nanoparticles were able to enhance light oil recovery by reducing interfacial tension and reversing wettability. 21][24] In this work, a novel "green" nanofluid composed of chemically modified nano-cellulose showing potential in enhancing oil recovery, was provided.The physical properties and micro oil recovery mechanisms of the nano-cellulose fluids were comprehensively investigated in our previous report. 25It was observed that the TEMPO nano-cellulose would easily aggregate with the exposure to the electrolyte due to the weak repulsive forces.This issue may render this nanofluid problematic in injection and propagation in porous media. 206][27][28][29] Therefore, the primary objective of this work was to investigate the physical properties of the chemical modified nano-cellulose.The variations under the impact of thermal, salinity and shear after grafting were specified.

Materials
The samples of nano-cellulose were provided by Tianjin Woodelf Biotechnology Co. Ltd. (Tianjin, China).The chemical structures of the samples evaluated in this work are depicted in Fig. 1.The average size of the nano-cellulose is 900nm.

Thermogravimetric analysis
Thermogravimetric measurements were carried out using a DSC823 TGA/SDTA 851e analyzer (Mettler-Toledo Crop., Switzerland).Prior to the analysis, the nano-cellulose samples were completely dried for a couple of days in a 100 ℃ oven.Approximately 7mg of the samples were heated from 25 to 400 o C under nitrogen atmosphere at a constant heating rate of 10 o C/min.The weight as a function of temperature was recorded.

Scanning electron microscopy
The morphology of nano-cellulose was examined with a Quanta 450 SEM.Small amounts of samples were placed on a cooper grid and evaporated using liquid nitrogen before being subjected to observation.

2.4Rheological analysis
The steady viscosities of the nanofluids as a function of shear rate (0.1~1000s -1 ) were measured using an Anton Par MCR 302 rheometer at a constant temperature of 25 o C. Dynamical shear measurements were performed with the same rheometer and measuring system at 25 ℃ in order to determine the loss (G″) and storage (G′) moduli of the nanofluids as a function of frequency ranging from 0.1 to 100 Hz.It should be noted that a strain sweep test was conducted to distinguish the linear viscoelastic region of the nanofluids prior to the dynamical measurements.All of the tests were performed in the linear response region.

3.1Thermal stability
The comparison of thermal stability of the nano-cellulose was conducted using TGA/DTG.Fig. 2 depicts the thermal responsive behavior of three samples upon heating.It can be seen that the TGA curves can be generally subjected into two regions.The first weight loss (13%) of the nano-cellulose occurred at the temperature below 250℃ can be attributed to the evaporation of the absorbed water.The second significant weight loss took place at the temperature between 250 and 330 ℃, caused by the rupture of carbon bonds.Fig. 2 also indicated that the thermal stability of NC-KY and NC-KYSS was slightly higher than NC, which probably resulted from the presence of the AMPS and alkyl chains.

3.2Morphology of the grafted nano-cellulose
The morphological features of the grafted nano-cellulose were examined using SEM as shown in Fig. 3.The new morphology of the nano-cellulose after modification was observed to remain unaltered.Generally, the nano-cellulose maintained the fibrous structure with the exception of slight roughness surface of NC-KY and NC-KYSS, probably due to the localized cellulose degradation during the grafting process.

3.3Dispersity of nano-cellulose
As mentioned above, the stability of nanofluid is one of the major concerns for EOR use.The presence of cations usually causes nano-cellulose to aggregate, and accordingly detracts the fluidity and also results in mass loss in porous media. 17,30,31In this section, the stability of the nano-cellulose dispersion was comprehensively studied as a function of salinity and storage time, from which the limits of cation concentration were identified.Fig. 4(a) clearly illustrates that the nanofluid of the ungrafted nano-cellulose remained homogeneous for 30 days without the presence of cations.However, once cations were introduced, nano-cellulose aggregation was observed as indicated in Fig. 4(a).The aggregation of NC under the effect of cations can be ascribed to the adsorption of Na + on the surface of nano-cellulose, which subsequently resulted in the reduction of Zeta potential and repulsive forces between molecules. 32he advanced properties of AMPS such as salinity resistance and thermal stability have been extensively studied previously. 33Therefore, the nano-cellulose is supposed to be able to withstand cations after AMPS grafting as shown in Fig. 4(b).Nevertheless, it can be seen that the phase separation took place after 3 days when the NaCl concentration was 0.5%, indicating AMPS grafting was still not sufficient to combat high salinity.
The stability of the nano-cellulose was considerably enhanced after alkyl chain grafting on the basis of NC-KY as depicted in Fig. 4(c).Moreover, the dispersity of the NC-KYSS was even promoted with the increasing cation concentration.As shown, slight aggregation indeed occurred at 0.7% of NaCl after 30 days, which was quite similarto NC and NC-KY.However, as indicated in Fig. 5, the NC-KYSS was still well dispersed in the aqueous solutions containing 1.0-1.5% NaCl.This improvement in stability in saline solution was associated with the hydrophobic groups grafted onto the NC-KY.It was known that the solvent polarity of the solution increased with the content of cations, which therefore made the amphiphilic nano-cellulose (NC-KYSS) well dispersed in aqueous solution.However, as the polarity of the aqueous increases, the amphiphilic nano-cellulose will be repulsed towards the gas-water surface, which is the so-called "salting out effect".Therefore, the limit of NaCl for NC-KYSS is 1.5% based on our experimental results.The schematic representation of nano-cellulose dispersity is shown in Fig. 6.

3.4Rheological properties of the nanofluid
The viscoelastic properties of the injectants are crucial parametersto the performance in enhancing oil recovery.In this section, two sets of tests: 1) steady viscosity and 2) linear viscoelastic oscillatory were performed to study the rheological behavior of the nanofluids.Furthermore, the viscosity of NC-KYSS increased slightly when the shear rate was higher than 600s -1 , which was probably caused by the re-entanglement of the fibrils.
The viscosity of the nanofluids versus the concentrations at a constant shear rate (10s -1 ) and room temperature 25 o C is also shown in Fig. 7.It was found that the effective viscosities of the nanofluids significantly increased with the increasing concentration.However, the grafting of AMPS and alkyl chains imposed a negative impact on the thickening capabilities of the nano-cellulose, likely caused by the enlarged molecule space. 35,36The comparison of nanofluid viscoelasticity was performed as shown in Fig. 8.The storage G' and loss G" of the nanofluid as a function of frequency were plotted.It can be seen that both G' and G" increased with nano-cellulose concentration and the values of G' are higher than G" values, illustrating the gel-like behavior of the nanofluids.The effect of thecations (Na + ) on the rheology of the nanofluids is illustrated in Fig. 9.In the cases of NC and NC-KY, the addition of cations significantly reduced the G" and G' of the nano-fluids.On the contrary, for NC-KYSS, the viscoelasticity was improved with the addition of the cations, which might be caused by the increased solvent polarity of the aqueous solution and thus provided additional repulsive forces to the NC-KYSS.This result can also be understood by the dispersity of NC-KYSS as shown in Fig. 4 and Fig. 5.

CONCLUSIONS
In this work, the physical properties of AMPS and alkyl chain grafted nano-cellulose were comprehensively investigated.Based on the experiments, some main conclusions can be stated as follows: (1) The thermalstability of nano-cellulose was slightly enhanced due to the presence of the functional groups.After grafting, the morphological features of the nano-cellulose were not altered.
(2) The AMPS and alkyl chain grafted nano-cellulose exhibited superior stability in the presence of electrolyte upon storage.As a result of the hydrophobic effect, this nano-cellulose remained homogenous for a duration of 2 months at 1.5%NaCl concentration.
(3) The nanofluids showed a gel-like behavior.The viscoelasticity of the nano-cellulose fluids was reduced after chemical grafting.The presence of cations can enhance the viscoelasticity of the AMPS and alkyl chain grafted nano-cellulose.

Fig. 4
is presented to show the nano-fluids at different NaCl concentrations upon storage.

Fig. 7 .
Fig. 7. Steady shear viscosity of the nanofluids Fig. 7 plots the steady viscosity of the nanofluids at different concentrations (0.2, 0.3, 0.4 and 0.5 wt%) as a function of shear rate.The shear dependence of the viscosity profiles clearly indicated that the viscosity of these nanofluids decreased nearly linearly with shear rate, suggesting the non-Newtonianfeature of nano-cellulose.Furthermore, the viscosity of NC-KYSS increased slightly when the shear rate was higher than 600s -1 , which was probably caused by the re-entanglement of the fibrils.The viscosity of the nanofluids versus the concentrations at a constant shear rate (10s -1 ) and room temperature 25 o C is also shown in Fig.7.It was found that the effective viscosities of the nanofluids significantly increased with the increasing concentration.However, the grafting of AMPS and alkyl chains imposed a negative impact on the thickening capabilities of the nano-cellulose, likely caused by the enlarged molecule space.35,36

Fig. 8 .
Fig. 8. Viscoelastic properties of the nano-fluids as a function of frequency